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Eriophorum vaginatum

Table of Contents

• INTRODUCTORY
• DISTRIBUTION AND OCCURRENCE
• BOTANICAL AND ECOLOGICAL CHARACTERISTICS
• FIRE EFFECTS AND MANAGEMENT
• MANAGEMENT CONSIDERATIONS
• APPENDIX: FIRE REGIME TABLE
• REFERENCES

INTRODUCTORY

		AUTHORSHIP AND CITATION SUMMARY FEIS ABBREVIATION COMMON NAMES TAXONOMY SYNONYMS LIFE FORM
Figure 1. Tussock cottongrass. Photo by Robert K. Mohlenbrock at USDA-NRCS PLANTS database / USDA NRCS. 1995. Northeast wetland flora: Field office guide to plant species. Northeast National Technical Center, Chester. Courtesy of USDA NRCS Wetland Science Institute.

DISTRIBUTION AND OCCURRENCE

• GENERAL DISTRIBUTION
• SITE CHARACTERISTICS AND PLANT COMMUNITIES

Figure 2. Distribution of tussock cottongrass by state and province. Green indicates that tussock cottongrass is present. Map courtesy of the PLANTS Database [344].

Tussock cottongrass is native to northern North America and Eurasia. In North America, it occurs from Alaska south to British Columbia, east to New England, and north to Greenland [112,113,177,225,230,290,366]. Tussock cottongrass is very common in the Low Arctic [19,348] but absent in the High Arctic [116].

States and provinces [344]:
United States: AK, CT, IN, MA, ME, MI, MN, NH, NJ, NY, PA, RI, VT, WI
Canada: AB, BC, MB, NB, NL, NS, NT, NU, ON, PE, QC, SK, YT

Elevation: Across its distribution in North America, tussock cottongrass occurs from 0 to 5,000 feet (0-1,524 m) [5,113,154,193,232]. It occurs from coastal up to montane and alpine zones [161,186,193].

• Texture and depth
• pH
• Moisture
• Nutrients

Patchy or continuous permafrost commonly underlies tussock cottongrass communities. Depth of the active layer varies. Seasonal thawing of the active layer in arctic tundra, where tussock cottongrass is widespread, typically reaches only 4 to 24 inches (10-60 cm) below the soil surface [18,20,316]. In subarctic areas, depth of the active layer in tussock cottongrass communities ranges from 5 to >39 inches (12-100 cm) [51,317]. Organic horizons may constitute most of the active layer [348]. Consequently, tussock cottongrass roots may not penetrate far into mineral soil, if at all. However, organic horizons in some areas of permafrost are thin, so tussock cottongrass is more deeply rooted in mineral soil [51,157,338,371]. Viereck and others' [348] claim that tussock cottongrass tussocks in Alaska "are always rooted in mineral soil so that the organic mat is never thicker than the active layer" appears to be erroneous. Tussock cottongrass was common at a Dempster Highway site, Yukon, where an average of 2 inches (5 cm) of organic matter remained permanently frozen below the active layer [371]. At 3 sites in central and northern Alaska (Eagle Creek, Meade River, and Cape Thompson), most tussock cottongrass roots did not extend below the structured dead layer (Shaver and Cutler unpublished data cited in [299]). Near Fairbanks, tussock cottongrass stands were most common where silt was abundant within the upper 12 inches (30 cm) of the soil profile. However, in some areas it occurred where the active layer consisted entirely of organic material, and in other areas layers of organic and mineral material alternated [51]. In a sphagnum moss-tussock cottongrass-leatherleaf (Sphagnum spp.-Eriophorum vaginatum-Chamaedaphne calyculata) mire near Warsaw, Poland, only in some years did the roots of tussock cottongrass reach near the underlying sand at about 39 inches (100 cm) deep [338]. In the southern Pennines, England, tussock cottongrass communities occurred on 2- to >5-feet (0.6-1.5 m) deep peat, and tussock cottongrass roots were confined to the peat substratum [1].

Depth of peat in tussock cottongrass communities varies. Tussock cottongrass occurred on "thin" peat deposits in Belgium [87], Finland [237], and northern England [130], but it also occurred where peat deposits were 10 feet (3 m) deep or more [366]. For example, a review noted that tussock cottongrass bogs in the Pennines usually have 2- to 30-feet (0.6-9 m) deep peat [366]. In a Finnish bog, peat cores (taken at 43 and 75 feet (13.1 and 22.9 m) deep) indicated that tussock cottongrass cover increased with increased peat depth [341]. For more information, see Seedling establishment and plant growth.

pH: Tussock cottongrass is very common in acidic soils (3.0-6.5 pH) (e.g., [43,112,125,129,154,216,293,316,333,366]). It occasionally occurs in neutral or mildly alkaline soils [93,317,375]. In northern Minnesota, tussock cottongrass was restricted to bogs with pH below 4.2 [125]. In the Firth River Basin of Alaska and Yukon, well-developed tussock cottongrass tussocks covered about 40% of the strongly acid soils of the upland tundra. Tussocks were less extensive and poorly developed on the mildly alkaline calcareous soils of a Carex spp. meadow terrace. The authors hypothesized that the relatively high amounts of available phosphorus and potassium in the strongly acid upland tundra soils may be important in the growth of large tussocks [93]. For more information on soil nutrients, see Seedling establishment and plant growth.

Moisture: Tussock cottongrass occurs in mesic and wet soils [86,97,98,183,207,263]. Soils may be moderately well-drained (e.g., [175]) to poorly drained (e.g., [243,263,309]). In the Arctic, tussock cottongrass roots grow to the bottom of the active layer, where melting permafrost keeps the entire root profile moist [366]. Tussock cottongrass is considered a facultative (occurs in wetlands 68%-99% of the time) or obligate (occurs in wetlands >99% of the time) wetland species throughout its range in the United States [272,344].

Tussock cottongrass appears to prefer sites where the water table is near to but below the soil surface [43,46,51,85,183,207,247,253,366]. However, it appears to be a "generalist" that can grow in a variety of soil moisture conditions, including where the water table is above the soil surface [131,183,319,374]. Many researchers described tussock cottongrass as tolerant of annual and seasonal flooding and drying (e.g., [285,319,338,348,366,366]). Tussock cottongrass tolerates occasional drought, apparently because of its deep roots and the high moisture-holding ability of the organic substrates within which it occurs. However, prolonged drought may kill tussock cottongrass [366].

Tussock cottongrass cover tends to decrease when bogs are drained [319] and increase when bogs are rewetted [165,189,207,343]. Laiho [199] attributed declines in tussock cottongrass cover following peatland drainage to shading by trees rather than to water level drawdown.

Growth of tussock cottongrass is usually best in areas with ground water movement [63,71] or where soil water flows rapidly ([51], Kriuchkov 1968 cited in [63]). Near Toolik Lake, Alaska, tussock cottongrass aboveground biomass was 10 times greater in water tracks (areas of above-average surface and subsurface water flow [301]) than in adjacent, undisturbed tundra [63]. In tussock tundra in the Philip Smith Mountains, Alaska, however, tussock cottongrass aboveground biomass was lower in water tracks than undisturbed areas [143]. Hastings and others [143] attributed differences between studies to the later successional stage of the water tracks in the Philip Smith Mountains, but did not provide details.

Nutrients: Tussock cottongrass commonly grows on nutrient-poor sites. : Nitrogen, phosphorus, and/or potassium may be limiting, depending on the site [297,298]. See Nutrient effects on plant growth for more information.

Climate: Tussock cottongrass occurs in climates with extreme seasonal variations in light and temperature [19]. The growing season may be 3 months or less [11,19,60,240]. Permafrost may be continuous, discontinuous, or nonexistent in tussock cottongrass communities. See Moisture for more information.

Plant communities: Tussock cottongrass occurs in tundra, marshes, bogs, fens, wet to mesic meadows and shrublands, swales, conifer swamps, peatlands, and muskegs (e.g., [39,44,80,105,186,193,294,348,355,366]). See the Fire Regime Table for a list of plant communities in which tussock cottongrass may occur and information on the fire regimes associated with those communities.

Tussock cottongrass occurs in upland and lowland tundra. Upland tundra in Alaska may be divided into moist tussock tundra, dry or alpine tundra, and low or tall shrub tundra. Over much of arctic and western Alaska, tussock cottongrass dominates moist tussock tundra. In moist tussock tundra, shrubs—commonly arctic dwarf birch (Betula nana), northern Labrador tea (Ledum palustre), mountain cranberry (Vaccinium vitis-idaea), bog blueberry (V. uliginosum), and black crowberry (Empetrum nigrum)—total <5% cover. Mosses and lichens are common. Sphagnum moss may be locally abundant but more commonly is absent or sparse. Bigelow sedge (Carex bigelowii) tundra is much less common in moist tussock tundra than tussock cottongrass and usually occupies slightly steeper and better-drained sites when the 2 occur in the same area. Tussock tundra communities with >25% shrub cover are classified as birch-ericaceous (Betula spp.-Ericaceae) shrublands or mixed shrub-tussock cottongrass tundra. Lowland tundra occurs primarily on the coastal plain in northern Alaska and in low-lying deltas and other coastal areas in western Alaska. The dominant vegetation in lowland tundra is a wet sedge meadow of tall cottongrass and water sedge (C. aquatilis) interspersed with many lakes. Tussock cottongrass occurs in these communities on relatively dry sites. Composition of tussock cottongrass communities ranges from pure tussock cottongrass with no shrubs to ≥50% shrub cover [19,20,114,348].

Tussock cottongrass is a component of many cold, moist shrublands and forested swamps. Throughout Alaska, scattered tussock cottongrass tussocks may be present in open low alder (Alnus spp.) shrublands, open low alder-willow (Salix spp.) shrublands, and on wet sites in open low birch (dwarf birch (Betula glandulosa) or arctic dwarf birch)-willow shrublands [348]. Tussock cottongrass often occurs in boreal black spruce (Picea mariana), white spruce (P. glauca), and tamarack (Larix laricina) communities (e.g., [216,242,328,348,349,350]). In Michigan, it forms large tussocks in open tamarack and black spruce swamps [355].

Tussock cottongrass is common in sphagnum moss (e.g., [176,216,242,256]), leatherleaf [216,293,333], and sheep-laurel (Kalmia angustifolia) [216,293] bogs. In parts of New England, New York, and the Canadian Maritime provinces, tussock cottongrass is a characteristic species in ombrotrophic sheep-laurel-leatherleaf-black spruce/reindeer lichen (Cladina spp.) dwarf-shrubland. It also occurs in nutrient-poor, acidic, weakly minerotrophic leatherleaf/tussock cottongrass/red sphagnum (Sphagnum rubellum) dwarf-shrubland fens and in the oligotrophic ombrotrophic black crowberry-dwarf huckleberry-cloudberry/sphagnum moss (Gaylussacia dumosa-Rubus chamaemorus/Sphagnum spp.) dwarf-shrubland bogs [216]. Throughout its range it is common in ombrotrophic mires (e.g., [6,216,256,335,375]). In Finland, tussock cottongrass preferred ombrotrophic peatlands to minerotrophic oligotrophic peatlands, minerotrophic mesotrophic peatlands, or forests [6].

BOTANICAL AND ECOLOGICAL CHARACTERISTICS

• GENERAL BOTANICAL CHARACTERISTICS
• SEASONAL DEVELOPMENT
• REGENERATION PROCESSES
• SUCCESSIONAL STATUS

Form and architecture: Tussock cottongrass is a densely tufted [8,44,126,161,186,278,290,294], tussock-forming [105,113,126,161,186], perennial sedge [44,186]. Some tussocks may be vertically elongate [157] or crescent shaped, perhaps due to wind (Forrest personal communication cited in [366]).

Aboveground structures: Leaves are 0.02 to 0.05 inch (0.5-1.2 mm) wide [113,186], triangular in cross section and channeled, and the upper sheaths are loose and inflated [44,105,113]. Some literature describes tussock cottongrass as evergreen because the basal portions of some leaves and stems remain green over winter [225,301].

