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Composite fire interval for Limestone Flats, Long Valley Experimental Forest, Arizona. Time span: 110 years; fire years: 51; composite fire interval: 1.8 years [35].

• Fire Group Zero: Miscellaneous special habitats. Includes scree, forested rocklands, wet meadows, mountain grasslands, quaking aspen groves, and deciduous riparian communities.
• Fire Group One: Dry savanna habitat types. Limber pine or ponderosa pine may dominate.
• Fire Group Two: Warm, dry, woodland habitat types. These types support ponderosa pine or Douglas-fir. The generally sparse understories are composed of scattered low shrubs and graminoids. Mature stands are open, with an undergrowth of dry-site grasses and forbs.
• Fire Group Three: Warm, moist ponderosa pine habitat types and warm, dry Douglas-fir habitat types. This group is usually dominated by ponderosa pine. Understories consist of low-lying shrubs and grasses, except where Douglas-fir regeneration forms thickets of young trees. In the absence of fire, Douglas-fir advanced regeneration successionally replaces ponderosa pine in Douglas-fir habitat types.
• Fire Group Four: Cool, dry Douglas-fir habitat types. These stands generally occur in the continental climate of the Northern Rocky Mountains and elsewhere, above the cold limits of ponderosa pine. Douglas-fir is often the only conifer on the site.
• Fire Group Five: Moist to mesic habitat types dominated by Douglas-fir or subalpine fir. Fires are infrequent and often severe, with long-lasting effects. Stands progress successionally to mature climax stands in the absence fire.
• Fire Group Six: Cool, moist habitat types such as grand fir. These productive sites often support diverse stands of seral tree species. Fire is infrequent but has played an important role in promoting these mixed-conifer stands.
• Fire Group Seven: Cool habitat types. These occur on slopes with a sparse understory and variable tree species composition. Ponderosa pine, Douglas-fir, lodgepole pine, and subalpine fir are typical components. This group includes stands in which fire-maintained lodgepole pine is a dominant seral species as well as stands in which it is a dominant climax species.
• Fire Group Eight: Dry, lower subalpine habitat types. This is a heterogeneous grouping of subalpine habitat types that range from moist to dry. Douglas-fir or a mixture of Douglas-fir and lodgepole pine are the major trees in this group; sometimes spruce follows successionally.
• Fire Group Nine: Moist, lower subalpine habitat types. These sites are often dominated by Engelmann spruce. Fires are infrequent but often severe, with long-lasting effects. Spruces are often the climax species; Engelmann spruce is a persistent seral species in subalpine fir habitat types.
• Fire Group Ten: Cold, upper subalpine and timberline habitat types. Fires are generally infrequent; the fires that do occur are often limited in extent by discontinuous fuels. Postfire succession is slow due to the harsh environment [29,125,138].

• According to Agee [2], "A fire regime is a generalized way of integrating various fire characteristics. The organization may be according to the characteristics of the disturbance..., dominant or potential (climax) vegetation on the site..., or fire severity, the magnitude of effects on dominant vegetation".
• According to Mutch [89], "A natural fire regime is the total pattern of fires over time that is characteristic of a natural region or ecosystem. The classification of fire regimes includes variations in ignition, fire intensity and behavior, typical fire size, fire return intervals, and ecological effects".
• According to Johnson and Van Wagner [61], "fire regime is a multivariate system characterized by (i) the fire history measured in fire frequency or fire return period, (ii) fire intensity measured in kW/m, and (iii) depth of burn (duff removed) measured in kg/m, or percent".
• Sugihara et al. [132,133] describe fire regimes based on three groups of seven attributes of fire patterns that are commonly considered to be important to ecosystem function:

fire regime condition class (FRCC):
A standardized, interagency index to measure the departure of current conditions from reference or historical conditions [74]. The fire regime condition classes are [9]:

FRCC 1: ecosystems with low (<33%) departure from a defined reference period; i.e., landscapes still within the natural or historical range of variation
FRCC 2: ecosystems with moderate (33%-66%) departure
FRCC 3: ecosystems with high (>66%) departure from reference conditions

fire regime group:
A categorization of historical fire regimes that describes the general frequency and severity of fires (based on Heinselman [52]). Fire regime groups referenced in FEIS reviews are those used in LANDFIRE products, which were first defined in 2010 [9] and later refined in 2020 [76]:

fire-resistant species:
Species with morphological characteristics that give it a lower probability of being injured or killed by fire than a fire-sensitive species [84]. Implies that the organism does not get injured by things that would seem able to injure it [61]. Rowe [119] uses a more restrictive definition of resistance, relating it only to plants with aboveground parts that survive fire. According to Levitt [78], there are 2 kinds of fire resistance: (1) Tolerance, which describes species that mitigate dangerous, often lethal conditions. In regard to fire, tolerance means that living cells are severely heated but survive anyway; such traits are rare. (2) Avoidance, which describes ways of preventing cells from heating to lethal temperatures. Most plant cells that survive fire do so through "avoidance", because of insulating tissues, for example, or because of an insulated microenvironment. Since Rowe [119] uses avoidance with a meaning different from this one, FEIS usually uses "resistance" to indicate both "fire tolerance" and "fire avoidance".

fire-return interval:
See fire interval.

fire rotation:
The time required to burn the equivalent of a specified area, calculated as: (total time period) ÷ (proportion of area burned in period) [1,17].

fire-sensitive species
A species with a "relatively high" probability of being injured or killed by fire [84]. Compare with fire-resistant species

fire severity:
Fire severity generally indicates the degree of environmental change caused by fire [94,120]. Scott and Reinhardt [123] provide a slightly more detailed definition: the effect of a fire on ecosystem properties, usually described by the degree of soil heating or mortality of vegetation. Terms describing levels of fire severity are inconsistent and can be very confusing. While terms from different sources may be similar in meaning, none are exactly equivalent. If you try to make them so, you will surely get a headache. It is best, when trying to describe levels of severity, to use terms from one source. As of 2023, three main sources describing fire severity levels were in use:

1) LANDFIRE data products (e.g. LF 2020 and LF 2008 used in FEIS) use fire severity classes defined in the Interagency Fire Regime Condition Class (FRCC) Guidebook version 3.0. The FRCC defines fire severity as "the effect of fire in terms of upper layer canopy replacement", or "top-kill". Replacement-severity fire may or may not kill canopy vegetation. For example, replacement-severity fire in grasslands removes aboveground biomass of grasses, which usually resprout, whereas replacement-severity fire in most conifer forests kills the trees. The FRCC guidebook classifies levels of fire severity as low, mixed, and replacement [9]:

Diagram of a drupe fruit and its seed. Image by LadyofHats.

