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Research Project Summary: Understory recovery after low- and high-intensity fires in ponderosa pine forests of northern Idaho

RESEARCH PROJECT SUMMARY CITATION:
Gucker, Corey L.; Walsh, Roberta A.; Fryer, Janet L., comps. 2017. Research Project Summary: Understory recovery after low- and high-intensity fires in ponderosa pine forests of northern Idaho. In: Fire Effects Information System, [Online]. Missoula, MT: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: www.fs.usda.gov/database/feis/research_project_summaries/Armour84/all.html [].

Revisions:
On 19 May 2017, the 2005 Research Project Summary by Gucker and the 1994 Fire Case Study by Walsh were combined by Fryer into this Research Project Summary. The 2005 Fire Study was previously published as an FEIS monograph, and the 1994 Fire Study was previously published in the FEIS Species Review of Columbia brome.

Sources:
Unless otherwise indicated, the information in this Research Project Summary comes from the following papers:

Armour, Charles D.; Bunting, Stephen C.; Neuenschwander, Leon F. 1984. Fire intensity effects on the understory in ponderosa pine forests. Journal of Range Management. 37(1): 44-48.

Armour, Charles D.; Bunting, Stephen C.; Neuenschwander, Leon F. (n.d.). The effect of fire intensity on understory vegetational development. Unpublished report on file with: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory, Missoula, MT. 17 p.

STUDY LOCATION:
The study area was on the Coeur d'Alene Indian Reservation in Benewah County, Idaho [1,2,3].

SITE DESCRIPTION:
The plant community studied occurs in the following vegetation classifications:

FRES20 Douglas-fir
FRES21 Ponderosa pine [5]
K011 Western ponderosa forest
K012 Douglas-fir forest [6]
SAF 210 Interior Douglas-fir
SAF 237 Interior ponderosa pine [4]
BpS 1010451 Northern Rocky Mountain dry-mesic montane mixed-conifer forest - ponderosa pine-Douglas-fir [7]

Study sites are on gentle slopes (0%-30%) at elevations of about 880 m. Average annual precipitation is 560 mm, most of which falls from October through March. Soils are fine silts, and study sites received approximately 10 cm of ash from the Mount St. Helens volcanic eruption in May 1981. The eruption occurred prior to the 2nd postfire vegetation sampling [1,3].

PREFIRE PLANT COMMUNITY:
Sites were within the Douglas-fir/mallow ninebark (Pseudotsuga menziesii/Physocarpus malvaceus) habitat type; however, seral ponderosa pine (Pinus ponderosa) dominated the plant community, with a shrub and perennial bunchgrass understory. Basal area of ponderosa pine before burning was 37.48 m2/ha. The most common shrub was common snowberry (Symphoricarpos albus), but chokecherry (Prunus virginiana), oceanspray (Holodiscus discolor), and dwarf rose (Rosa gymnocarpa) were also important. Pinegrass (Calamagrostis rubescens) dominated the groundlayer vegetation; bluegrass (Poa spp.) and blue wildrye (Elymus glaucus) were also important. Typical forbs on the study sites were northern bedstraw (Galium boreale), twin clover (Trifolium latifolium), Virginia strawberry (Fragaria virginiana), and common yarrow (Achillea millefolium) [1,2,3].

The estimated mean fire-return interval is 22 years in this area. However, study sites had not burned for 44 to 62 years, and they had not been grazed by livestock for more than 30 years. Some selective logging (removal of 11%-35% of canopy) occurred in the study area. Sites were logged 3 times in approximately 30 years. The last logging was in 1977 [1,2].

Three study sites were selected for study. The sites were located within 25 km of each other and had similar stand structures, land use histories, species compositions, and species abundances. Each site was divided into 0.2- to 1.0-ha plots, with 3 treatments per site: unburned, low-intensity burn, and high-intensity burn [1].

PLANT PHENOLOGY
Phenology of the prefire vegetation was not described. Since burning was conducted in the fall, plants were probably approaching senescence.

FIRE SEASON/SEVERITY CLASSIFICATION:
High- and low-intensity prescribed fires were ignited in 1978:

Fall/High intensity
Fall/Low intensity

FIRE DESCRIPTION:
Researchers compared early postfire vegetation recovery on sites burned with different intensities in seral ponderosa pine communities of the Douglas-fir/mallow ninebark habitat type. The plots were burned over 30 days burned under varying conditions of temperature, fuel moisture, and relative humidity, resulting in fires of varying intensity [1,2].