Tussock cottongrass culms are stiff, 4 to 28 inches (10-70 cm) tall [8,105,113], and sheathed to half their lengths [109]. Spikelets are solitary, terminal, and erect [44,105,113,126,186,278,355]. They are 0.4 to 0.8 inch (1-2 cm) long in flower and 0.7 to 2.0 inches (1.7-5.0 cm) long in fruit [113,186]. Each flower has ≥10, typically white, perianth bristles (strong, stiff, slender hairs [220]) that are 0.25 to 0.75 inch (6-19 mm) long and cottony, becoming more conspicuous as fruits mature [8,44,113,186,278]. Fruits are small nutlets [113,126,186,366].

Tussock cottongrass has tillers (e.g., [11,71,106,108,225,226,276,360]). Some researchers described tussock cottongrass as having rhizomes (e.g., [10,17,58,71,82,89,157,366,371]).

Tussock cottongrass has aerenchyma in its stems and roots [52,312]; this facilitates movement of oxygen into roots and methane and other gases out to the atmosphere [223]. This allows tussock cottongrass to tolerate flooding [312] and anoxia [52,82]. See Value for Rehabilitation of Disturbed Sites for information about greenhouse gas emissions by tussock cottongrass.

Belowground structures: Tussock cottongrass has an annual, fibrous root system that dies each winter and regrows each summer [71,240]. Freezing kills roots. Roots can live for up to 9 months if not frozen. Unfrozen, overwintering roots produce new shoots in spring [60]. Tussock cottongrass roots are slender and dense [36,128,225,226,240]. Plants in Alaska produced adventitious roots [225]. For more information, see Seasonal Development.

Bliss [36] described tussock cottongrass as relatively deep rooting, while Shaver and Cutler [299] described its rooting depth as intermediate compared with other species in tussock tundra in Alaska. Roots may grow up to 3.3 feet (1 m) long in a single growing season [60,112,306,338], but most roots are shallow [84,371]. Tussock cottongrass roots penetrated to the maximum active layer depth at Eagle Creek and Dempster Highway sites [71,371], but the phytomass per 2-inch (5 cm) depth increment at that point was very small (<1 g/m²) [371].

Roots closely follow the thawing front and rapidly colonize newly thawed soil [36,240]. Near Toolik Lake, roots averaged 12 inches (30 cm) long, extending 4 inches (10 cm) below ground until reaching permafrost [240]. In Atkasook, Alaska, tussock cottongrass roots were approximately 20 inches (50 cm) above permafrost, with most roots <10 inches (25 cm) deep [11]. For more information, see Moisture.

Although tussock cottongrass roots can occur in mineral soil (see Texture and depth), most roots (≥70%) are found in the elevated and organic-rich tussock [63,65,71]. Because roots are elevated above the ground surface, tussock cottongrass's roots have a deeper organic horizon to exploit than roots of plants located between tussocks [71].

Tussock cottongrass roots may be dense [10,240,324]. An average tiller initiated 4.3 roots at the end of one growing season in Atkasook [10].

Many researchers stated that tussock cottongrass does not form mycorrhizal associations (e.g., [35,69,70,78,100,234,366]). Lavoie and others [207] suggested that tussock cottongrass is a successful early colonizer because it is a nonmycorrhizal plant that absorbs organic nitrogen directly (see Successional Status for more information). However, some researchers found either arbuscular-mycorrhizal [78,112] or endomycorrhizal [63] associations with tussock cottongrass.

Several researchers described tussock cottongrass as having a corm [42,74,128,305].

Stand structure: Less than 1-year-old tussocks consist of small tufts without winter-killed leaves. Older tussocks range from 2 inches (5 cm) tall to 31 inches (80 cm) tall (e.g., [36,71,112,157,158,238,276,298,318,325]) and have abundant, persistent dead leaves [128,172] (see Fuels). Tussocks may be larger in southern populations than northern populations [29,303] and taller in boreal forest bogs than open tundra [238]. At Eagle Creek and Toolik Lake, tussocks in bulldozed-scraped areas in postdisturbance year 15 and vehicle track areas in postdisturbance year 8 were smaller than tussocks in undisturbed tundra [108]. Growing-season temperatures (the number of thaw degree-days) at 6 sites in central Alaska were strongly positively correlated with tussock cottongrass tussock height (r=0.956) [226].

Tussock cottongrass density varies among sites from sparse to dense. Density in 34 plots in central and northern Alaska ranged from <1.25 tussocks/m² to >11.25 tussocks/m² [303]. On the Seward Peninsula, in a tussock-dwarf shrub community dominated by tussock cottongrass and/or Bigelow sedge, there were usually 2 to 4 tussocks/m²; tussocks made up about 20% of the plant cover [263]. In the Low Arctic subalpine zone of the Richardson and British mountains of the western Canadian Arctic, tussock cottongrass density was 5 to 8 tussocks/m² [200]. In southern Québec, one peat field had >3 tussocks/m² [207].

Tussock cottongrass tussocks are typically closely spaced at approximately regular intervals [117,157]. In the Fairbanks area, the distance between tussock cottongrass tussocks was usually <20 inches (50 cm) [51]. In the Imuruk Lake area, Northwest Territories, tussock cottongrass tussocks were more or less evenly spaced at intervals of a few inches to 24 inches (61 cm) [157].

Some authors suggested that tussock cottongrass tussocks are created by clonal growth [71,179,325]. Several researchers assumed that tussocks are comprised of the vegetative offspring of a single individual [158,226,238]. Keatinge [179] hypothesized that small (5.2 feet (1.6 m diameter)) tussock cottongrass patches were the result of the "continued centrifugal growth of one tussock separating at some stage into smaller units". However, the author observed no direct evidence of the formation of circular clumps [179]. Because tussock cottongrass seedlings established on the tops, on the sides, and between tussock cottongrass tussocks following a fire on the Elliott Highway [370], at least some tussocks may not be made up entirely of one individual [366].

Tussock cottongrass tussocks throughout central Alaska averaged 158 years old [226]. Mark and Chapin [225] attributed tussock cottongrass's longevity to the species' relatively conservative use of nutrient stores, minimal allocation to reproduction, and relatively stable habitats. Polozova (1970 cited in [366]) concluded that tussock cottongrass in northern Russia remains in the "juvenile stage" for 20 years and in the "generative stage" for 50 to 55 years. The author suggested that maximum productivity is at 40 to 60 years old, but that plants may remain active for >100 years. For information on tiller longevity, see Seedling establishment and plant growth.

Phenology varies with latitude. At 9 sites located from the southern arctic foothills to the coast of Prudhoe Bay, there was a steady progression in plant phenology from south to north [378]. Tussock cottongrass phenological development was reported as follows:

Tillers: Tussock cottongrass produces tillers early in the growing season [108], while daughter tillers generally develop late in the season [172]. At Eagle Creek and Toolik Lake, plants produced tillers continuously during June and July, with some production into August [108]. The emergence and growth of daughter tillers continued throughout fall in Wales, especially after October, until mid-December. Shortly thereafter air and soil temperatures fell below 32 °F (0 °C) and growth ceased. By February, all leaves on tillers had died back [128,276]. Not all tillers produce daughter tillers in a given year. In Westmoreland, England, <20% of tillers produced daughter tillers between April and October [276]. For more information, see Vegetative regeneration.

Leaves: Tussock cottongrass leaves are produced in sequence throughout the growing season [89,168]. As older leaves die, nutrients are translocated to new developing tissues[30,64,89,172,298,301,302,346]. As a result, tussock cottongrass has high nutrient resorption efficiency values compared with other species [172,302,346]. Reviews of nutrient use and nutrient cycling in tussock cottongrass and compiled resorption data for many arctic and subarctic plant species, including tussock cottongrass, are available in these sources: [2,30,182].

A second advantage of sequential leaf development in tussock cottongrass is that there is always some green leaf tissue present on the plant. Overwintering green tussock cottongrass leaves can photosynthesize at the start of the growing season at nearly half of seasonal peak rates [11,89,168,276].

Several researchers described the following pattern of leaf development in tussock cottongrass [17,168,172,243,276]: The first leaves appearing in spring and early summer die (complete senescence) in fall or winter. The tips of leaves produced in midsummer die but the basal portions of the leaves survive through winter. These leaves complete growth the next growing season and die that summer. Late summer and fall leaves die back at the tip, the basal portions of the leaves survive through winter, and the leaves live until the next fall. Thus, spring leaves die at a younger age than other leaves [17]. At Smith Lake, Alaska, the oldest leaf cohorts began to senesce in mid- to late June and the youngest ones, initiated in July, partially senesced in early to mid-August. However, all green leaves initiated during the growing season retained green basal portions throughout the fall and winter and died the following year [225]. In another interior Alaskan site near Fairbanks, tussock cottongrass plants produced leaves sequentially at about 1.5-month intervals, and each leaf remained active for 2 growing seasons [172]. The net increase in biomass over the growing season is typically small [11,301]. Warm weather in spring may accelerate leaf growth [183], and warm weather in fall may delay senescence [245].

When dormant in winter, plants store nutrients and carbohydrates in stems and leaf sheaths to support early growth. Roots do little to support early growth [30,64,89,172,298,301,306]. Dead leaves usually persist intact on the plant for several years [128,172]. For more information, see Fuels.

Culms: Plants flower only once a season [366], typically in spring and summer (Table 1). Fall flowering is unusual but was observed in bogs of the southern portion of the Canadian boreal forest [366]. Culms and spikes are formed the growing season prior to flowering [157,225,360]. In arctic tundra, culms are produced about 3 weeks before growth stops in fall [366]. Culm elongation the following spring begins as soon as free water is available. Flowering usually occurs while there is snow on the ground and the soil is frozen [40,225,366]. At Latnjajaure, Sweden, prefloration (the time between snowmelt and flowering) ranged from 9.6 to 13.4 days during 3 years, while postfloration (the time from flowering to seed dispersal) ranged from 59.8 to 69.0 days [238]. Timing of snowmelt may affect flowering date [45,56,75,207,358]. Flowering takes place with minimal nutrient uptake from soils [74,360].

Seed production: Tussock cottongrass reproduces sexually by seed [366]. The inflorescence is indeterminant, so the proportion of developing flowers towards the apex may increase under favorable conditions [238]. In central Finland, tussock cottongrass plants first produced seeds at age 3 [285].

Flowering occurs much less frequently than tillering [11,72,106,118,359]. In tussock tundra near Toolik Lake, 96.5% of annual production was allocated to vegetative growth, with only 3.5% allocated to sexual reproduction [72]. Only 4% of adult tillers examined at Eagle Creek and Toolik Lake produced inflorescence buds, but 31% produced daughter tillers [106]. In Atkasook, only 4% of the aboveground biomass of tussock cottongrass was invested in reproductive structures; the rest was contained in leaves. Less than 3% of tussock cottongrass tillers flowered [11]. Even though the number of flowering shoots on an individual plant may be small, high plant density may make flowers appear common [359] (Figure 5).

Tussock cottongrass seed production may be high. In a vacuum-mined peatland in southern Québec, tussock cottongrass plants produced as many as 2,400 seeds/m² [207]. Maximum seed production was 1,380 seeds/m² and 460 seeds/m² at undisturbed sites in Tuktoyaktuk and Inuvik, Northwest Territories, respectively (Table 2) [372].

Flowering and seed production vary greatly annually and regionally ([122,300], Polozova 1970 cited in [366]). At Eagle Creek, flowering varied from 0.04 to 19.0 inflorescences/m² between 1976 and 1982 [300]. At Kuparuk Ridge, Alaska, tussock cottongrass flowering varied from 0.7 to 33.5 inflorescences/m² between 1977 and 1983 [122]. Inflorescence density varied from approximately 0.5 to 18.0 inflorescences/m² at 34 locations in central and northern Alaska [303].

Flower and seed production are influenced by soil moisture, nutrients, weather, and terrain. For example, soil moisture may affect flower density. Within the Kuparuk Oilfield in Alaska, flower density was highest in moist tussock cottongrass tundra and lowest in wet sedge tundra during both early and late snowmelt periods [246]. On Nimrod Hill in the central Seward Peninsula, Alaska, tussock cottongrass seedlings were densest in burned moist sedge tussock-shrub tundra, followed by burned dry shrub tundra, and lastly by burned wet sedge shrub tundra. Of the 3 plant communities, spikelets were only found in burned moist sedge tussock-shrub tundra [260]. See Seed production after fire for more information.