• 1-hour timelag fuels: Dead fuels comprised of herbaceous plants or woody plants less than about 0.25 inch (6.4 mm) in diameter and the surface layer of litter on the forest floor.
• 10-hour timelag fuels: Dead fuels comprised of wood from 0.25 to 1 inch (0.6-2.5 cm) in diameter and the litter from just beneath the surface to around 0.75 inch (1.9 cm) below ground.
• 100-hour timelag fuels: Dead fuels comprised of wood from 1 to 3 inches (2.5-7.6 cm) in diameter and litter from around 0.75 to about 4 inches (1.9-10 cm) below ground.
• 1,000-hour timelag fuels: Dead fuels comprised of wood from 3 to 8 inches (7.6-20.3) in diameter and the forest floor layer >4 inches (10 cm) below ground.

fuel continuity:
A qualitative description of the distribution of fuels both horizontally and vertically. Continuous fuels readily support fire spread. The larger the fuel discontinuity, the greater the fire intensity required for fire spread [20].

fuel loading:
The weight per unit area of fuel, often expressed in tons/acre or tonnes/hectare. Dead woody fuel loadings are commonly described for small material in diameter classes of 0 to 1/4, 1/4 to 1, and 1 to 3 inches and for large material in one class greater than 3 inches [20].

fuel moisture:
Expressed as a percent or fraction of oven-dry fuel weight. It is the most important fuel property controlling flammability. In living plants it is fluctuations vary considerably by species but are usually above 80% to 100%. As plants mature, moisture content decreases. When herbaceous plants cure, their moisture content responds as dead fuel moisture content, which fluctuates according to changes in temperature, humidity, and precipitation [20].

full-tree harvesting:
Cutting and removing an entire upper portion of a tree consisting of trunk, branches, and leaves or needles [97].

gametophyte:
The sexual stage in the life cycle plants, when the chromosomes in each cell are reduced to half the usual number, typically diploid (2n) reduced to haploid (1n) (compare sporophyte) [49,55].

geitonogamy:
Pollination between flowers on the same plant [49].

gemma:
A cell or cluster of often bud-like cells, borne on the gametophyte, that can reproduce the plant vegetatively. Plural: gemmae [27].

genet:
A unit or group derived by asexual regeneration from a single original zygote; a clone [79].

genotype:
The hereditary or genetic constitution of an individual; all the genetic material of a cell (usually only nuclear material); all the individuals sharing the same genetic constitution [79].

geophyte:
Plant with perennating tissue below the soil surface; may possesses tuberous underground stems filled with stored food (e.g., bulb, corm, rhizome, stem tuber) or sprout from root tissue [110].

geotrophism:
The response of plants in either growth or position to the pull of gravity. Geotrophic response is positive when the response is towards the earth's center of gravity (e.g., early root growth) and negative when away from it (e.g., early stem growth) [53].

glume:
One of the pair of bracts at the base of a spikelet in Poaceae, or the single bract suntending the flower in Cyperaceae [55].

graminoid:
A grass or grasslike monocot; includes the grass (Poaceae), sedge (Cyperaceae), rush (Juncaceae), cattail (Typhaceae), and arrowgrass (Juncaginaceae) families [55,68].

grass/fire cycle:
An altered fire regime that may result when nonnative invasive grass species dominate the herbaceous layer in a plant community. The process occurs in this way: the nonnative grass colonizes an area and provides a continuous fine fuel that is readily ignited and facilitates fire spread. Larger and possibly more severe fires then occur more frequently in the invaded area than in similar, uninvaded communities. Following these grass-fueled fires, nonnative grasses typically recover more rapidly than native species, further increasing the probability of fire and the possibility of greater fire size and severity and decline of native species (adapted from [31]; also see [18].

grassland:
A formation dominated by grasses (>25% cover), with shrubs and trees constituting <25% of total cover [75]. Compare with shrubland, woodland, wooded shrubland, savanna, forest.

gravid:
Carrying eggs or young [79].

greenstrip:
A long, narrow band of fire-retardant vegetation that is created by seeding [34].

ground fire:
Fire that burns in the organic material below the litter layer, mostly by smoldering combustion. Fires in duff, peat, dry or dead moss and lichens, and/or punky wood are typically ground fires [20].

ground residual colonizer:
A plant species that establishes after a disturbance such as fire from soil-stored seed that was already on site prior to the disturbance [131].

gynodioecious:
Plant species with female and perfect flowers on separate plants [49].

gynomonoecious:
Plant species with female and perfect flowers on the same plant [49].

habitat type:
A land or aquatic unit capable of producing similar plant communities [53]. It describes areas in which similar environmental conditions allow succession to proceed toward the same climax association. Seral species may dominate such areas, but the habitat types are named for the climax species [33]. Contrast with cover type.

hammock (hummock):
1) In the southern United States, especially Florida, a tract of hardwood forest that rises above an adjacent marsh.
2) A ridge or hill of ice or an ice field [95].

haplotype:
The collective genotype of a number of closely linked loci; the constellation of alleles present at a particular region of a chromosome [79].

hardwood:
1) Angiosperm trees or
2) the xylem of angiosperm trees.
The wood of hardwood trees may be physically hard (high specific gravity) or soft (low specific gravity) [53].

headfire:
A fire spreading or set to spread with the wind [94].

heath:
Species within the plant families Ericaceae, Empetraceae, and/or Diapensiaceae. A heathland is dominated by species in those families [16].