Mean prefire fuel load on burned sites was 58,200 kg/ha, nearly half of which was duff [3]. Three replicated high-intensity burns, low-intensity burns, and unburned sites were sampled. High- and low-intensity fires had significantly different (P<0.05) fireline intensities. Energy released by high-intensity fires was between 30 to 3,034 kcal/m/s and averaged 781 kcal/m/s; energy released by low-intensity fires averaged 127 kcal/m/s and ranged from 25 to 194 kcal/m/s. Flame lengths averaged 0.9 m and ranged from 0.1 to 1.7 m on all sites. On average, duff smoldered longer on high-intensity plots than on low-intensity plots. Significantly more duff was consumed on the high-intensity fire plots (80%) than on low-intensity fire plots (40%). Postfire depth of duff averaged 6.6 cm on unburned plots, 4.0 cm on low-intensity plots, and 1.3 cm on high-intensity plots [1,2].

FIRE EFFECTS ON PLANT COMMUNITY:
Researchers measured cover and frequency on burned and unburned plots for 3 consecutive posttreatment years (1979-1981). Shrub canopy estimates came from line intercept sampling using five 10-m transects in macroplots (15 × 15 m). Cover and frequency of herbaceous vegetation was measured in 10 microplots (20 × 50 cm) at the edge of the shrub transects. Level of significance was set at P<0.1 for all variables that follow [1,2].

Graminoid coverage was significantly lower on high-intensity plots than on unburned plots during all 3 years of treatments [1,2], although it increased significantly from the 1st to the 3rd postfire year on burned plots. Significantly more duff was removed on plots burned at high intensity. The authors attributed low coverage of graminoids on high-intensity plots to prolonged duff smoldering, which killed graminoid rhizomes and root crowns [1]. Forb coverage was greatest on low-intensity plots, and researchers noted that increases in forb coverage on burned plots were significantly greater than on unburned plots for all 3 postfire years. Shrub coverage was not significantly different between treatments [2].

Table 1—Mean canopy cover (%) of shrubs, graminoids, and forbs across 3 postfire years [2].
Life form Unburned Low intensity High intensity
Shrubs 31 23 27
Graminoids 20 ~19 11
Forbs 17 25 20

When researchers assessed individual species coverages, they found significant differences between at least 2 treatments for 5 shrub, 3 perennial graminoid, 8 perennial forb, and 4 annual species. These species are identified in the table below. Most of the species with significant coverage differences between treatments showed significant differences in frequency as well. Conifer seedlings were absent from all 3 study sites for all 3 postfire years of the study [1,2].

Fire-sensitive species: During all 3 years of treatments, Columbia brome, dwarf rose, and mallow ninebark had significantly greater coverage on unburned plots than on burned plots [1,2]. Columbia brome was 3 times more frequent on unburned than on high-intensity plots, and 2 times as frequent on unburned than on low-intensity plots [1].

Species sensitive to high-intensity fires: During all 3 years of treatments, coverage of creeping barberry (Mahonia repens), field woodrush (Luzula campestris), Woods' rose (Rosa woodsii), and Virginia strawberry was significantly lower on high-intensity plots than on low-intensity or unburned plots. Creeping barberry was nearly absent from high-intensity plots [1,2].

Fire-adapted species: During all 3 years of treatments, coverage was highest on low-intensity plots for Scouler willow (Salix scouleriana), American vetch (Vicia americana), Jessica's aster (Symphyotrichum jessicae), old man's whiskers (Geum triflorum), and slender cinquefoil (Potentilla gracilis). Fireweed (Chamerion angustifolium), maiden blue-eyed Mary (Collinsia parviflora), prickly lettuce (Lactuca serriola), spreading dogbane (Apocynum androsaemifolium), tall annual willowherb (E. paniculatum), thistle (Cirsium spp.), and tiny trumpet (Collomia linearis) were most common on high-intensity plots. The researchers noted that redstem ceanothus (Ceanothus sanguineus) seedlings occurred only on plots burned at high intensities. Fireweed was 2 to 5 times more common on high-intensity plots than on low-intensity plots, and was nearly absent from unburned plots [1,2].

The following table provides plant percent cover and frequency in unburned, low-intensity, and high-intensity treatments by life form [1,2].