Nutrient availability may alter flower and seed production [205,212,297,300,371]. Addition of phosphorus and potassium at Eagle Creek increased inflorescence density 25-fold (Chapin unpublished data cited in [62]). However, nutrients did not affect flowering date in the Imnavait Creek watershed in the northern foothills of the Brooks Range, Alaska. Flowering occurred from about 17 to 23 June during 3 years in control plots and 15 to 25 June in fertilized plots [205]. At Eagle Creek, fertilization increased flowering, but the effect was greatest in high flowering years. In years with low flowering, fertilization had little or no statistically significant effect [300]. Along the Dempster Highway, culms were 8 times more abundant in winter tractor tracks (32 culms/m²) and 44 times more abundant along drainage ditches (176 culms/m²) than in undisturbed tussock tundra (4 culms/m²). This was attributed to high nutrient contents along the tractor tracks and drainage ditches [371]. For more information, see Nutrients.

Warm weather may increase flower and seed production [238,245]. In the northern foothills of the Alaska Range, snow fences and spring snow removal increased soil temperatures and thaw depth, which tended to increase tussock cottongrass flower production (P=0.053) [245]. However, there was no simple correlation with flower production and either current or previous-year local weather over 4 years at 34 sites spanning 5.50° latitude and 3,440 feet (1,050 m) elevation in northern and central Alaska. Yearly variation in weather appeared to affect plants on a broad, regional scale rather than on a local scale [303].

Within the Kuparuk Oilfield in Alaska, an index of terrain ruggedness was positively correlated with the density of tussock cottongrass flowers within moist Eriophorum spp. tundra (r=0.86, P<0.05), probably due to shallow snow on windblown slopes, high solar radiation on south-facing slopes, and deep snow on leeward slopes [246].

Tussock cottongrass tussock density does not appear to influence flower density. In central and northern Alaska, there was no correlation between tussock density and inflorescence density among 34 plots [303].

Flowering after disturbance: Disturbances that do not kill plants may stimulate tussock cottongrass flower and seed production. One year after a low-severity fire in Alaskan tussock tundra that burned all litter and aboveground vegetation but little peat, tussock cottongrass flowering increased "dramatically". There were 4 times more culms on a burned than an unburned area in Inuvik and 5 times more seeds (Table 2). These increases were attributed to [365,366,372]:

1) nutrients released by fire,
2) translocation of stored nutrients within plants,
3) warmer soils, and
4) a deeper active layer.

The active layer was 35% to 50% deeper in burned than unburned areas in spring, and 15% to 20% deeper in fall. Thus, the growing season was longer on burned than unburned areas [365,366,372]. In Westmoreland, England, tussock cottongrass flower density was higher in a bog burned every 20 years (23.2 inflorescences/m²) than in unburned bogs (3.2 inflorescences/m²) [270]. See Seed production after fire for more information.

Severe mechanical disturbances may temporarily reduce tussock cottongrass cover and flower density. In tussock tundra on the Tuktoyaktuk Peninsula region, Northwest Territories, tussock cottongrass cover and flower density were greater in an undisturbed community (19.5% cover, 10.8 flowerheads/m²) than 1 or 2 years after disturbance on both a rutted, eroded vehicle trail (<0.5% cover, 0.2 flowerhead/m²) and a seismic line that was bladed to permafrost and had many overturned tussocks (<0.5% cover, 0.06 flowerhead/m²). Bare ground on the disturbed sites was high: 83% on the trail and 70% on the seismic line. On another seismic line where only a few tussock tops were sheared off, tussock cottongrass cover was 32.6% 8 years after the disturbance, nearly 3 times that on undisturbed control sites. The disturbed sites had nearly 58 times more flowerheads than controls (61.7/m² and 1.07/m², respectively). The author hypothesized that the increased thaw after disturbance might have resulted in a larger soil volume for root exploration, which might have stimulated greater microbial activity. Thus nutrient release, availability, and/or uptake might have increased, enhancing tussock cottongrass flowering along the seismic line where only a few tussocks were disturbed [146].

Seed dispersal: Wind and water disperse tussock cottongrass seeds [9,54,112,366]. Salonen and others [285] stated that seeds are "highly dispersible". Long bristles and very low wing loading promote dispersal [54]. Seeds are buoyant and long-floating in water [9]. Andersson and others [9] reported that tussock cottongrass seeds collected along the Vindel River in Sweden floated for 3.5 days. Tussock cottongrass seeds may also germinate without dispersing. For example, Sernander (1901 cited in [17]) observed tussock cottongrass culms bent to the ground, where seedlings developed while still within the inflorescence.

Seed banking: Tussock cottongrass has both a transient and persistent soil seed bank [112]. Seeds may germinate shortly after dispersal without after-ripening [112,229,300,372] or remain dormant in the seed bank, sometimes for long periods [123,227]. A flora reported 80 to 443 tussock cottongrass seeds/m² in European seed banks [112].

Tussock cottongrass seeds that are potentially several decades old may germinate from the seed bank. Viable seeds are most numerous in the organic and surface soil horizons [132,164,227]. Ninety-seven percent of the viable, buried seed found in organic soil horizons at a site on Kuparuk Ridge was tussock cottongrass and Bigelow sedge [123]. An average of 345 buried tussock cottongrass seeds/m² germinated from soils collected from tussock cottongrass tundra at Eagle Creek. Germinants emerged from the upper 6 inches (16 cm) of soil cores. Seeds were most numerous in the structured dead layer at the top of the organic horizon (Table 3). Seedling mortality and growth appeared similar among depth classes [227]. Many authors noted that tussock cottongrass seeds germinated from seed banks following disturbances that exposed seeds (e.g., [73,123,227,300]). In undisturbed tussock tundra at Kuparuk Ridge, viable tussock cottongrass seeds were present primarily in the upper 0.4 inch (10 cm) of organic soil, although viable seeds were found as deep as 11.4 inches (29 cm). Buried seed was most abundant in moss mat and tussock microhabitats, while buried seed was absent from frost boils and hepatic mats. In a bulldozed area 2 years after disturbance, organic soils contained 32 viable tussock cottongrass seeds/m², while viable buried seed was absent from mineral horizons beneath organic layers (Table 4) [123].

Some studies reported few tussock cottongrass seeds in seed banks, however [97,233]. Tussock cottongrass was scarce in the soil seed bank of mire communities in Scotland despite being prominent components of the surface vegetation; only 3 individuals germinated from 13 mire soil samples [233].

Seed bank size in tussock cottongrass tundra apparently decreases with latitude [97]. No tussock cottongrass seeds germinated from seed banks from a tussock cottongrass-diamondleaf willow (Salix pulchra) community at Oumalik, Alaska, on the Arctic Coastal Plain [97], despite being present in the seed bank and in similar communities further north at Kuparuk Ridge (Gartner and others 1983 cited in [97]) and Eagle Creek (McGraw 1980 cited in [97]). The decrease in the number of viable buried seeds from Eagle Creek to Kuparuk Ridge to Oumalik suggested that the number of viable buried seeds decreases with latitude [97].

Germination: Tussock cottongrass seeds may germinate shortly after dispersal [229,300,372]. For example, current-year seeds collected on 9 July at Eagle Creek germinated after being cooled on ice for 2 weeks then placed on moist filter paper in petri dishes and exposed to light [229]. Wein and MacLean [372] stated that seeds collected from Rat River, Yukon, germinated "immediately" after dispersal when placed on moist filter paper.

Seeds may also remain dormant in the seed bank for relatively long periods [227]. Tussock cottongrass seeds collected near Umiat, Alaska, were viable after cold storage for 30 months [37]. At Kuparuk Ridge, tussock cottongrass seeds germinated after 5 years of cold storage in the dark [123]. Gartner [121] stated that tussock cottongrass has enforced dormancy, defined as the inability to germinate due to an environmental restraint that is "easily broken" once the environmental constraint (i.e., lack of light, shortage of water, low temperatures, poor aeration, etc.) is removed. Noting differences among studies, she hypothesized that in some populations, germination of tussock cottongrass seeds may be controlled by intrinsic dormancy mechanisms as well as enforced dormancy mechanisms. Intrinsic dormancy mechanisms include elapsed time and chilling [121].

Cold stratification may enhance germination in tussock cottongrass. In a greenhouse in Newfoundland, cold stratification enhanced tussock cottongrass seed germination compared with unstratified seeds (stratified: 25% germination, unstratified: 0% germination; P<0.05) [222]. However, at Kuparuk Ridge, cold stratification did not affect the germination rate of tussock cottongrass seeds in the light (germination of cold-stratified and unstratified seeds was approximately 60%), but cold stratification increased germination rate in the dark (cold-stratified seeds: about 40%, unstratified seeds: about 10%) [122].

At Kuparuk Ridge, seed coat type influenced tussock cottongrass seed dormancy: 15% of seeds that germinated in the year they were produced had a light brown seed coat rather than the typical hard, black seed coat; only seeds with a black coat germinated in experiments with 5-year-old seeds [123].

Tussock cottongrass seeds may germinate quickly after being exposed to suitable conditions [37,164,227]. Some tussock cottongrass seeds—collected in September from 20-inch (50-cm) deep peat cores, stored at 41 °F (5 °C) over winter, and planted in a greenhouse on peat and sand on 18 April—germinated on the second day. Many more germinated in the first 7 days [164]. Most germination of cold-stratified tussock cottongrass seeds collected at Eagle Creek and planted in a greenhouse occurred within the first 20 days; germination had ceased by day 60 [227]. In contrast, at Kuparuk Ridge, only 4% of the 1,401 seeds that germinated within 5 years of sowing in field plots did so in the year of sowing. Most (80%) germinated in the second growing season after sowing. Seeds were sown on 26 June [122].

Viability of tussock cottongrass seeds appears to be high [72,238,284,372]. In Finland, 100% of seeds collected during 2 years in an abandoned peatland mine were viable [284]. At Eagle Creek, 76% of seeds were viable [72]. Seed weight may affect germination rate, with heavier seeds having higher germination rates [238].

Though tussock cottongrass can form a persistent seed bank, seed viability and germination rates are typically highest immediately after dispersal and decline over time. Seeds from Rat River, Yukon, that were collected within 2 weeks of initial seed dispersal in early July showed 75% germination, while seeds collected in August or September showed <25% germination. Seeds from a 3-year-old burn and an unburned site at Inuvik, collected 1 week after initial dispersal, showed >90% germination. Viability was reduced by 50% within 16 months for seeds from the burned site, while a 50% reduction had not been reached for seeds collected from the unburned site at the end of the 19-month study. Seeds from the unburned site were heavier than those from the unburned sites, so they may have retained viability longer (Table 2) [372]. On field plots in tussock tundra at Kuparuk Ridge, seeds collected prior to 30 June did not germinate. Only 12% of the sown tussock cottongrass seeds germinated the year of sowing; germination of current year's seeds declined from 25% for seeds collected 30 June to 0.5% for seeds collected on 21 July. Seed number peaked on 23 June and remained high until early July, then declined in late July. This suggested that most viable seeds were produced and dispersed in late June and early July [123]. In contrast, germination rates were similar for a seed lot collected near Umiat, even though part of the lot was stored for 6 to 7 months and the other part for 30 months (both about 23%) [37].

Light: In general, tussock cottongrass seed germination is best in high light [37,366,372]. Bliss [37] tested 479 tussock cottongrass seeds collected near Umiat, and 23% germinated in continuous light in the laboratory; of 286 seeds, 0% germinated in continuous darkness. Although the latter seeds did not germinate in the dark, 22% germinated when placed in continuous light [37]. Wein and MacLean [372] found that light was not an absolute requirement for germination in growth chambers, but percent germination was "drastically" reduced without light.

Temperature: Optimal germination of tussock cottongrass seeds was 73 to 86 °F (23-30 °C) when tested at constant temperatures in laboratories, but tussock cottongrass seeds germinated over a wide range of temperatures in light (range: 55-95 °F (13-35 °C)). They only germinated at 64 to 81 °F (18-27 °C) in the dark [73,122,372]. Optimum germination temperature of seeds collected from Rat River, Yukon, from burned and unburned sites near Inuvik, Northwest Territories, and from Scotland was close to 86 °F, the minimum temperature was close to 59 °F, and the critical maximum temperature was somewhat over 95 °F [372].

Water: Tussock cottongrass requires water saturation for germination [49,249].