helophyte:
Freshwater plant with perennating tissue in water or saturated soil beneath water [110].

hemicryptophyte:
Herbaceous perennial or biennial plant with perennating tissue at the soil surface [110].

herb:
A nonwoody, vascular plant such as a graminoid, forb, or fern [53].

herbaceous:
A class of vegetation dominated by nonwoody plants (herbs). Herbs generally have at least 25% cover, while shrubs and trees have <25% cover [53].

hermaphrodite:
Having both male and female reproductive organs in the same individual [79]. For plants, also see perfect.

hibernaculum:
A shelter in which animal(s) hibernate or overwinter [7,79].

hibernation:
The act or condition of passing the winter in a dormant state characterized by a slowing of metabolic processes that is more dramatic than torpor; typically involving the abandonment of thermal homeostasis in mammals [39,79,83]. Compare to aestivation.

high-severity fire:
May refer to either high soil burn severity, high vegetation burn severity, or replacement-severity fire. As used in LANDFIRE, refers only to replacement-severity fires, which kill or top-kill more than 75% of the upper canopy layer [9]. Also see discussion of fire severity.

historical fire regime:
See fire regime.

Holocene:
The current geological epoch, which spans about 10,000 years before present (BP) to the present. Also known as the Postglacial, it was preceded by the Pleistocene [100].

homogamous:
Having pistils and stamens that mature at the same time [49].

hybrid swarm:
A group of morphologically distinctive individuals that results from the creation of hybrids between 2 parent species, then the backcrossing of the offspring to members of the parent species and the interbreeding among the hybrid individuals [13].

hydroperiod:
The frequency and duration of inundation or saturation of an ecosystem. In the context of characterizing wetlands, the term describes that length of time during the year that the substrate is either saturated or covered with water.

hydrophyte:
Freshwater plant with perennating tissue below the surface of the water; vegetative shoots are submerged and leaves are submerged or floating; only flowers rise above the water surface [110].

hypanthium:
A cup-shaped extension of the floral axis, usually formed from the union of the basal parts of the calyx, corolla, and stamens, commonly surrounding or enclosing the pistils. Plural: hypanthia [49].

hypocotyl:
The portion of the embryonic shoot below the cotyledon and above the radicle [5].

hypogeal:
A mode of seed germination in which the cotyledons remain below ground [5].

imperfect:
A flower having either pistils or stamens, but not both; unisexual [49].

importance value:
A measure of overall importance of a given species in a community [79]. Definitions are inconsistent. Importance value is often calculated as the sum of relative frequency, relative density, and relative dominance, where relative dominance is synonymous with relative basal area [45,79] or some similar measure [5,88]. When importance value is defined otherwise, FEIS defines the term as used by the author cited.

indehiscent:
Not opening at maturity [49].

indeterminant flowering:
Inflorescence in which the outer or lower flowers open first [49].

indicator value:
Product of the relative abundance and relative frequency of occurrence for a species in group samples [36].

inhibition model of succession:
The hindrance of subsequent species colonization or suppression of growth of species already present due to establishment of earlier colonists; opposite of the facilitation model of succession [28].

initial off-site colonizer:
A plant species that establishes in early succession from seed dispersed onto the disturbed site [131].

interference:
The negative effect of one organism upon another, regardless of the presence of a limiting resource. Competition is one facet of interference; allelopathy is another [14,47]. Note, however, that use of this term in the ecological literature is inconsistent; Harper [48], Hall [46], and Radosevich et al. [107] use "interference" to refer to both positive and negative interactions between organisms.

introgression:
The spread of genes of one species into the gene pool of another by hybridization and backcrossing [79].

invasibility:
Susceptibility of a plant community to invasion [150].

invasive species:
A species that can establish, persist, and spread in an area [81,121]. In addition, the species must cause, or have potential to cause, harm; in natural areas, "harm" usually occurs in the form of significant changes in ecosystem composition, structure, or function [142]. Randall [108] states this idea pragmatically: A plant species must interfere with management goals to be considered invasive. A nonnative species is not invasive simply because it is present in a wildland ecosystem; it must also have impacts on the ecosystem that interfere with attainment of management objectives. Fire-related impacts of invasive plants may include changes in the species composition or structure of postfire plant communities, especially when these changes occur at the expense of native species, and changes in fuel properties that alter fire behavior or fire regimes.

involucre:
A whorl of bracts below an inflorescence or cone [5,55].

irruption:
An irregular, periodic migration where large parts of a population leave their home region and move into unusual areas for a season (that is a "bursting in", immigration, or invasion into other regions). It may be triggered by high population densities, food shortages, or both. An irruption differs from true migration because it is irregular and does not necessarily have a return movement matching the outward movement (also see eruption) [12].

karyotype:
The appearance of the chromosomal makeup of a somatic cell in an individual or species, including the number and arrangement and size and structure of the chromosomes [79].

Kuchler potential natural vegetation (Kuchler plant associations):
A vegetation classification system of the conterminous United States that groups vegetation into physiognomic units based on potential natural vegetation [69,72].

ladder fuels:
Shrubs and young trees that provide continuous fine material from the forest floor into the crowns of dominant trees [124].

latewood:
An annual ring of secondary xylem formed late in the growing season, with relatively smaller, thicker, denser, and darker cells than cells formed early in the growing season [53].

layering:
A form of vegetative reproduction in which an intact branch develops roots as the result of contact with soil or other media [53].

leaf area index (LAI):
Ratio of total leaf area (one side of leaf only) to total ground surface, a unitless measure [88].

liana:
A climbing woody plant [68].

lignotuber:
A woody storage structure forming a swelling, more or less at ground level, from which dormant buds can develop [53]. Functionally and ecologically similar to burls that occur at ground level of some woody species [59].

litter:
Recently fallen plant material, including leaves, needles, fine twigs, and other organic material on the forest floor, that is only partially decomposed and is still discernible [79,123]. Also see duff.

locus:
The position of a given gene on a chromosome. Plural: loci [79].