Table 2—Percent cover and frequency of shrubs, perennial graminoids, perennial forbs, ferns, and annuals. Data are averaged over 3 postfire years of data collection [1,2]. Links go the FEIS Species Reviews.
Common name Scientific name
Cover (Frequency)
Unburned Low intensity High intensity
Shrubs
Saskatoon serviceberry Amelanchier alnifolia 0.5a 0.2a 0.1a
redstem ceanothus Ceanothus sanguineus 0.2a 0.1a 0.8a
black hawthorn Crataegus douglasii 1a 0a 0.2a
oceanspray Holodiscus discolor 3.9a 1a 2.7a
creeping barberry* Mahonia repens 1.1a (8.6ab) 1.2a (11.1a) 0b (0.2b)
mallow ninebark* Physocarpus malvaceus 2a 0b 0.4b
bitter cherry Prunus emarginata 0a 0a 0.1a
chokecherry Prunus virginiana 3.7a 5a 2.1a
dwarf rose* Rosa gymnocarpa 3.1a 0.6b 0.9b
Nootka rose Rosa nutkana 0.5a 0.1a 0.2a
Woods' rose* Rosa woodsii 0.8a 0.7a 0.2b
Scouler's willow* Salix scouleriana 0.1b 1.1a 0b
white spirea Spiraea betulifolia 0.6a 1.8a 2.1a
common snowberry Symphoricarpos albus 17.3a 14.3a 20.4a
trailing snowberry Symphoricarpos hesperius 0a 0a 1a
Perennial graminoids
Columbia brome* Bromus vulgaris 0.7a (15.5a) 0.3b (7.7b) 0.3b (4.6b)
pinegrass Calamagrostis rubescens 29.1a (44a) 27.8a (48.1a) 16.1a (25.7a)
blue wildrye Elymus glaucus 2.7a (46.1a) 1.7a (36.4a) 1.2a (24.4a)
Altai fescue* Festuca altaica 0b (24.8a) 0.6a (20.4a) 0.2ab (10.8a)
Idaho fescue Festuca idahoensis 1.9a (0.6a) 1a (5.3a) 0.4a (2.1a)
field woodrush* Luzula campestris 0.5a (14.5a) 0.4a (11.6a) 0.1b (2b)
bluegrass Poa spp. 2.8a (29a) 2.1a (29.3a) 1.8a (22.2a)
bluebunch wheatgrass Pseudoroegneria spicata subsp. spicata 1.4a (25.7a) 2.1a (30.4a) 0.8a (11.8a)
Perennial forbs
common yarrow Achillea millefolium 1a (28.2a) 1.3a (28.3a) 0.5a (12.1a)
Piper's anemone Anemone piperi 0a (1.9a) 0.1a (3.3a) 0.1a (1.5a)
spreading dogbane* Apocynum androsaemifolium 0b (0.7b) 0b (0.1b) 1a (9.2a)
fireweed* Chamerion angustifolium
(Epilobium angustifolium)		 0b (0.2b) 0.3b (27ab) 1.2a (7.6a)
thistle* Cirsium spp. 0.1b (1b) 0.1b (2.8b) 0.4a (8.7a)
Virginia strawberry* Fragaria virginiana 2.9ab (47.6a) 3.4a (53.6a) 1.6b (24.2b)
northern bedstraw Galium boreale 2.7a (24.4a) 3.5a (25.3a) 1.9a (13a)
sticky purple geranium Geranium viscosissimum 0.9a (12.7a) 1.7a (17.9a) 1.1a (11.3a)
old man's whiskers* Geum triflorum 0.3b (4.2b) 0.8a (11.8a) 0.1b (2.3b)
white hawkweed Hieracium albiflorum 0.7a (2.7a) 0.8a (3.7a) 0.4a (1.9a)
drypark pea Lathyrus bijugatus 0.7a (13.9a) 0.4a (11a) 0.6a (12a)
nineleaf biscuitroot Lomatium triternatum 0.4a (4.5a) 0.4a (3.6a) 0.4a (3.6a)
lupine Lupinus spp. 0.1a (1.4a) 0.6a (6.9a) 0.4a (5.4a)
largeleaf sandwort Moehringia macrophylla
(Arenaria macrophylla)**		 1.3a (11a) 2.1a (16.8a) 0.6a (3.9a)
sweetcicely Osmorhiza berteroi 0.1a (2.7a) 0.2a (5.6a) 0.2a (4.8a)
beardtongue Penstemon spp. 0.5a (6.2a) 0.2a (3.3a) 0a (1.2a)
sticky cinquefoil Potentilla glandulosa 0.4a (5.6a) 0.4a (5.1a) 0a (0.6a)
slender cinquefoil* Potentilla gracilis 1.4ab (20.2c) 2.2a (31.6a) 0.8b (10.2b)
Canada goldenrod Solidago canadensis 0.5a (3.2a) 1.1a (6.2a) 0.1a (0.9a)
starwort Stellaria spp. 0.6a (4.4a) 0.6a (5.9a) 1.2a (11.7a)
Jessica's aster* Symphyotrichum jessicae
(Aster jessicae)		 0.3b (3.9b) 2.2a (22.2a) 1b (7.6b)
common dandelion Taraxacum officinale 0.1a (3.6a) 0.1a (3.3a) 0.2a (4.3a)
twin clover Trifolium latifolium 1.9a (25.5a) 1.2a (18.4a) 2.3a (20.1a)
American vetch* Vicia americana 0.1b (1.2b) 0.7a (11.1a) 0.1b (2.1b)
Ferns
western brackenfern Pteridium aquilinum 1.3a (6.5a) 2.3a (8.9a) 2.5a (8.7a)
Annuals
Japanese brome Bromus japonicus 0.1a (3.6a) 0.1a (1.6a) 0.1a (2.6a)
maiden blue-eyed Mary* Collinsia parviflora 0.1b (3b) 0.1b (2.3b) 0.4a (11.5a)
tiny trumpet* Collomia linearis 0.2b (6.8b) 0.4b (11.3b) 0.9a (22.9a)
tall annual willowherb* Epilobium paniculatum 0.1b (2.4b) 0.2b (5.9b) 1.3a (27.3a)
sweet fennel Foeniculum vulgare 0.2a (5a) 0.3a (7.8a) 0.4a (8.7a)
prickly lettuce* Lactuca serriola 0b (1.2b) 0.1b (2.7b) 0.8a (17.5a)
*Species with a significant change in coverage between at least 2 treatments.
**Nomenclature follows PLANTS Database [8]. Names in parentheses are those used in the original sources.