Substrates: Tussock cottongrass seeds germinate on almost any moist substrate, including mosses, lichen mats, sides and tops of burned tussocks, litter, burned and unburned organic soils such as peats, and mineral soils [53,123,228,366,370]. Most tussock cottongrass seedlings at Eagle Creek occurred in the dead leaves of tussock cottongrass and Bigelow sedge and on mosses (Table 5) [228]. A study at Kuparuk Ridge found that germination of sown tussock cottongrass seeds was significantly faster in frost boils than moss mats prior to 25 June (P<0.05), but after 25 June, germination was significantly greater on moss mats than frost boils (P<0.05). This result was attributed to the partial flooding of moss mats early in the growing season combined with their high ice content and slow soil thaw in the early season, followed by more favorable soil temperature and moisture conditions in later summer as the surface soil thawed. The final number of tussock cottongrass seeds to germinate on the field-sown plots did not differ significantly among frost boils, moss mats, or lichen mats [122]. Germination in the Alaskan arctic tundra occurs where soils are frequently disturbed, such as on cryopedologic features (patterned ground) [157].

Some studies reported that tussock cottongrass germination is higher on organic than mineral soils [123,370]. On field plots in Alaskan tussock tundra at Kuparuk Ridge, germination percentages were 4.5 times higher on organic soils (18%) than mineral soils (4%) [123]. In a comparison of different substrates in a laboratory in Alaska, tussock cottongrass emergence was >2 times higher on burned peat than on mineral soil; germination on peats occurred within 1 week, while germination on mineral soil did not begin until the end of the 2nd week [370].

Tussock cottongrass seeds germinate best at the soil surface [370], but some tussock cottongrass seeds may germinate at soil depths up to 0.6 inch (15 mm) [53]. Tussock cottongrass seeds collected near the Elliott Highway germinated on mineral and peat surfaces but not when sown 0.4 inch (1 cm) deep [370]. In contrast, germination of tussock cottongrass seeds collected from harvested and undisturbed bogs in Québec and planted in a greenhouse was similar among 4 depths ranging from 0 to 0.6 inch [53]. A study in southern Germany found no significant differences in germination rates between covered and uncovered seeds in an abandoned peatland mine (range: 75%-98%) [311].

Seedling establishment and plant growth: Tussock cottongrass establishes and grows best in relatively warm, wet to moist soils [123,343], in full sun [12,66,112,179,250], and in areas with low shrub and moss abundance [231,289]. Tussock cottongrass tillers produce only a few leaves each year. In general, tillers produce from 1 to 4 leaves sequentially during the growing season [17,89,108,168,172,243,243,276,301,305]. Individual tussock cottongrass tillers produced leaves for 3 or 4 years, after which time tillers initiated inflorescences and died [128].

Temperature: Tussock cottongrass has a broad temperature range for growth, although low temperatures may reduce tussock cottongrass growth. Tussock cottongrass can grow at very low temperatures (<36 °F (2 °C)) [36,194], although growth tends to be higher at higher temperatures [36,99,194,195,366]. Tillers collected near Fairbanks and grown in a laboratory at optimal temperature (54 °F (12 °C)) produced about 6 times more biomass than tillers grown at low temperature (36 °F) [194]. A laboratory study of tussock cottongrass collected at Eagle Creek determined that the optimum temperature for leaf growth was above 68 °F (20 °C), while root growth peaked between 59 °F (15 °C) and 68 °F. Tillering was most active at 59 °F [196].

Late spring frosts may kill newly formed tussock cottongrass leaves [366] and flowers [238]. However, no effect of early spring frost on tussock cottongrass was detected in central Sweden [17].

Frost heaving may kill tussock cottongrass seedlings. In Québec, numerous tussock cottongrass seedlings and small tussock cottongrass tussocks were found uprooted in abandoned peatland mines. Frost heaving was the most probable cause of uprooting and may be why so few new plants established even though numerous seeds were produced and germinated each year [209].

Soil moisture: Tussock cottongrass seedling establishment appears to be optimum in wet or moist, but not flooded, soils [123,131,343]. In tussock tundra at Kuparuk Ridge, soils with naturally established tussock cottongrass seedlings had continuously high moisture content in surface horizons regardless of substrate (mineral or organic soil) [123]. However, new seedlings only established where the soil surface was above the water level in a rewetted mined bog in southern Finland [343]. Tussock cottongrass growth in flooded soils may depend on nitrogen availability, with reduced growth in flooded soils with low nitrogen availability [124].

A review noted that tussock cottongrass seedlings are generally sensitive to drought [366]. However, one study found 3,685 tussock cottongrass seedlings/m² 7 years after peat mining despite low soil moisture in the uppermost layer of the peat, low soil pH, and low ambient temperatures early in the fall [284]. For more information about soil moisture effects on tussock cottongrass, see Soils.

Soil type: A review stated that tussock cottongrass seedlings are more abundant on organic than mineral substrates. This is in part because few or no tussock cottongrass seeds are stored in mineral soils [300] (see Seed banking) and germination rates are lower [123,370]. Although seedling densities may be higher on organic than mineral substrates, seedling mortality may be similar between substrates. Two years after tussock cottongrass established on a bulldozed site in tussock tundra at Kuparuk Ridge, seedling mortality in organic substrates did not differ from that in mineral substrates (43%) [123]. Results of a study on tussock cottongrass growth in different substrates were equivocal. In potted plants in a greenhouse, tussock cottongrass growth rates were twice as high on organic than mineral soils, but growth rates in field plots were similar between organic and mineral soils (Gartner 1982 cited in [300]).

Soil depth: Tussock cottongrass growth may be greater in thick than thin organic soils. In Alaska, Eriophorum spp. tundra was "well developed" at Anaktuvuk Pass, where the organic mat was approximately 4 inches (10 cm) deep, and at Eagle Creek, where the organic mat was 8 inches (20 cm) deep. At Prudhoe Bay, however, tussocks were scattered on rims of low-centered polygons where the organic mat was only 2 inches (5 cm) deep [60]. This suggested better tussock growth where the organic mat was thicker. See Texture and depth for more information.

Tussock cottongrass growth may be greatest where the active layer is deep. Two years after fire in Inuvik, the "most vigorous" tussock cottongrass plants growing in tussock-shrub tundra were rooted in mineral soil at the outer edges of frost boils, where the active layer was twice as deep as under peat or dead moss [370].

Seedling establishment after disturbance: Tussock cottongrass seedlings are often abundant on disturbed sites (e.g., [72,122,300]). Fire, frost activity, erosion, and animal disturbances provide opportunities for tussock cottongrass seedling establishment in arctic tundra [122,300] (see Seedling establishment and plant growth after fire). However, tussock cottongrass seedlings also occur on undisturbed sites (e.g., [17,133,231]).

Seedling mortality is often high on disturbed sites [73,207,229,311,365,370]. One year after a low-severity fire in Alaskan tussock tundra that burned all litter and aboveground vegetation but little peat, there were over 200 tussock cottongrass seedlings/m². However, 2 years after fire, "very few" of the seedlings had survived [365]. In an abandoned peatland mine in southern Germany, tussock cottongrass seeds were sown on bare peat surfaces in fall and covered with translucent fleece, jute fiber mat, or tussock cottongrass leaf mulch. The following summer, there were 30 to 39 seedlings/400 cm² on all sites. In the second summer, there was a 66% to 82% reduction, with only 5 to 12 seedlings/400 cm² on all sites, so establishment of tussock cottongrass seedlings was low. Nonetheless, the authors stated that tussock cottongrass seedlings showed the highest tolerance of all the 5 species tested to the "extreme" environment of bare peat surfaces [311].

Seedling mortality may also be high on undisturbed sites. For example, in undisturbed tussock cottongrass tundra at Eagle Creek, there were 55.1 tussock cottongrass seedlings/m². Tussock cottongrass, northern Labrador tea, and black crowberry accounted for 97% of all seedlings found. Less than 1% of tussock cottongrass seedlings had produced a second tiller, and no tussock cottongrass plants >6 years old were found. The estimated mortality rate for tussock cottongrass was 63% per year. The authors concluded that high mortality and failure of seedlings to produce new tillers indicated that although tussock cottongrass seeds germinate readily, establishment is rare in undisturbed, closed tussock cottongrass tundra [228].

McGraw and Fetcher [231] hypothesized that tussock cottongrass may establish infrequently on undisturbed tundra because of interspecific competition for light and other resources. Near Healy, Alaska, the proportion of total aboveground biomass of tussock cottongrass was negatively correlated with the aboveground biomass of shrubs, including cloudberry (r = -0.650, P<0.001) and bog blueberry (r = -0.526, P=0.001). Aboveground biomass of tussock cottongrass was greatest in a thermokarst area disturbed by recent thawing of ground ice and least in an area disturbed by thermokarst activity 30 years prior; it was intermediate in undisturbed tundra. Apparently, shrubs displaced tussock cottongrass as they became increasingly more developed [289]. However, in bulldozed tussock tundra at Kuparuk Ridge, a seedling removal experiment found that competition for nutrients by other plant species did not significantly reduce tussock cottongrass seedling recruitment or growth [123]. For more information, see Successional Status.

Timing may affect tussock cottongrass seedling establishment after disturbance. In central Finland, seedlings that established soon after disturbance survived longer than those establishing later [285].

Tussock cottongrass seedlings may not survive the freezing and thawing of mineral soils. Hopkins and Sigafoos [158] observed dead seedlings on bare mineral soil frost-heaved in the fall. They stated that seedlings survived the freezing and thawing of the mineral soil only if a thin layer of peat covered the mineral soil. Following natural revegetation of bulldozed tussock tundra at Kuparuk Ridge, 1- and 2-year-old tussock cottongrass seedlings died mostly during the growing season but also died in winter. Mortality of small seedlings was due to dislodging by surface runoff and frost action in summer, while mortality of large seedlings was due primarily to winter grazing [123]. In another study in the same area, the greatest tussock cottongrass seedling densities in Alaskan tussock tundra were found where a hepatic mat stabilized the soil surface and reduced frost heaving [122]. In southern Québec, the number of tussock cottongrass tussocks decreased and ericaceous shrub cover increased on mined sites abandoned for 14 years then monitored for 5 years. Frost heaving and the low water table level (<12-16 inches (30–40 cm) below the soil surface) apparently deterred tussock cottongrass establishment [209].

Plant growth after disturbance: Tussock cottongrass leaf production may be higher in disturbed than undisturbed sites. In disturbed tundra at Eagle Creek and Toolik Lake, leaf production was 3.0 leaves/tiller/year during 3 years. In undisturbed tundra, leaf production was 2.52 leaves/tiller/year at Eagle Lake and 2.23 leaves/tiller/year at Toolik Lake. Leaf production rate for a given site was nearly constant from year to year [108].

Nutrient effects on plant growth: Seedling growth rate may be in part controlled by nutrient availability, especially nitrogen and phosphorus. At Kuparuk Ridge, density and size of tussock cottongrass seedlings were greater in hepatic mats than in other microhabitats (frost boils, moss mats, lichen mats, and tussocks) (P<0.05). This was attributed to lower nitrogen concentrations in the other microhabitats [122]. High ammonium nitrogen availability in particular may benefit tussock cottongrass establishment and growth [224,283,286]. In central Finland, abandoned peat mines with a thick peat layer and high ammonium nitrogen and phosphorus contents were usually densely revegetated by tussock cottongrass [283].

Tussock cottongrass plants tolerate nutrient-poor soils by remobilizing and translocating nutrients [48]. Its high resorption efficiency results from a combination of its sequential leaf growth and nutrient translocation within leaves [172] (see Seasonal Development). Tussock cottongrass commonly grows under limiting nutrient conditions, but the specific nutrient that is deficient varies [297,298]. Permafrost-dominated ecosystems are often nitrogen limited. Occasionally phosphorus has also been found to be limiting in Alaska, but not as strongly as nitrogen (e.g., [2,89,240,295,297,298]). Three growing seasons after nitrogen-phosphorus-potassium fertilization in tussock tundra at Toolik Lake, the total biomass per unit area of tussock cottongrass tussocks more than doubled. The increase was due mainly to higher tiller density, although tiller size also increased. Nitrogen was apparently most strongly limiting [298]. However, factors other than nutrients may limit growth in tussock tundra. Shaver and Chapin [297] stated that air and soil temperature, water, light, wind, snow distribution, soil pH, soil aeration, and overall soil fertility also influence the composition and distribution of tundra plant communities. However, the authors noted that based on nitrogen:phosphorus ratios in Alaskan moist tussock tundra sites, nitrogen limitation was 3 times as frequent as phosphorus limitation [297].