lodging:
A permanent disconnection of the stem from its upright position [5].

long-term effects:
Effects lasting more than 10 years (personal communication 21 October 1998 with Wendell Hann, Fire Ecologist and assistant to National Fuels Specialist, USDA Forest Service).

low-severity fire:
A fire that has little effect on soil heating or on vegetation, especially the overstory vegetation [123]. As used in LANDFIRE, a low-severity fire is a surface fire that replaces less than 26% of the upper canopy layer, thus maintaining the site in a given successional stage [9]. Similar to understory fire, but can refer to grasslands and shrublands as well as forests and woodlands. See discussion of fire severity.

low-severity fire regime:
General pattern in which most fires are of low severity and do not substantially change the aboveground vegetation structure. Similar to understory fire regime.

mafic:
Containing or relating to a group of dark-colored minerals, composed predominantly of the ferromagnesium rock-forming silicates, such as olivine and pyroxene [100].

marsh:
A low-lying wetland that has shallow water; water levels that fluctuate daily, seasonally, or annually due to tides, flooding, evapotranspiration, groundwater recharge, and/or seepage losses; is vegetated with monocots; and does not accumulate appreciable peat deposits. Marshes often form a transitional zone between water and land [95,141].

mast:
1) Exceptionally high seed production [79].
2) Seeds of all plants used by wildlife, including seeds with fleshy exteriors (such as berries) and seeds with dry or hard exteriors (such as nuts and cones). Collectively, the fruit of masting species [85].

mean fire-free interval:
See mean fire interval.

mean fire interval:
Average of all fire intervals determined, in years, for a specified area during a specified time period. The size of the area and the time period must be given [84]. Mean fire intervals are usually composite fire intervals but may be point fire intervals.

mean fire-return interval:
See mean fire interval.

meristem:
A group of plant cells that divides indefinitely, producing new growth. It occurs at the growing tip of roots or stems (apical meristem), in cambium (lateral meristem), and in the stems and leaves of graminoids (intercalary meristem) [5].

mesic:
1) Pertaining to conditions of moderate moisture or water supply [124].
2) One of six soil temperature regimes.

metapopulation:
A population structure in which individual populations exist on patches that are dynamic in space and time [53]. These individual or subpopulations are connected by pathways of immigration and emigration, and exchange of individuals occurs between subpopulations. Emigrating individuals are able to colonize currently unoccupied patches of suitable habitat, including previously occupied patches from which the species has recently become extinct [79].

milk stage:
Developmental stage of plant in which seeds are well formed but soft and immature; precedes dough stage [90].

minerotrophic:
A site with high dissolved mineral content, with the nutrients intruded from groundwater flow in addition to precipitation. Compare with ombotrophic [141].

mire:
A bog or fen; also referred to as a carr [95].

mixed-severity fire:
A fire that exhibits a wide range of fire severity as a result of surface fire in some patches, burning others with stand-replacement severity, and thinning the overstory in other patches. An equal proportion of low-, moderate-, and high-severity burning clearly fits into the mixed-severity fire class, but there is currently no standard by which to define what mix of fire severities should be classified as a mixed-severity fire [123]. As used in LANDFIRE, mixed-severity fire refers to fires that cause partial replacement of the upper canopy layer (26%-75%). According to this definition, mixed-severity fire is not the same as mosaic fire [9]. However, it has occasionally been used with that meaning. See fire severity.

mixed-severity fire regime:
General pattern in which fires tend to be of mixed severity, cause selective mortality in the upper canopy layer (depending on different species' susceptibility to fire), or vary in time or space between low-severity and stand-replacement [124]. Note that mixed-severity fire regime has a broader definition than mixed-severity fire and may include a general pattern of mosaic fires or of fires with varying severities over time.

mixing depth
Vertical distance between the ground and the altitude to which pollutants are mixed by turbulence caused by convective currents or vertical shear in the horizontal wind [143].

moderate-severity fire:
Fire that causes moderate soil heating. Occurs where litter is consumed and duff is charred or consumed, but the underlying mineral soil is not visibly altered [94,150]. Although thresholds are subjective, fire that kills from 30% to 70% of the upper canopy layer is generally considered moderate severity [123].

moisture control section (soil):
The moisture control section depends on particle-size class of the soil: It extends approximately from (1) 10 to 30 cm below the soil surface in fine-loamy, coarse-silty, fine-silty, or clayey soils; (2) 20 to 60 cm in coarse-loamy soils; and (3) 30 to 90 cm in sandy soils. The limits are deeper in soils with rock fragments that do not absorb and release water [136].

monocarpic:
Flowering and bearing fruit only once and then dying; term can apply to annuals, biennials, or perennials [49].

monoecious:
Male and female flowers borne on the same plant [49].

monoestrous:
Having one reproductive cycle or breeding period per year [79].

monopodial:
Having branches arise from a single main axis [49].

mosaic fire:
A fire that produces patches of burned and unburned vegetation across the landscape. Note that mixed-severity fire has been used in the past to describe mosaic fires [9]. See fire severity.

muskeg:
A swamp or bog formed by an accumulation of Sphagnum moss, leaves, and decayed matter that resembles peat. Prevalent in Alaska and Canada; part of the North American boreal forest biome [95].

mutualism:
An interdependent relationship in which both organisms benefit; frequently a relationship of complete dependence [79].

natural:
Ecosystems, plant communities, or processes that still function as they did in pre-Columbian times [144].

natural fire rotation:
See fire rotation.

nidicolous:
Living in a nest; also used of young animals, especially birds, that remain in the nest for a prolonged period [79].

nonnative species:
An introduced species that evolved elsewhere and has been transported and purposefully or accidentally disseminated by humans [78].

nonrefractory:
In FEIS, seeds that germinate in the absence of fire-related stimuli; seeds may germinate readily upon wetting or they may have a dormancy that is overcome by some factor unrelated to fire [62].

old-growth stage:
Fourth stage of forest stand development following major disturbance, as described by Oliver [96]. In this stage, stems in the overstory gradually die out and stems in the understory slowly replace them. Also see stand initiation stage, stem exclusion stage, and understory reinitiation stage.