FIRE MANAGEMENT IMPLICATIONS:
In this study, postfire plant species composition was largely determined by composition of the prefire plant community. Comparisons between unburned plots, low-intensity fires, and high-intensity fires in this ponderosa pine-dominated community showed that for most species, cover and frequency remained unchanged regardless of fire intensity. Also, most species that were impacted by the fires showed increased coverage and frequency on burned plots compared to unburned plots. The decrease in graminoid frequency and small increase in forb coverage on high-intensity plots may impact forage availability for native ungulates and livestock in the short term. The authors noted, however, that graminoid coverage increased over 3 postfire years, and postfire growth of surviving graminoids would likely compensate for graminoid mortality due to smoldering duff. They also noted that duff reduction correlated to low duff moisture content, not flame length or fireline intensity [1]. Prescribed burning that is conducted when duff moisture is high can help protect graminoids from fire damage or mortality.

REFERENCES:

1. Armour, Charles D.; Bunting, Stephen C.; Neuenschwander, Leon F. 1984. Fire intensity effects on the understory in ponderosa pine forests. Journal of Range Management. 37(1): 44-48. [6618]
2. Armour, Charles D.; Bunting, Stephen C.; Neuenschwander, Leon F. [n.d.]. The effect of fire intensity on understory vegetational development. Unpublished report on file with: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory, Missoula, MT. 17 p. [30855]
3. Bakken, Stephen. 1981. Predictions of fire behavior, fuel reduction, and tree damage from understory prescribed burning in the Douglas-fir/ninebark habitat type of northern Idaho. Moscow, ID: University of Idaho. 137 p. Thesis. [43780]
4. Eyre, F. H., ed. 1980. Forest cover types of the United States and Canada. Washington, DC: Society of American Foresters. 148 p. [905]
5. Garrison, George A.; Bjugstad, Ardell J.; Duncan, Don A.; Lewis, Mont E.; Smith, Dixie R. 1977. Vegetation and environmental features of forest and range ecosystems. Agric. Handb. 475. Washington, DC: U.S. Department of Agriculture, Forest Service. 68 p. [998]
6. Kuchler, A. W. 1964. United States: Map, [Potential natural vegetation of the conterminous United States]. Special Publication No. 36. New York: American Geographical Society. 1:3,168,000; colored. [3455]
7. LANDFIRE Biophysical Settings. 2009. Biophysical setting 1010451: Northern Rocky Mountain dry-mesic montane mixed conifer forest - ponderosa pine-Douglas-fir. In: LANDFIRE Biophysical Setting Model: Map zone 10, [Online]. In: Vegetation Dynamics Models. In: LANDFIRE. Washington, DC: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory; U.S. Geological Survey; Arlington, VA: The Nature Conservancy (Producers). Available: https://www.landfire.gov/national_veg_models_op2.php [2017, May 16]. [91883]
8. USDA Natural Resources Conservation Service. 2017. PLANTS Database, [Online]. U.S. Department of Agriculture, Natural Resources Conservation Service (Producer). Available: https://plants.usda.gov/. [34262]
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