Northern bogs without permafrost are sometimes phosphorus limited [331]. Tamm [331] found that the growth of tussock cottongrass in a southern Swedish peat bog was phosphorus limited. Bogs with thick peat deposits may be phosphorus limited because of the distance to the mineral subsoil [28]. Potassium was most limiting in England, Wales [127,128,129,130], and Finland [305].

In contrast to the above studies, some researchers found little or no effects of nitrogen, potassium, and/or phosphorus fertilization on tussock cottongrass (e.g., [57,62,133,166,301,339]). The diverse growth responses reported for fertilized tussock cottongrass may result from local differences in nutrient availability [211].

Seedling growth may be rapid after fire has released nutrients in the soil. For more information, see Plant response to fire.

Roots: Tussock cottongrass roots are annual (see Roots) and are rapidly initiated following soil thaw in spring [71,105]. In Westmoreland, England, roots had a life span of only 2 months, and the rate of elongation of roots was nearly 0.4 inch (1 cm)/day [117]. Wein [366] hypothesized that growth rates of roots in the Arctic would be slower than that reported by Forrest [117] in Westmoreland. In a growth chamber in Québec, seedling roots grew an average of 0.9 inch (2.3 cm)/day during the first 3 weeks after emergence [53].

Vegetative regeneration: Tussock cottongrass reproduces vegetatively by tillering (e.g., [11,71,106,108,225,226,276]). It has "extremely plastic" tillering rates and tiller mortality [108]. Vegetative reproduction appears to be more common than sexual reproduction [72] (see Seed production).

Tussock cottongrass may have dense tillers. In Alaska, individual tussock cottongrass tussocks had <100 to >600 individual tillers [109]. In a sphagnum moss bog in central Sweden, density was typically <10 tillers/100 cm²; the highest density observed was 26 tillers/100 cm² [17]. Tiller density per unit area of tussock tends to decline with increasing tussock diameter. Mosses and shrubs may invade large tussocks, which reduces tiller density [109]. According to Chapin and others [71], a tussock cottongrass plant can contain between 100 and 200 tillers after only 3 growing seasons in favorable field conditions.

Daughter tillers form directly adjacent to parent tillers. As daughter tillers form, parent tillers are pushed out to the edge of the tussock, and daughter tillers are squeezed upward in the center [71,157]. In Westmoreland, England, daughter tillers may form the second year of a tiller's life [276]. In undisturbed tundra at Eagle Creek and Toolik Lake, no <1-year-old tillers and only 3% of 1-year-old tillers produced daughter tillers [106]. The mean interval between the production of one tiller and that of its daughter tiller was 5.33 and 4.75 years at Eagle Creek and Toolik Lake, respectively [108]. During the course of a growing season, a tiller usually produces 1 but may produce 2 or 3 daughter tillers. It may also produce an inflorescence bud, which will be exserted the following year. Some tillers form both inflorescence buds and daughter tillers; others form either inflorescence buds or tillers, but not both [108]. The capacity to produce flowers seems to be related to the production of daughter tillers. Seventy-five percent of the tillers that produced inflorescence buds also produced daughter tillers [106]. If a tiller forms an inflorescence bud, it dies after flowering the next year [108,132].

Soil moisture and nutrients affect tiller production. In a Finnish peatland, the total number of tillers produced during a growing season decreased with decreasing moisture and nitrogen and phosphorus concentrations (P<0.001) [306].

Early-season growth of tussock cottongrass is supported largely by photosynthesis and by stored nutrient reserves. Early-season growth is not strongly limited by available carbon or nutrients, and late-season nutrient uptake mainly replenishes reserves and supports growth [59]. Jonasson and Chapin [172] showed that tillers can survive a whole growing season without nutrient absorption and still have high levels of nutrients in fall.

Fertilization often increases tillering in tussock cottongrass [108,296,305], but tiller mortality typically increases with fertilization [67,108]. Nitrogen, phosphorous, and potassium may limit tussock cottongrass growth (e.g., [2,89,106,127,240,295,297,298,305,331]). See Nutrient effects on plant growth for more information.

Tussock cottongrass tillers may live ≥15 years in tussock cottongrass tundra [108]. Estimated tiller longevity was 8 years throughout central Alaska [226]. Average life expectancy was 7.06 years at Eagle Creek [108], 4.61 at Toolik Lake [108], and 3 to 4 years at Atkasook [10].

Vegetative growth after disturbance: Disturbance may increase the rate of tillering. In Eagle Creek and Cape Thompson, Alaska, the overall tillering rate on disturbed sites (a bladed area and vehicle tracks) was much higher than on undisturbed sites [109]. Several researchers found increased tillering after fire [163,266,267,370] (see Vegetative growth after fire). However, disturbance may reduce tiller longevity [106,108,109]. Tiller life expectancy was >2 times longer in undisturbed than disturbed tundra at Eagle Creek [108].

Competition with shrubs for light and other resources may reduce tillering. Tussock cottongrass tussocks that were almost completely covered by other plant species had fewer daughter tillers, lower tiller mass, and lower phosphorus content than uncovered plants (P<0.05) [109]. For more information, see Successional Status.

For information about vegetative growth following defoliation by herbivores, see Rangeland management.

Arctic tundra: Several researchers considered tussock cottongrass "unusual" in that it dominates large areas in both undisturbed and disturbed tundra [72,108,109,231]. Tussock cottongrass tussocks often recover quickly after tundra fires. Jandt and others [162] suggested that the "competitive advantage of the tussock growth form" (i.e., tussocks create favorable microclimates that become snow-free sooner than inter-tussock areas [71]), plus warmer soils and improved plant nutrient status after fire [370], may explain dominance of tussock cottongrass in burned tussock tundra [162]. For more information, see Plant response to fire.

Some tussock cottongrass communities may be stable for many decades, while other tussock cottongrass communities may be fire-maintained, with tussock cottongrass eventually replaced in the absence of disturbance [51]. Tussock tundra, especially in the Arctic foothills and the hilly parts of the Arctic Coastal Plain, is "very stable" and may represent "climax" vegetation on poorly drained flats, plateaus, benches, and gentle slopes. However, in sedge tussock-shrub tundra, lack of fire or other disturbance for long periods may result in succession to other communities such as ericaceous shrub/sphagnum moss [261,265,266,348]. Racine and others [261,265] hypothesized that without fire or other disturbances, such as frost action, dwarf shrubs, mosses, and lichens invade tussock cottongrass tussocks. Over time, dwarf shrubs and mosses eventually into and around the tussocks, killing the tussocks. The authors stated that fire would slow these successional trends by reducing vegetation and organic soil around tussocks and increasing tussock growth and reproduction, unless the fire was so severe that there was no tussock recovery [261,265]. At Eagle Creek, Alaska, removing moss from "heavily infested" tussock cottongrass tussocks produced little effect, but removing shrubs resulted in tussocks producing many more daughter tillers and smaller adult tillers than controls. Results were attributed to increased light and temperatures caused by shrub removal. Lichens and Bigelow sedge were removed from all tussocks [111]. In contrast, Hobbie and others [153] did not find increased tillering in response to plant removal experiments at Toolik Lake. Differences between studies could have been due to location or plant removal methods.

Tussock cottongrass colonizes bulldozed and scraped areas of tundra (e.g., [62,122,123]). An area of tussock tundra at Eagle Creek was bladed free of vegetation with a bulldozer, leaving a frozen, 6-inch (15 cm) thick organic mat in place. Within 5 years, this area had 95% cover by tussock cottongrass and Bigelow sedge. By 9 years after the disturbance, the aboveground biomass in the bladed area was equal to that in undisturbed tundra. By 16 years after the disturbance, individual tussocks in the bladed area were roughly equal in size to those in surrounding undisturbed tundra [229].

Soil moisture may affect succession in arctic tussock cottongrass communities. In tundra along the Meade River near Atkasook, tussock cottongrass was absent from polygon troughs and polygon centers, which are seasonally flooded, but occurred on polygon rims, which are higher, drier, and geomorphologically older [253].

In tussock tundra, the seed bank is usually sufficient to completely revegetate disturbed sites [123], so productivity returns to that of undisturbed tundra within about 10 years [62]. For more information, see Seedling establishment and plant growth after fire.

Figure 3. Tussock cottongrass grows from postfire nutrient-rich soils in Gates of the Arctic National Park. Photo courtesy of the National Park Service.

Boreal and subarctic forests: Tussock cottongrass often increases after fires that reduce the forest canopy, perhaps because of its shade intolerance (e.g., [12,244]). Archibold [12] stated that tussock cottongrass "can only establish immediately after fire" in conifer forests. Further, it is a "rapid-growth pioneer" that tends to decline as conifer cover increases. Kryuchkov (1968 cited in [369]) observed that tussock cottongrass increased after forest fires in eastern Siberia that reduced trees and shrubs. Prior to the 2001 Survey-Line Fire southwest of Fairbanks, Alaska, the area was a low-lying, open-canopy black spruce forest with an understory of tussocks. Two and 3 years after the fire, which killed many of the black spruce, the dominant vegetation was tussock cottongrass, grasses, birch, willow, purple marshlocks (Comarum palustre), bog Labrador tea (Ledum groenlandicum), bog blueberry, mountain cranberry, and leatherleaf. Organic matter thickness 2 and 3 years later was highly variable (1-16 inches (2–40 cm)) because of tussock-hollow microtopography, patchy fuel consumption, and variable soil subsidence [244]. Pre- and postfire images taken by Weber [362] illustrate the "dramatic proliferation" of tussock cottongrass in burned black spruce forests. In contrast, in forests in interior Alaska, tussock cottongrass was present only in soils with permafrost regardless of time since fire [327]. See Foote’s [115] Research Paper for general information on postfire succession in black spruce communities of interior Alaska.

British Isle heathlands: Immediately after fire in British Isle heathlands, tussock cottongrass may be reduced (McFerran 1991 cited in [320]). However, tussock cottongrass cover typically increases quickly thereafter [51,271]. Tussock cottongrass communities may eventually be replaced by heath in the absence of fire (e.g. [14,154,155,179]).

Northern bogs and peatlands: Tussock cottongrass may establish soon after fire in bogs. For example, in northern Alberta, tussock cottongrass was more abundant in recently burned bogs (wildfire 6-27 years prior) than unburned bogs [159]. In a 30-year study on an abandoned, mined bog in southeastern Québec, vegetation succeeded from shade-intolerant species associated with open and moist conditions (e.g., tussock cottongrass, leatherleaf, and sheep-laurel) to species tolerant of shade and drought (e.g., black spruce, rhodora (Rhododendron canadense), and Russow's sphagnum (Sphagnum russowii)) [250]. Fire in bogs and fens may favor tussock cottongrass by reducing trees. In the Great Lakes region, fire reduced encroaching tamarack, eastern white pine (Pinus strobus), and red pine (Pinus resinosa) in a muskeg bog dominated by Carex spp., tussock cottongrass, leatherleaf, bog rosemary, dwarf birch, and bog laurel (Kalmia polifolia) [354]. However, tussock cottongrass also occurs in bogs in the absence of disturbance. For example, it was present soon after fire in a Finnish bog but also persisted for years after fire [341]. Mean relative cover of tussock cottongrass in 3 bogs in the Swiss Jura Mountains was 28% 29 years after peat harvesting and abandonment, 32% after 42 years, and 25% after 51 years [287]. In a sphagnum moss bog in southern Sweden, tussock cottongrass frequency increased by 11% (P≤0.001)—from 44% to 55%—over 54 years without disturbance [176].

Peatland mining is a major disturbance affecting many tussock cottongrass communities [207]. Tussock cottongrass readily colonizes abandoned or newly restored peatland mines (e.g., [34,54,102,207,208,258,285]), but Pouliot and others [258] stated that it is not abundant in undisturbed peatlands in eastern Canada. Lavoie and others [207] stated that tussock cottongrass "never" dominates plant assemblages in undisturbed ombrotrophic peatlands. In southern Québec, tussock cottongrass cover in such bogs usually ranges from 1% to 5%, and it very rarely exceeds 25% [207].