oligohaline:
1) An organism that is tolerant of only a moderate range of salinities.
2) Brackish water with a salinity from 0.3 to 3.0 parts per thousand, or sea water with a salinity from 17 to 30 parts per thousand [79].

oligotrophic:
A lake or other body of water with extremely low dissolved mineral content, resulting in very moderate productivity [95].

ombotrophic:
A site with low dissolved mineral content, with the nutrients coming exclusively from precipitation. Compare with minerotrophic [141].

ontogeny:
The growth and physical development of an individual [79].

organic soils:
Deep layers of organic matter that develop in poorly drained areas such as bogs, swamps, and marshes [20].

orthotrophic:
An orientation or growth response in a straight line. Also see phagiotrophic [79].

ovoviviparous:
Producing fully formed eggs that are retained and hatched inside the maternal body, with the release of live offspring [79].

paludification:
The process of bog formation resulting from the gradual rising of the water table as accumulation of peat impedes water drainage [79].

pappus:
The modified calyx of the Asteraceae, consisting of awns, scales, or bristles at the apex of the achene [49].

parturition:
The act of giving birth [79].

passive crown fire:
A type of crown fire in which the crowns of individual trees or small groups of trees burn, but solid flaming in the canopy cannot be maintained except for short periods. Passive crown fire encompasses a wide range of crown fire behavior, from occasional torching of isolated trees to nearly active crown fire. Passive crown fire is also called torching or candling. A fire in the crowns of the trees in which trees or groups of trees torch, ignited by the passing front of the fire. The torching trees reinforce the spread rate, but these fires are not basically different from surface fires [123]. Also see active crown fire.

peat:
Organic soil material formed by partial decomposition of plants, especially Sphagnum spp. mosses, in water [95,127].

peatland:
An ecosystem in which organic matter is produced faster than it decomposes, resulting in an accumulation of partially decomposed vegetative matter [95]. To distinguish between peatlands and peaty mineral soils, minimum depths of organic deposits are sometimes set at 8 to 12 inches (20-30 cm) in the United States [51]; in Canada, the minimum depth of organic deposits required for classification as peatland is 16 inches (40 cm) [126].

permafrost:
1) Permanently frozen material underlying the solum.
2) A perennially frozen soil horizon [127].

perfect:
Flowers or cones with both male and female reproductive organs; can also be called bisexual or hermaphroditic [49].

pericarp:
The fruit wall, often with 3 distinct layers: endocarp, mesocarp, and outer exocarp [5] (see fruit for a diagram of these layers).

perigynium:
A scale-like bract enclosing the pistil in Carex [49].

phanerophyte:
Woody plant with perennating tissue more than 10 inches (25 cm) above the soil surface [110].

phase:
A category that provides functional or technical detail at any level (formation, subformation, series, and association) of the vegetation classification system hierarchy [57].

phenology:
The study of the relationship between weather and climate and the timing of periodic natural phenomena such as bud bursting and flowering of plants or migration of birds [79].

phenotype:
The sum total of observable structural and functional properties of an organism; the product of the interaction between the genotype and the environment [43,79].

phenotypic plasticity:
The capacity for marked variation in the phenotype as a result of environmental influences on the genotype during development [79].

philopatric:
Exhibiting a tendency to remain in the native locality. Used of species or groups that show little capacity to spread or disperse and of individuals that tend to remain in, or return to, their home areas or domiciles [79].

phreatophyte:
A plant that absorbs water from the permanent water table [79].

plagiotrophic:
An orientation or growth response at an oblique angle to the vertical. Also see orthotrophic. [79].

Pleistocene (Ice Age):
The geological epoch that preceded the current epoch (the Holocene). The Pleistocene spanned about 18,000 to 10,000 years before present (BP). It was an epoch when the earth entered its most recent phase of widespread glaciation [100].

ploidy level:
The number of sets of chromosomes in a cell or an organism. Haploid is having one chromosome set, or one copy of each chromosome per cell. Diploid is having two sets of chromosomes, usually with a set of chromosomes from each parent. Polyploidy is having three or more sets of chromosomes. There are many types or variations of polyploidy. For example, triploid is having three times the haploid number of chromosomes; tetraploid is having four times the haploid number of chromosomes. Polyploidy is common in plants [77,148].

point fire interval:
A composite fire interval over a relatively small area. The "point" may be a single tree or a small area (e.g., 1 or 2 ha). Agee [3] stated that "Although a tree is the best 'point' on the landscape, it is usually not the best sample unit to use to derive a 'point estimate' of fire. Usually the combination of cross-dated records from two or more closely spaced trees are used. Each sample tree is itself a point sample, and as the number of trees whose records are combined grows, two things usually happen: the fire record becomes more complete, so that the fire interval becomes shorter; and the point frequency tends to become an area frequency (i.e., composite fire interval) as the area over which records are combined expands" [3]. Because a fire can burn through a small area without scarring any trees, the point fire interval may underestimate fire frequency [103,135]. Also see fire interval and composite fire interval.

pollinia:
Masses of waxy pollen grains transported as a unit in many orchids (Orchidaceae) and milkweeds (Asclepiadaceae). Milkweed pollinia are joined by a corpusculum [49].

polycarpic:
Producing flowers or spores more than once during a life cycle [79].

polygamodioecious:
A plant that is mostly dioecious but with some perfect flowers [49].

polygamomonoecious:
A plant that is mostly monoecious but with some perfect flowers [49].

polygamous:
A mating system in which each adult may mate with more than one member of the opposite sex [83].

polygyny:
A mating system in which males mate with a large number of females and father a large number of offspring [83].

polyploid:
Having three or more complete sets of chromosomes in each cell [49].

polyestrous:
Having more than one breeding period per year [79].

population index:
Any indicator of the size of a population (e.g., mountain bluebird nests/km²) [43].

potential natural vegetation:
The vegetation that would occur on a given site if disturbance by humans was excluded. It is a reflection of the environmental setting, or the biological potential of a land area to support a specific vegetation type within the constraints of the nonanthropogenic disturbance regime of that site ([9,71], Tüxen 1956 as cited by [70]).

potential natural vegetation group (PNVG):
A grouping of ecologically similar vegetation types that are likely to exist under the natural range of variability in biophysical environments and ecological processes, including fire and other disturbances [74]. These groups are modeled with disturbance inputs (fire, insect and disease mortality, grazing, and drought) that are characteristic of the group and used for Fire Regime Condition Class assessments [9].

precocial:
Offspring that exhibit a high degree of independent activity at hatching or birth (also see altricial) [79].

prescribed burn:
See prescribed fire.

prescribed fire:
Any fire intentionally ignited by management in accordance with applicable laws, policies, and regulations to meet specific objectives. Also called a controlled burn or prescribed burn [93,149]. An escaped prescribed fire is considered a wildfire [93].

prescribed natural fire:
Naturally ignited wildland fire that burns under specified conditions where the fire is confined to a predetermined area and produces the fire behavior and fire characteristics required to attain planned fire treatment and resource management objectives [94]. This term was not in official use as of 2010. See use of wildland fire.

presence:
A constancy class derived from samples of indefinite area [43]. FEIS usually cites the original author's definition when this term is used.

presettlement fire regime:
As generally used, the characteristic fire regime prior to the mid- to late 1800s, before extensive settlement by European Americans in most parts of North America, before extensive conversion of wildlands for agricultural and other purposes, and before fires were effectively suppressed and excluded from many areas [124]. However, many authors assign less precise or different meanings to this term. FEIS generally uses the definition given here for presettlement fire regime and related terms (e.g., historical fire regime, reference fire regime).

protandrous:
Mating system by which anthers release pollen before the stigma is receptive [49].

protogynous:
1) Mating system by which the stigma is receptive before the release of pollen [49].
2) A plant with perfect flowers that assumes a functional female condition before changing to a functional male state [79].

radial spines:
Cacti spines that radiate from the margins of the areole [11].

radicle:
The embryonic root [5].

ramet:
A member or unit of a clone, which may follow an independent existence if separated from the parent [79].

rangeland condition:
A rating of rangeland condition based on total percentage of native "climax" vegetation within a given habitat type. This approach assumes that climax vegetation can be determined for the habitat type [37,38].

reaction intensity:
The energy release rate of the fire front. The energy is released when burning gases are released from combustable organic matter in fuels; therefore, fuel parameters including particle size, bulk density, moisture, and chemical composition are factors determining reation intensity. Expressed as the amount of heat released/unit area² [117].

reburn:
Repeat burning of an area over which a fire has previously passed [94].

reclamation:
Restoration of biophysical capacity [40].

recruitment:
The influx of new members into a population by reproduction or immigration [79].

reference fire regime:
May refer to presettlement fire regime or to the fire regime of any other period chosen as a basis for comparison to the present.

refractory:
A term used to describe seeds that require a fire related stimulus, alone, or in conjunction with other conditions such as cold stratification to germinate [62].

relative density:
Number of individuals of a given species per unit area expressed as a percentage of total number of individuals of all species per unit area [88,128].

relative dominance:
Basal area of a species expressed as percentage of total basal area [88,128].

relative frequency:
Frequency of a species divided by sum of frequencies of all species, expressed as a percent [88].

relict:
A biotic community or fragment of a community that has survived some important change, often to become in appearance an integral part of existing vegetation [24].

replacement-severity fire:
A fire that causes >75% kill or top-kill of the upper canopy layer (>80%, according to Smith's [124] definition of stand-replacing fire), reverting vegetation to an earlier successional stage. Can be applied to all vegetation formations (forests, woodlands, shrublands, and grasslands). Replacement-severity fire may kill or only top-kill the plants in the upper canopy layer. LANDFIRE documentation states explicitly that replacement of the upper canopy can occur in any vegetation formation: "replacement fire in grassland removes the leaves, but leaves sprout from the basal crown, whereas replacement fire in most conifers causes mortality of the plant" [9]. See stand-replacement fire and fire severity.

restoration:
Restoration of biophysical capacity by returning sites to previous, desired conditions [40].

rhizoid:
In mosses, liverworts, and ferns, a thread-like growth, simple or branched, that provides anchorage and water and nutrient absorption for the gametophyte [5,27,55].

rhizome:
A horizontal underground stem with of a series of nodes that commonly produce roots [20,49].

root collar:
See root crown.

root crown:
The point at which the root and stem of a plant meet and the primary vascular anatomy changes from that of a stem to that of a root. Transition point between stem and root. It may be clearly or vaguely apparent [20].

root graft:
1) The natural growing together or joining of the roots of nearby plants.
2) Horticulture: the process of grafting a shoot or stem of one plant onto the root section of another [109].

root sprout:
A stem shoot arising from a shoot-forming bud on the root [134].

root sucker:
A root sprout [134].

rootstock:
1) Roots and/or rhizomes capable of sprouting.
2) A stock for grafting consisting of a root or a piece of root [134].

savanna:
A formation in which the overstory is dominated by woody vegetation growing as scattered individuals or clusters. The dominant life form may be trees (i.e., tree savanna; dominants are >6.5 feet (2 m) tall) or shrubs (i.e., shrub savanna; dominants are ≤6.5 feet (2 m) tall) [114]. The understory is composed of grass; shrubs are sparse to absent [115,122]. When describing savannas, use of a hyphen indicates about equal cover of dominant overstory and understory species (e.g., Oregon white oak-California brome savanna). Compare with grassland, shrubland woodland, wooded shrubland, forest.

scarification:
1) Heat (usually fire), mechanical, chemical, or moisture treatment of seeds to make the seed coat permeable, overcome dormancy, and improve germination.
2) Mechanical removal of vegetation or debris, or disturbance of the soil surface, to facilitate reforestation [53].

scatter-hoard:
Seed cached in scattered shallow holes, a common caching behavior for some rodents [124].

schizocarp:
A dry, indehiscent fruit that splits into separate one-seeded segments at maturity [49].