Tussock cottongrass has several characteristics that make it a successful early colonizer. Lavoie and others [207] listed the following characteristics of tussock cottongrass that may facilitate its establishment and survival in nutrient-poor, drained, mined peatlands:

1) tolerates prolonged drought periods because of its deep root system,
2) produces numerous seeds each year that are easily dispersed by wind,
3) seeds germinate at high (73-95 °F (23-35 °C)) temperatures and such temperatures are common on bare peat surfaces,
4) forms long-lived tussocks,
5) uses nutrients efficiently (i.e., the growth of new leaves is supported almost entirely by nutrients retranslocation from older leaves that are senescing), and
6) it is a nonmycorrhizal plant that absorbs organic nitrogen directly [207].

FIRE EFFECTS AND MANAGEMENT

• FIRE EFFECTS
• FUELS AND FIRE REGIMES
• FIRE MANAGEMENT CONSIDERATIONS

Tussocks of young tussock cottongrass are not well developed (see Fire adaptations); thus, young tussocks may be most susceptible to fire-caused mortality [12]. Mature tussock cottongrass tussocks often survive surface and ground fires because of their dense tussock growth form (meristems are insulated by tightly bunched tillers) and elevated position [370].

Soil moisture may influence the extent to which fire damages tussock cottongrass plants. Wein and Bliss [370] concluded that decadent tussocks were consumed by fire to a greater extent on dry sites near Inuvik than moist sites near the Elliott Highway.

Burn season may affect postfire mortality. "Intense" fires in February and March in a heather (Calluna vulgaris)-tussock cottongrass blanket bog in Westmoreland, England, burned the dead leaves of tussock cottongrass. However, the species is usually dormant at that time of year and was "not apparently affected" [270].

Immediate fire effects on seeds: No field studies have examined the effects of fire on tussock cottongrass seeds. Wein and McLean [372] concluded from germination tests in a laboratory that tussock cottongrass seeds likely would not survive burning because the critical maximum temperature for germination of tussock cottongrass seeds was relatively low (95 °F (35 °C)). A laboratory experiment in Alaska in which tussock cottongrass seeds were exposed to various temperatures in a furnace showed that most seeds were killed by exposure to high temperatures (212, 392, and 572 °F (100, 200, and 300 °C)), even at short time exposures (30-150 seconds). Seeds showed some germination for exposure up to 90 seconds at 212 °F. The authors concluded that seeds within organic soils would not survive burning [370]. Seeds stored in unburned organic soils may germinate after fire, however. For more information, see Seed banking and Germination.

Postfire regeneration strategy [321]:
Chamaephytic root crown in organic soil or on soil surface
Tussock graminoid
Corm and/or an herbaceous root crown, growing points in soil
Ground residual colonizer (on site, initial community)
Initial off-site colonizer (off site, initial community)
Secondary colonizer (on- or off-site seed sources)

Figure 4. Burned tussock cottongrass tussocks 1 year after the Anaktuvuk River Fire, North Slope, Alaska. Photo courtesy of Adrian Rocha.	Figure 5. Flowering tussock cottongrass 2 years after the Anaktuvuk River Fire, North Slope, Alaska. Photo courtesy of Adrian Rocha.

Fire adaptations and plant response to fire:

Plant response to fire: Tussock cottongrass may establish from soil-stored and/or wind-blown seeds after fire [73,123,227]. If tussocks survive fire, they typically tiller, and cover reaches unburned control levels in as little as 2 years [288]). Viable tussock cottongrass seeds are often buried in organic soil horizons [123,227] (see Seed banking). Since viable tussock cottongrass seeds are located up to 11 inches (29 cm) deep in organic soils [123], some viable seed is usually available after fire that does not burn down to mineral soil [265]. However, Gartner and others [123] stated that because viable tussock cottongrass seed was present only in the uppermost soil horizons on Kuparuk Ridge, only shallow disturbances would leave tussock cottongrass seeds in the soil seed bank. The authors noted, however, that fires in tundra often do not burn organic soils deeply and leave some organic soil remaining. For example, tussock-shrub tundra fires in western Alaska in 1977 burned most vegetation but removed ≤2 inches (5 cm) of the approximately 11-inch (28 cm) thick wet organic horizon [261]. Tussock cottongrass seeds are unlikely to survive fire if burned [370] (see Immediate fire effects on seeds). However, wind may disperse seeds from nearby sources, and surviving on-site plants may produce abundant seeds soon after fire [73] (see Seed production after fire). Tussock cottongrass seedlings are often some of the first to appear on burned sites, but seedling establishment may be low (see Plant establishment and plant growth after fire).

• Seed production after fire
• Seedling establishment and plant growth after fire
• Vegetative growth after fire
• Plant nutrients and depth of thaw after fire

Seedling establishment and plant growth after fire: Fire provides a favorable seedbed for tussock cottongrass establishment [370], and increased availability of nutrients after fire stimulates postfire growth [51,370]. In a comparison of tussock cottongrass seedling emergence on different substrates in a laboratory in Alaska, burned peat showed highest rates of emergence for surface-sown seeds; emergence was more than twice as great on burned peat as on mineral soil. Mean seedling fresh weights per pot were 191, 154, 104, and 7 mg for burned peat, raw peat, decomposed peat, and mineral soil, respectively (P<0.05). The authors suggested that seedling fresh weight was greatest on burned peat because the fire released nutrients and enriched the peat [370] (see Germination).

Tussock cottongrass seedlings are often abundant after fire. Racine [260] noted the occurrence of "abundant" tussock cottongrass seedlings on charred peat following the 1977 Seward Peninsula fires. On Nimrod Hill, tussock cottongrass seedlings were densest in burned moist sedge tussock-shrub tundra, followed by burned dry shrub tundra, and lastly by burned wet sedge-shrub tundra [260]. Thirty-eight days after the 21 June Kungiakrok Creek Fire (1982) in tussock cottongrass-shrub tundra on the Noatak National Preserve, Alaska, tussock cottongrass averaged almost 30 seedlings/m². It presumably germinated from the seed bank [266]. On 4 burned field sites in Alaska and the Northwest Territories, establishment of tussock cottongrass seedlings was "dramatic" [370].

Postfire mortality of seedlings may be high. Two months after a late June fire near at the Elliot Highway site, tussock cottongrass seedling density was 198 seedlings/m² on burned peat between tussocks. Seedlings emerged on sides and tops of burned tussocks but at an unreported, but lower density. Although "substantial densities" of seedlings survived winter, very few of these lived until the following fall [370]. One and 2 growing seasons after the 1977 fire on Nimrod Hill, seedlings of tussock cottongrass were "fairly abundant" in intertussock spaces; however, 2-year-old tillering seedlings were found only occasionally, suggesting low seedling survival from postfire year 1 to 2 [261]. Twenty-four years after the fire, tussock cottongrass density had increased by 0.3 to 0.4 tussock/m² from 4 years before the fire [267]. Although tussock cottongrass seedlings established within a few weeks after the moderate severity 1982 Kungiakrok Creek Fire, few or none survived [266]. By midgrowing season 1 year after a low-severity fire in Alaskan tussock tundra that burned all litter and aboveground vegetation but little peat, there were >200 tussock cottongrass seedlings/m², but by the next spring, very few of the seedlings had survived [365]. For more information, see Seedling establishment after disturbance.

New tussock cottongrass growth may appear soon after fire (e.g., [361,365]), and tussock cottongrass is often one of the first species to recover after fire [370]. New growth may be evident within 3 weeks of fire [361,365]. For example, new tussock cottongrass growth was evident on 17 July in a wet tundra community in the Mackenzie River Delta region of the Northwest Territories that was burned under prescription on 23 June [361].

Fire generally reduces tussock cottongrass cover and biomass, but tussock cottongrass usually recovers quickly. After the low- to moderate-severity 1977 wildfire on Nimrod Hill, tussock cottongrass cover decreased on the shallow (2%-3% slope) footslopes from 34% 5 years before the fire to 16% to 20% in postfire year 1. Between postfire years 1 and 3, tussock cottongrass cover increased on the footslope by 9% to 20%. At one site, tussock cottongrass cover continued to increase until postfire year 24, when the study ended; at 2 other sites, tussock cottongrass cover remained the same (Table 9) [267]. Twenty-three years after the Kungiakrok Creek Fire, the total vascular cover was 98%, an increase of 50% since 38 days after the fire. The increase resulted primarily from increases in tussock cottongrass and Bigelow sedge [266].

In Alaskan tundra, tussock cottongrass cover and/or biomass on burned sites can exceed that of unburned control sites in as little as 1 to 13 years and remain higher than unburned controls for more than 20 years [107,162,265]. For example:

• Two and a half years after the 1988 Selawik National Wildlife Refuge wildfire burned 209,088 acres (84,615 ha) in northwestern Alaska, tussock cottongrass biomass and relative frequency were greater in burned plots in tussock tundra than in adjacent unburned plots [288].
• One year after a fire on an Elliott Highway site, the proportion of the total aboveground vascular plant biomass comprised of tussock cottongrass was greater on burned than unburned control plots; production of tussock cottongrass harvested in late August was 15.0 g/m² on a burned site and 17.8 g/m² on an unburned site [370]. Tussock cottongrass contributed 54% of the total aboveground vascular plant biomass in the burned site 13 years later [107]. The fire apparently "released tussock cottongrass from competition" and provided a short-term nutrient pulse [370]. By postfire year 24, the proportion of total plant biomass comprised of tussock cottongrass on burned plots (24%) was similar to that before fire, but the proportion in control plots had decreased to only 3% of total aboveground vascular plant biomass [347] (Table 10). The decline in the proportion of tussock cottongrass biomass between postfire years 13 and 24 may have been the result of continued invasion by other plants and thus, increased interspecific competition within tussocks. [347].
• Studies of the 1977 burns on the Seward Peninsula and the Noatak and Kokolik rivers found that vascular plant cover returned to unburned control or prefire levels in 6 to 10 years. At all sites, most regrowth was tussock cottongrass [265].
• Following the 1981 Ulukluk Creek Fire in northwestern Alaska (a 5-acre (2 ha) wildfire in low shrub-tussock tundra), average cover of tussock cottongrass rose from <1% 1 month after the fire to >30% 3 years after the fire, while unburned control plots ranged from 13% to 20% tussock cottongrass cover. Even 14 years after the fire, tussock cottongrass cover was 11% higher on burned plots than unburned plots [162,163]. By 24 years after the fire, cover was similar: 43% in burned plots and 42% in unburned plots [163].
• On the Seward Peninsula, tussock cottongrass cover in burned areas exceeded cover in unburned areas by 19% 20 years after the 1977 Wagon Wheel Fire, and by 20% 25 years after the 1972 Pargon River Fire. The authors concluded that "large, vigorous tussocks of tussock cottongrass were a finding common to tundra burn sites aged 14 to 25 years" on the Seward Peninsula [162].
• A wildfire of "unusually" high severity, with deep charring of organic soils, burned across the forest-tundra ecotone near Inuvik, Northwest Territories, in 1968. Tussock cottongrass cover was relatively similar 5 and 22 years after the fire in both forest-tundra and tundra. Unburned controls were only measured in postfire year 5 because visual observations suggested that unburned controls "changed very little" between 5 and 22 years after the fire [204] (Table 11).

Little information was available on tussock cottongrass response to fire in the Great Lakes region. Two years following prescribed burning in a muskeg in north-central Wisconsin, average frequency of tussock cottongrass was 27.5% in burned areas and 35.0% in adjacent unburned areas; the difference was not statistically significant [353].

For information about tussock cottongrass seedling establishment and plant growth after fire in Europe, see these sources: [154,155,269,270].

Vegetative growth after fire: Tussock cottongrass often produces tillers after fire. Tussock cottongrass plants recover quickly from unburned live stem bases. In tussock-shrub tundra, they may account for most vascular plant cover during the first 4 to 5 years after fire [266]. Twenty days after the Kungiakrok Creek Fire, tussock cottongrass growth in tussock cottongrass-shrub tundra was already "well underway", with tillers about 6 inches (15 cm) long. Fire had removed about 2 to 4 inches (5-10 cm) of the 5- to 6-inch (12-14 cm) organic horizon between tussocks, and thaw depth was about 4 inches deeper on burned than unburned areas. Standing water was present at the bottom of the intertussock spaces on the burned area [266]. Soon after the low- to moderate-severity 1977 fire in tussock-shrub tundra on Nimrod Hill, new leaves developed "rapidly" from tussock bases [267]. On the Elliot Highway site, tussock cottongrass tillers sprouted during the first postfire growing season following a 25 June prescribed fire in tussock cottongrass tundra [370]. Tussock cottongrass recovered "quickly" during the first 3 years after the 1981 Ulukluk Creek Fire that burned in lichen-tussock tundra, "demonstrating vigorous basal sprouting from tussock bases and heavy flowering" [163]. For more information on this topic, see Vegetative growth after disturbance.