sclerophyllous:
Having tough, leathery, usually evergreen leaves [55].

scoria:
Vesicular, cindery dark lava formed by the escape and expansion of gasses in basaltic and andesite magma; generally denser and darker than pumice [100].

secondary colonizer:
A plant species that establishes from seed after early succession; establishment may be from soil-stored seed, seed dispersed from initially colonizing plants, or seed dispersed from off-site sources [131].

seed:
The discrete body from which a new plant develops. Formed from a fertilized ovule, the seed is comprised of an outer seed coat (testa) that encloses a food store and an embryo plant. The food may be stored in the cotyledons of the embryo itself or around the embryo in the endosperm [5] (see fruit for a diagram).

seed tree:
1) A tree left standing for providing seed.
2) A method of natural regeneration [53].

selection:
Used in animal ecology to indicate use of a resource in higher proportions than its availability [60,116].

sere:
A succession of plant communities leading to a particular plant association [124].

series:
In the vegetation classification hierarchy, the level naming the dominant overstory species (e.g., red fir series, Jeffrey pine-white fir series, or low sagebrush series) [57,104]. The "series" category does not imply greater heterogeneity than the plant association [104]. Also see formation, subformation, and association [57].

serotinous:
Pertaining to fruit or cones that remain on a tree without opening for one or more years. In some species (e.g., lodgepole pine), cones open and seeds are shed when heat is provided by fire or hot, dry conditions [53].

serpentine soils:
Soils weathered from parent materials rich in ferromagnesian hydrous silicate rock-forming minerals that typically have high levels of magnesium, iron, nickel, and chromium and low levels of calcium, molybdenum, and other essential nutrients [67,100,140]. Many plant species are intolerant of serpentine soils, and vegetation is usually sparse or stunted and characterized by the presence of many endemic spceies [5,140].

severity:
See fire severity.

short-term effects:
Effects lasting less than 10 years (personal communication 1998 October 21 with Wendell Hann, Fire Ecologist and assistant to National Fuels Specialist, USDA Forest Service).

shrubland:
A formation in which the overstory is dominated by shrubs 1.6 to 16 feet (0.5-5 m) tall, and trees have <5% total cover [75]. Compare with grassland, woodland, wooded shrubland, savanna, forest.

silique:
A dry, dehiscent fruit of the Brassicaceae with 2 valves that separate at maturity; fruit is typically less than twice as long as wide [49].

sink habitat:
A site that does not support self-sustaining populations and relies on immigration from populations from source habitats [39].

sinker root:
1) a root other than a taproot that grows straight downward
2) a secondary root from the cortical system in some semiparasitic plants (e.g., Viscum and Loranthus) that grows directly downward into the tissues of the host [53].

snag:
A standing dead tree from which the leaves and some of the branches have fallen [124].

sobol:
A shoot arising from underground stem tissue [49].

soboliferous:
Having several loosely clumped, principal stems that arise from a common underground stem system, each distinct above ground level [56].

softwood:
1) Gymnosperm or conifer trees.
2) The xylem of conifer trees [53].

soil burn severity:
The effect of a fire on ground surface characteristics, described in terms of char depth, organic matter loss, altered color and structure of soil, and reduced infiltration [101,120]. Also see fire severity and vegetation burn severity.

soil moisture regimes:
Soil moisture regimes are defined in terms of ground water level and seasonal presence or absence of water held at a tension of less than 15 bars (1500 kPa) in the moisture control section [136], as follows:

• Aquic: A reducing regime in which the soil is nearly free of dissolved oxygen due to saturation by groundwater during periods when the soil temperature at 20 inches (50 cm) is >41 °F (5 °C).
• Udic: Soil is neither dry for as long as 90 cumulative days nor for as long as 60 consecutive days in the 90 days following summer solstice at periods when the soil temperature at 20 inches (50 cm) is >41 °F (5 °C).
• Ustic: A limited amount of water is available for plants but occurs at times when the soil temperature is optimum for plant growth.
• Xeric: A limited amount of water is present but does not occur at optimum periods for plant growth; common in mediterranean climates with moist, cool winters and warm, dry summers.
• Aridic: No water is available for plants for more than half the cumulative time that the soil temperature at 20 inches (50 cm) is >41 °F (5 °C), and no period as long as 90 consecutive days when water is available for plants while soil temperature is continuously >46 °F (8 °C) [127,136].

• Pergelic: Mean annual soil temperature is <32 °F (0 °C). Permafrost is present in moist soils; dry frost is present in dry soils [127].
• Cryic: Mean annual soil temperature is >32 °F (0 °C) but <46 °F (8 °C), and summer temperatures are cold.
• Frigid: Mean annual soil temperature is >32 °F (0 °C) but <46 °F (8 °C), and summer temperatures are warm.
• Mesic: Mean annual soil temperature is ≥46 °F (8 °C) but <59 °F (15 °C).
• Thermic: Mean annual soil temperature is ≥59 °F (15 °C) but <72 °F (22 °C).
• Hyperthermic Mean annual soil temperature is ≥72 °F (22 °C) [127,136].

succession:
The gradual, somewhat predictable process of community change and replacement leading toward a climax community; the process of continuous colonization and extinction of populations at a particular site [124].

suffrutescent:
Woody only at the stem base or root crown [55].

surface fire:
Fire that burns in litter and other fuels at or near the surface of the ground, mostly by flaming combustion [20]. Usually refers to fire behavior rather than fire effects. However, the Rapid Assessment phase of the LANDFIRE program [74] used "surface severity" to mean low severity; LANDFIRE no longer does so [9].

surface root system:
That part of a tree's root system that lies within approximately 10 inches (30 cm) of the soil surface (review by [134]).

swale:
A slight depression, sometimes swampy, in the midst of generally level land [100].

swamp:
A wetland with seasonal water-level fluctuations and relatively strong water flows influenced by minerotrophic groundwater; sometimes inundated and characteristically dominated by trees or shrubs; may have a mineral, organic, or peat substrate. Swamps may be fresh- or saltwater and tidal or nontidal. Not as wet as marshes, fens, and bogs [95,141].

taiga:
The northern coniferous forest biome adjacent to arctic tundra [79].

thalweg:
The line of maximum depth in a stream. The thalweg is the part that has the maximum velocity and causes cutbanks and channel migration [91].

thermokarst:
An irregular land surface resulting from the melting of excess ground ice and subsequent thaw settlement [50].

therophyte:
Annual plant, with perennating tissue contained only in the seed [110].

tiller:
A basal or subterranean lateral shoot; common in bunchgrasses (Poaceae) and other monocotyledons [5,49,56].

tolerance model of succession:
An intermediate between the facilitation model of succession and the inhibition model of succession, in which modifications made to the environment by earlier colonists neither increase or decrease rates of recruitment and growth to maturity of later colonists [28].

top-cover:
The proportion of ground for which a species provides the uppermost cover; cannot exceed 100% [45]. Differs from cover.

top-kill:
To kill aboveground plant tissues without killing underground parts from which the plant can produce new stems and leaves [124].

torpor:
A state of reduced activity and metabolism in which organisms can save energy; not as deep a reduction in metabolic activity as hibernation [39].

total heat release:
The heat released by combustion during burnout of all fuels, expressed in BTU/foot² or Kcal/meter² [20].

tree:
The U.S. Forest Service's Forest Inventory and Analysis (FIA) program defines a tree as any perennial woody plant species with central stems and distinct crowns that can attain a height of 15 feet (4.6 m) at maturity [30].

tree-length harvesting:
Removing the merchantable portion of a tree to the roadside [97].

trichasium:
A cyme with 3 branches [49].

trichome:
A hairlike outgrowth of the epidermis [49].

tundra:
The zone of treeless, low arctic vegetation between taiga to the south and the arctic region of perpetual snow and ice to the north [53].

tussock:
A clump or tuft, especially of a graminoid [55].

type conversion:
A change in vegetation type due to unnatural disturbances. This term is usually applied when there has been a fundamental change in vegetation types (e.g., from shrubland to annual grassland) [31,65].

ultramafic soils:
Soils derived from ferromagnesian rocks such as serpentinite, dunite, and peridotite (which contain >70% mafic minerals such as olivine and pyroxene) and have high levels of nickel, chromium, and other heavy metals and low levels of calcium and other essential nutirents [67,100,147].

umbel:
A determinate or indeterminate flat-topped or convex inflorescence with the pedicels arising at a common point. A compound umbel is branched [106].

umbellet:
The secondary umbel in a compound umbel [106].

underburn:
See understory fire.

understory fire:
A fire in forests or woodlands that is not lethal to the dominant, overstory vegetation and thus does not change stand structure substantially [19,124]. Most of the dominant vegetation survives (>80% according to Smith [124], >75% according to Barrett et al.'s [9] definition of low-severity fire). Similar to low-severity fire, but application limited to forests and woodlands. See fire severity.

understory fire regime:
General pattern in which most fires are understory fires. Applied mostly to forest and woodland vegetation types [19,124].

understory reinitiation stage:
Third stage of forest stand development following major disturbance, as described by Oliver [96]. In this stage, brush and advanced regeneration reinvade the understory (following the stem exclusion stage) as the overstory becomes very mature. Also see stand initiation stage and old-growth stage.

upper canopy layer:
The uppermost layer of vegetation in a plant community. In a forest, the layer formed by the crowns of the tallest trees. As used by LANDFIRE, synonymous with dominant overstory vegetation and applied to all plant communities, not just forests and woodlands [9].

use of wildland fire:
Management of either wildfire or prescribed fire to meet resource objectives specified in land or resource management plans [93].

utricle:
A small, thin-walled, one-seeded, bladder-like fruit [49].

variety:
1) A category in the taxonomic hierarchy of botanical classification below the species and subspecies levels [49,79].
2) A rank used to designate a group of plants varying in flower color, habit, or some other way [55].

vegetation burn severity:
The effect of a fire on vegetation, often described by the degree of scorch, consumption, and mortality of vegetation [101,120] and the projected or "ultimate vegetative recovery" [101]. Depending on measurement methods, vegetation burn severity may or may not include mortality of belowground plant tissues. See also fire severity and soil burn severity.

vernalization:
A process of thermal induction in plants, in which growth and flowering are promoted by exposure to low temperatures [79].

warm-season:
A plant that makes most of its growth during spring and summer and sets seed in late summer or early fall [44]. It is normally dormant in winter. Also see cool-season.

wildfire:
An unplanned ignition caused by lightning, volcanoes, or unauthorized or accidental human actions. Prescribed fires are not considered wildfires unless they exceed prescription or are reclassified for some other reason as "escaped" (see escaped prescribed fire) [93].

wildland fire:
A nonstructural fire that occurs in wildland vegetation and/or natural fuels. Includes both wildfire and prescribed fire [93].

wildland fire for resource benefit:
A wildland fire used to accomplish specific resource objectives [123]. This term was not in official use as of 2010 [93]. See use of wildland fire.

wildland fire use:
This term was not in official use as of 2010. See use of wildland fire.

wildland-urban interface (WUI):
An area where structures and other human developments meet or intermingle with undeveloped wildland or vegetative fuels [93].

wooded shrubland:
A formation dominated by woody species, in which the proportion of shrubs is generally greater than the proportion of trees [114]. Grass and bare ground are usually present but not required for defining this formation. Compare with grassland, shrubland, wooded shrubland, savanna, forest.

woodland:
A formation in which tree cover is greater than shrub cover, but the canopy is not closed [114]. Generally, overstory trees are >16 feet (5 m) tall and have 25% to 60% cover [75]. When describing woodlands, use of a dash indicates an overstory/understory relationship (e.g., bur oak/big bluestem woodland). Compare with grassland, shrubland, wooded shrubland, savanna, forest.

xenogamy:
Cross fertilization [79].

xeric:
1) Having very little moisture; tolerating or adapted to dry conditions [124].
2) One of five soil moisture regimes.

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