Plant nutrients and depth of thaw after fire: Tussock cottongrass nutrient content is often high after postfire nutrient flushes. Late summer regrowth of tussock cottongrass "proved to be relatively high in protein content", and therefore, in nitrogen, following a fire in Kotzebue Sound in 1977 [184]. On 4 burned field sites in Alaska and the Northwest Territories, tussock cottongrass plants in burned areas had higher nitrogen, potassium, calcium, and magnesium content than plants in unburned areas. This was attributed to release of nutrients, increased active layer depth, and greater microbial activity after fire [370]. Almost 2 years after the 1988 Selawik National Wildlife Refuge wildfire, late winter protein content and in vitro digestibility of tussock cottongrass were higher in samples collected from burned than unburned plots. Postfire increases in protein content, digestibility, and availability of tussock cottongrass may make burned tussock tundra an attractive feeding area for caribou in late winter [288]. For more information, see Importance to Wildlife and Livestock.

After fires, tussock cottongrass plants may benefit from a deepened active layer and warmer soils [365,370]. On 4 burned sites in Alaska and the Northwest Territories, tussock cottongrass seedling and culm densities were greater where fire had deepened the active soil layer. For example, in the Caribou Hills, Northwest Territories, the active layer was 160% deeper in burned than unburned areas in spring, and in the fall, it was 140% deeper (Table 8); thus, the growing season was longer on the burned areas [370]. Vavrek and others [347] proposed that because tussock cottongrass roots may grow to the bottom of the active layer (e.g., [71,371]) (see Roots), the species may obtain nutrients at greater depths than shallow rooting species. Thus, a persistent increase in the active layer may prolong dominance by tussock cottongrass in burned communities [347]. Brown and others [47] reported an increased thaw of 140% to 160% 4 years after a fire in a black spruce/Eriophorum spp. tussock community in eastern Alaska. They also reported a 141% and 152% increase in thaw depth in a 1-year-old burn in an Eriophorum spp. tussock community with scattered black spruce in central Alaska. Wein and Bliss [366] documented warmer soils, increased nutrient cycling, greater tussock cottongrass growth, and more abundant tussock cottongrass flowering for burned tussock cottongrass communities as well. Kryuchkov (1968 cited in [351]) reported that before wildfire in eastern Siberia, the upper permafrost layers were 20 to 28 inches (50-70 cm) thick. Fire thawed the upper permafrost layers; warmer soils and the resultant moisture release stimulated tussock cottongrass growth and increased tussock cottongrass cover soon after fire. A few years after the fire, however, the active layer was only 16 to 18 inches (40-45 cm) thick due to the insulating effect of thick postfire vegetation (Kryuchkov 1968 cited in [376]).

• Fuels
• Fire regimes

Tussock cottongrass aboveground biomass may exceed 400 g/m² in some areas [38], and aboveground annual production may exceed 60 g/m²/year [118,149] (Table 12). In tussock and heath tundra near Toolik Lake, annual production was 51.8 g/m²/year for leaf biomass, 11.6 g/m²/year for stem biomass, and 86.3 g/m²/year for root biomass [149]. Wein and Bliss [371] reported considerable year-to-year variation in tussock cottongrass production at several sites in Alaska. Differences among sites were attributed to latitude (with less production at more northerly sites) and soil moisture. At the Eagle Creek site, annual aboveground dry matter production was 14.3 g/m²/year in 1968, 10.7 g/m²/year in 1969, and 27.6 g/m²/year in 1970. A large increase in dry matter production at all sites in 1970 relative to 1968 and 1969 may have been due to "favorable growing conditions" in 1969; however, apparent differences may have been due to different sampling methods among years [371].

Tussock cottongrass may produce abundant litter [329]. Tussock cottongrass litter on burned areas may exceed that on unburned areas within 5 years. Five and 22 years after a severe forest-tundra wildfire near Inuvik, litter biomass on burned areas was >200% higher than on unburned controls. Most of the litter was tussock cottongrass and bluejoint reedgrass leaves [204].

Tussock cottongrass leaves are highly flammable [329]. The relative fuel-potential ratings of 12 northern tundra and forest-tundra vascular plant species of the Mackenzie River Delta region were evaluated from measured fuel characteristics by simulating a test fire with the Rothermel [279] fire behavior model. The relative importance of the fuel parameters were in decreasing order: moisture content, biomass, fineness (surface:volume ratio), packing ratio, silica-free ash content, and caloric content (Table 13). The fuel-potential ratings of the plant species and of the communities were differentiated primarily by their leaf characteristics. Dead leaves of tussock cottongrass and bluejoint reedgrass constituted the most flammable fuels of species measured. The extremely high, relative fuel-potential ratings for tussock cottongrass were expected because dead leaves composed 50% of the species' total dry weight. Based on small experimental fires, live tussock cottongrass material did not prevent fire spread. The dead material sustained combustion, and the fire merely burned around the live material [329]. During the 1977 fire on Nimrod Hill both live and dead leaves of tussock cottongrass plants burned [267].

Wein [363] reported that tussock cottongrass in the Mackenzie River Delta region, Northwest Territories, slowed its growth in August, and moisture content was low in mid-August (Table 14). This suggests that flammability increases throughout the growing season [329]. Using information from Sylvester and Wein [329], Johnson [169] provided the mean values of high heat of combustion for tussock cottongrass as 18,618 kJ/kg for live leaves and 19,110 kJ/kg for dead leaves.

Based on data from interior Alaskan black spruce communities, a model predicted that fuel and flammability traits of tussock cottongrass are expected to affect future fire probability only moderately [32].

Decomposition rates: Tussock cottongrass roots and other plant parts decompose slowly after dying, in part because of high mineral content of tussock cottongrass [79] and extremely slow rates of mineralization, nutrient turnover, and microbial activity in soils with tussock cottongrass [35,63,79,366]. Thus, substrate for tussock cottongrass roots is primarily dead tussock cottongrass plant parts [35,366] (see Texture and depth). The leaf sheaths and roots of tussock cottongrass are recognizable in peat layers formed thousands of years ago (Mäkilä 1994 cited in [306]). Turnover time under the surface of the intertussock area in the Dempster Highway soil profile was 70 years at 0 to 2 inches (5 cm) deep, 120 years at 2 to 4 inches (10 cm) deep, and 45 years at 4 to 6 inches (15 cm) deep. The slow decomposition rate at 2 to 4 inches deep could be a reflection of extremely or very strongly acid soils (pH 4.2-4.7). In the Eagle Creek soil profile, turnover times were as long as 260 years in the tussock center, but they were 40 years at the top of the tussock and in the 4- to 6-inch level under the surface of the intertussock area [371].

Culms and leaf sheaths may decompose more slowly than leaf blades [306]. Decomposition rates depend, in part, on soils. In Westmoreland, England, decomposition rates of tussock cottongrass leaves were 37.7% to 38.8% in mineral soils and 39.9% to 44.2% in peat soils during 1 year. Leaves of sheep fescue (Festuca ovina) had the slowest decomposition rates of leaves examined for the same period, followed by tussock cottongrass and timothy (Phleum pratense) [79]. These rates of decomposition were greater than those reported by Heal and others (1978 cited in [79]) at the same location. Heal and others found that leaves of tussock cottongrass had the slowest decomposition rates of leaves of 9 species examined (29% mass loss by the 2nd year) (Heal and others 1978 cited in [7]). The difference between studies was attributed in part to the age of the litter used. The younger plant material used by Coulson and Butterfield [79] had higher mineral content, which probably contributed to higher decomposition rates. In Fennoscandian lichen heathlands, decomposition of tussock cottongrass leaves was 37% in the 1st year compared with 42% in smooth black sedge (Carex nigra), 21% to 27% in birch, and only 4.9 to 5.6% in star reindeer lichen (Cladina stellaris) (Wielgolaski 1975a,b cited in [15]).

Fire regimes: Even though organic soils tend to be moist year-round, tussock cottongrass communities in the Arctic are fire prone [260,329]. Tussock-shrub tundra was one of the most frequently burned plant communities during the 1977 Seward Peninsula fires [260]. The Anaktuvuk River Fire in 2007 burned 401 miles² (1,039 km²) of Alaska's Arctic slope in late summer and fall, making it the largest fire on record to date (2014) for the tundra biome. Tussock cottongrass was the dominant species throughout the burned area [219]. In Fairbanks, 7 of 9 tussock cottongrass communities showed some evidence of fire history [51].

Tussock cottongrass communities may burn relatively frequently or not burn for long periods. Mean fire-return intervals for tussock cottongrass tundra and tussock cottongrass-shrub tundra range from 50 years to >1,000 years [94,147,148,163,201], although there are records of these communities burning 2 or more times at very short fire-return intervals during the late 1900s and early 2000s [142,171,264] (see the Fire Regime Synthesis on Alaskan tundra communities).

Fires in tussock and shrub-tussock tundra tend to be fast-moving, surface or crown fires [4,31,140,163,219,219,261,352,368]. During hot, dry years, ground fires may burn deeply into organic soils, though they typically do not burn down to mineral soil [173,261]. Low- or mixed-severity fires occur primarily in lowland, poorly drained sites [170,261]. Tussock cottongrass occurs in pockets of boreal conifer forests, such as black spruce forests, where stand-replacing, crown fires with accompanying surface and ground fires are common (e.g., [95,217,281]) (see the Fire Regime Synthesis of Alaskan black spruce communities). Based on organic matter consumed and plant survivorship immediately or 1 year after fire in Noatak, Bering Land Bridge, Denali, and Yukon-Charley National Parks, tussock tundra and low shrub-tussock tundra sites had the lowest ground burn severity when compared with white spruce, black spruce, and deciduous forests. The authors stated that this was not surprising because of rapid fire spread, less smoldering, and the generally mesic sites where tussock tundra occurs [4]. Fires in tundra communities often burn discontinuously, resulting in a mosaic of unburned, lightly burned, and severely burned areas (e.g., [4,142,213,260,261,368]).

Fires in tussock cottongrass communities range from very small to very large (e.g., [21,90,171,264,367]). Fires in tussock cottongrass communities may be lightning or human-caused, with human ignitions increasing from the 1950s to the 2000s (e.g., [50,83,90,178,364]). Fire typically occur from May through August (e.g., [90,91,264,281,368]).

See the Fire Regime Table for further information on fire regimes of vegetation communities in which tussock cottongrass may occur. Find further fire regime information for the plant communities in which this species may occur by entering the species name in the FEIS home page under "Find Fire Regimes".

Season of burning may affect tussock cottongrass postfire mortality. Rawes and Hobbs [270] reported that "intense" fires in February and March in a heath-tussock cottongrass blanket bog in Westmoreland, England, did not harm tussock cottongrass because it was dormant.

Fires that occur before plants are large enough to withstand fire and deposit seeds could be detrimental to tussock cottongrass [12,280]. According to Polozova (1970 cited in [366]), tussock cottongrass reaches maximum production at 40 to 60 years old (see Stand structure).

In tussock tundra, tussock cottongrass's seed bank is sufficient to revegetate disturbed sites [123], so seeding is likely unnecessary after fire as long as sufficient organic soils remain on site. Nonetheless, several authors recommended either planting or seeding tussock cottongrass to rehabilitate disturbed sites (e.g., [49,62,183,372]) (see Value for Rehabilitation of Disturbed Sites). Postfire seedling mortality may be high (see Seedling establishment and plant growth after fire).

Saperstein [288] commented that tussock cottongrass (important spring caribou forage) may increase after fire, but lichens (important winter caribou forage) are likely to be reduced for the long term (see Importance to Wildlife and Livestock).

MANAGEMENT CONSIDERATIONS

• FEDERAL LEGAL STATUS
• OTHER STATUS
• IMPORTANCE TO WILDLIFE AND LIVESTOCK
• VALUE FOR REHABILITATION OF DISTURBED SITES
• OTHER USES
• OTHER MANAGEMENT CONSIDERATIONS

Caribou and reindeer: Throughout their range, caribou and reindeer graze tussock cottongrass year-round, and in some areas it may form a considerable portion of their diet [3,76,309,310,340]. Tussock cottongrass often greens before snowmelt [309,360] (see Seasonal Development), and the nutritious early growth makes it an important early season forage. Tussock cottongrass floral parts (culms and spikes) are especially preferred at this time (e.g., [10,198,309,334,360,366,376,377,378]). Tussock cottongrass and other Eriophorum spp. comprised 77.5% of caribou diets during calving in spring in northern Yukon [334]. Because of high digestibility, caribou and reindeer often prefer tussock cottongrass floral parts to other tussock cottongrass plant parts and other plant species in spring. For example, tussock cottongrass floral parts comprised 90% of forage consumed by the Western Arctic caribou herd the first 2 weeks after snowmelt; immediately following tussock cottongrass flowering, the herd moved to other communities [198]. Tussock cottongrass may also provide important green forage in winter ([334,360], Karev 1961 cited in [309]).

Caribou may consume postfire new growth of tussock cottongrass. When midsummer tundra fires occurred on the winter range of the Western Arctic Herd, late summer postfire growth of tussock cottongrass was grazed by caribou as they moved through burned areas in late October [184,185].

Other livestock: Cattle and domestic sheep commonly consume tussock cottongrass on the British Isles, especially in winter and early spring (e.g., [11,135,136,156,257,313]). In North America, use of tussock cottongrass by livestock other than reindeer has not been reported.

Grizzly bear: Barren-ground grizzly bears on the North Slope foothills, Alaska, ate tussock cottongrass floral parts (10% frequency in scats). Tussock cottongrass floral parts ranked third in percent total volume of important grizzly bear foods. In summer, grizzly bears did not consume tussock cottongrass, but in fall, they occasionally dug up and consumed tundra vole caches of Eriophorum spp. tillers [145]. However, in Ivvavik National Park, northern Yukon [218] and in the Arctic National Wildlife Refuge, Alaska [255], grizzly bears did not eat tussock cottongrass, despite it being abundant in the areas.

Rodents: True lemmings, collared lemmings, and voles live in tussock cottongrass communities and eat tussock cottongrass (e.g., [10,179,273]).

Birds: Many kinds of waterfowl, wading birds, grouse, and passerines use tussock cottongrass communities as breeding grounds (e.g., [81,144,259,354,380]). In Wisconsin, sharp-tailed grouse preferred open tussock cottongrass muskeg communities [354].

Birds often eat tussock cottongrass [81,259]. For example, tussock cottongrass was the third most frequently consumed food by Canada geese in Winisk, Ontario [81]. Willow ptarmigan (22% of crops) preferentially consumed tussock cottongrass flower buds in Finland. The authors noted, however, that flower buds occurred only at low density and thus probably were only of supplemental and transient value [259].

Insects: In Atqasuk, Alaska, tussock cottongrass was moderately palatable to lepidopteran larvae [58].

Palatability: Tussock cottongrass is moderately or highly palatable to many animals. Generalist herbivores in Atqasuk, Alaska—including caribou, tundra voles, collared lemmings, true lemmings, arctic ground squirrels, and 4 lepidopteran larvae (Polia spp., Apentesis spp., Parasemia parthenos, and Gynaephora rossii)—grazed tussock cottongrass [58]. Person and others [251] ranked September-collected tussock cottongrass culms, spikes, and leaves 11th out of 30 plants used as reindeer forage in descending order of in vitro digestibility. Tussock cottongrass leaves were highly palatable to brown lemmings, collared lemmings, and tundra voles near Atkosook, Alaska [25]. Near Toolik Lake, green shoots of tussock cottongrass were highly palatable to tundra voles but only slightly palatable to singing voles [26].

Nutritional value: Nutrition of tussock cottongrass aboveground plant parts is variable (e.g., [43,56,88,128,151,211,211,292,303,359]). Typically, nitrogen concentrations increase over the growing season, while phosphorus concentrations decrease [63,291,295,322,335,360]. Nutrition in tussock cottongrass is highest in young plant parts and differs among plant parts. Young tussock cottongrass floral parts are typically the most nutritious and digestible [61,198,225,259,376,377,378]. Plant parts are relatively poorly defended by toxic or digestion-inhibiting secondary compounds [59,149,198]. At Eagle Lake, potassium and phosphorus contents were highest in stems and the youngest leaves, while calcium content was highest in older leaves [174]. In an interior Alaskan muskeg, the time since initiation of a given leaf, rather than time of year, was most closely related to nutrient status of individual leaves: Young leaves always had high concentrations of nitrogen, phosphorus, potassium, and magnesium and low concentrations of calcium, whereas old leaves had low concentrations of nitrogen, phosphorus, and potassium, intermediate concentrations of magnesium, and high concentrations of calcium [172]. Nutritional quality and digestibility of floral parts decreases following flowering [198,378]. Nutrient concentrations of tussock cottongrass plants are often low relative to other plants [24,64,135,360]. The nutrient content of mature tussock cottongrass leaves collected from Atkasook in early August was considered "low", with 1.5% nitrogen, 0.09% calcium, and 0.2% phosphorus [24].

Disturbance affects nutrient concentrations in tussock cottongrass plants. At sites along the Dempster Highway, tussock cottongrass tillers in "old" winter tractor tracks and 8-year-old drainage ditches had higher nitrogen, phosphorus, calcium, sodium, and iron content than undisturbed control sites [371]. The nutritional value of tussock cottongrass foliage may increase soon after fire [184]. For more information, see Plant response to fire.

See Leaves for information on resorption efficiencies of tussock cottongrass. See Nutrient effects on plant growth for information on nutrient limitation in soils.

Cover value: Tussock cottongrass tussocks are used as denning structures by rodents [134,179,273], as estivating structures by turtles [381], and as nesting structures for birds [380]. At Toolik Lake, tussock cottongrass plants often have extensive biomass clipped by voles following snowmelt during years with high vole populations, resulting in "haypiles". These haypiles may serve as shelter for voles [134,167]. Lapland longspurs commonly nest within tussock cottongrass-Bigelow sedge-shrub tundra. One year after fire on the Seward Peninsula, Lapland longspurs built nests between and against the sides of charred and uncharred tussock cottongrass tussocks [380].

VALUE FOR REHABILITATION OF DISTURBED SITES: Tussock cottongrass has potential for restoration of disturbed sites because of its success as a colonizer (e.g., [62,372]). For example, it is often one of the first species to colonize burns (e.g., [41,370]) or areas that were mined for peat (e.g., [102,208]). Tussock cottongrass also colonized denuded areas of an oil well site in Oumalik, Alaska [96]. See Successional Status for more information.

Tussock cottongrass establishment may create microclimatic conditions facilitating the establishment and growth of nonvascular plants, particularly sphagnum moss (e.g., [139,179,200,206,275,314,342,366]). Tussock cottongrass tussocks are substrate for several species of vascular plants as well. In the Low Arctic subalpine zone of northern British Columbia, dwarf birch, bog blueberry, and mountain cranberry were "constants" on tussocks. Other species of high constancy were cloudberry, black crowberry (Empetrum nigrum), Lapland lousewort (Pedicularis lapponica), and spruce muskeg sedge (Carex lugens) [200]. Tussock cottongrass tussocks may facilitate establishment of plants by providing a relatively dry microsite, litter cover, enhanced seed accumulation, increased germination and seedling survival, and a stable substrate for seeds and seedlings [190,191,192,366].

Several authors recommended either planting or seeding tussock cottongrass or allowing tussock cottongrass to naturally establish on disturbed sites (e.g., [49,62,183,372]) or surface-oiled areas [77,262]. Gartner and others [123] commented that tussock cottongrass seeds are suitable for sowing because 1) seeds mature synchronously, 2) plants seed prolifically for several years following fire or on fertilized undisturbed sites, 3) seeds can be easily hand collected, and 4) seeds retain viability under refrigeration for at least 5 years. Because of tussock cottongrass's presence in the seed bank, several studies recommended stockpiling organic soil layers for later revegetation efforts in disturbed areas (e.g., [62,123,227,300]). For information on using fertilizer in tussock cottongrass communities for rehabilitation, see these sources: [62,231,300,331].

Although tussock cottongrass may facilitate the establishment of Sphagnum spp. in bogs [160], tussock cottongrass roots vent methane from the anoxic peat layer. Thus, peatlands colonized by tussock cottongrass emit substantially more methane than peatlands colonized by other species, and this could enhance greenhouse gas emissions (e.g., [119,120,138,188,223,237,239,274]). Mahmood and Strack [221] stated that the potential benefits of tussock cottongrass establishment should be weighed against its high methane emissions. A review cautioned that dense cover of tussock cottongrass on abandoned mined bogs is not necessarily an indication of successful restoration of bog functions such as peat formation [254].

Because of tussock cottongrass's sequential leaf development, new leaves are available to herbivores throughout the growing season [172] (see Seasonal Development). However, at Atkasook, caribou only ate tussock cottongrass plants when flower buds were present (spring and early summer) and when new green leaves were available from tussocks that had been grazed previously. An accumulation of standing dead shoots appears to deter caribou from eating tussock cottongrass [172,376,377]. Overstocked reindeer have greatly reduced tussock cottongrass in Canada's Reindeer Grazing Preserve [76,372]. For information about livestock use of tussock cottongrass in Europe, see these sources: [17,135,137,156,270,319].

Climate change: Many researchers examined the responses of tussock cottongrass to experimental and observed changes in climate (e.g., [66,67,67,68,149,150,153,210,245,304,305,324,326,337,356]). In upland tussock tundra, 2 years of experimental warming increased the growth of dominant canopy shrubs (arctic dwarf birch and northern Labrador tea) but not the growth of tussock cottongrass and less dominant graminoids, shrubs, and mosses. Results suggested that warming in arctic tundra is likely to increase the growth of dominant canopy shrubs at the expense of tussock cottongrass [66]. Other tundra research also indicated that tussock cottongrass was likely to decrease and arctic dwarf birch to increase with climate warming [150]. Shrub expansion has been documented over Alaskan arctic landscapes without fire during the past 50 years (e.g., [68,323,332]). Using modeled increases in climate warming, fire, and drought on the Seward Peninsula, Rupp and others [282] predicted vegetation succession from upland tundra to deciduous forest or grassland steppe. However, the potential response of tussock cottongrass to climatic warming is complex. A warming, nutrient addition, and shading greenhouse experiment near Toolik Lake found that the response of tussock cottongrass to manipulations was complicated by interacting factors. For example, increased air temperatures had no effect on tiller mass, except when nutrients were added. When nutrients were added, increased air temperatures increased tiller mass (P<0.001) [66]. Leadley and Reynolds [210] used simulation modeling to examine the long-term (50-year) response of tussock cottongrass to climate change. The model predicted that climate change will indirectly affect tussock cottongrass through changes in nitrogen availability. Nitrogen availability may increase because of increased rates of nitrogen cycling with climate change. . However, if nitrogen availability increases to more than double 1992 levels,  plants become carbon limited rather than nitrogen limited. At this level of nitrogen availability, carbon dioxide concentrations would then play an important role in controlling tussock cottongrass productivity [210]. Reviews of potential and observed climate change effects that include information on tussock cottongrass include: [23,27,101,215,231,241].

The ability of populations to accommodate change by phenotypic plasticity may reduce the impact of climate change. Fetcher and Shaver [110] found that in Alaska, northern populations of tussock cottongrass were less phenotypically plastic than southern populations. They speculated that northern populations might therefore respond more slowly to climate warming than southern populations [110]. The long life span of tussock cottongrass (>100 years [226]) may mean they are relatively buffered from climate changes for decades or centuries [110,231].

Several researchers examined the response of bog and fen plant communities with tussock cottongrass to experimental warming and water table manipulations [103,304,374,379]. In Sweden, tussock cottongrass frequency increased in experimentally warmed plots after 8 years [379]. In Great Britain and Ireland, tussock cottongrass was predicted to lose habitat but remain widespread in a "high" temperature scenario. The losses would largely be those of wet or moist habitats. Predicted change in distribution was less for the "low" temperature scenario [33].

APPENDIX: FIRE REGIME TABLE

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