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After the FiresThe Ecology of Change in Yellowstone National Park$

Linda L. Wallace

Print publication date: 2004

Print ISBN-13: 9780300100488

Published to Yale Scholarship Online: October 2013

DOI: 10.12987/yale/9780300100488.001.0001

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Elk Biology and Ecology Before and After the Yellowstone Fires of 1988

Elk Biology and Ecology Before and After the Yellowstone Fires of 1988

Chapter:
(p.117) Chapter 6 Elk Biology and Ecology Before and After the Yellowstone Fires of 1988
Source:
After the Fires
Author(s):

Francis J. Singer

Michael B. Coughenour

Jack E. Norland

Publisher:
Yale University Press
DOI:10.12987/yale/9780300100488.003.0006

Abstract and Keywords

This chapter describes Elk biology and ecology before and after the Yellowstone fires of 1988. Elk occupy an intermediate position in post-fire vegetal succession. The fires of 1988 in Yellowstone National Park provided a unique opportunity to study the effects of large-scale wildfire on one of the largest migratory Elk populations in North America. Several large, hot, fall fires burned about one-third of the 590,000 ha summer range and about one-quarter of the 134,000 ha winter range for Elk. It was predicted that the 1988 fires would alter elk distribution and habitat preferences, due to these changes in forages and forest cover. Elk would possibly avoid burned forests in the winter because these burned forests accumulate more snow than unburned forests and the snow will be more crusted from wind and diurnal thawing.

Keywords:   Yellowstone National Park, Elk biology, vegetal succession, large-scale wildfire

Elk occupy an intermediate position in postfire vegetal succession. Neither recent burns nor unburned continuous forests support the highest density of elk (Lyon 1966, Leege 1969, Martinka 1976). Martinka (1976) concluded that the highest densities of elk were found in a complex of multiaged conifer stands used for thermal and predator avoidance, intermixed across the landscape with previously burned bunchgrass and seral shrub communities that were used primarily for feeding. The fires of 1988 in Yellowstone National Park (YNP) provided a unique opportunity to study the effects of large-scale wildfire on one of the largest migratory elk populations in North America. Several large, hot, fall fires burned about one-third of the 590,000 ha summer range and about one-quarter of the 134,000 ha winter range for elk (Singer and others 1989).

Fire may benefit elk through increased forage quality (DeWitt and Derby 1955, Hobbs and Spowart 1984, Leege 1969, Rowland and others 1983, Chapter 5), increased forage quantity (West and Hassan 1985, Chapter 5), earlier green-up and later plant senescence (Daubenmire 1968, Peet and others 1975, Skovlin and others 1983, Hobbs and Spowart 1984), and increased foraging efficiency (Canon and others 1987). Elk strongly prefer (p.118) forages from previously burned sites (Leege 1969, Nelson 1976, Davis 1977, Roppe and Hein 1978). Most of these prior studies of elk-fire relations were of relatively small, prescribed burns (tens of ha), and most were spring burns (Hobbs and Spowart 1984, Canon and others 1987). Other than Turner and others (1994) and Martinka (1976), few studies documented responses of elk to large-scale wildfires ignited during the more typical lightning season in later summer or autumn.

The generalization that fire benefits elk (Peek 1980) is supported by some but not all empirical studies. For example, herbaceous protein concentrations (and thus nitrogen concentrations) are sometimes enhanced following fire (Rowland and others 1983, Seip and Bunnell 1985, Singer and Mack 1993, Pearson and others 1995, Singer and Harter 1996) and sometimes are increased by only a few percentage points (Bendel 1974, Lyon and Stickney 1976, Hobbs and Spowart 1984). Protein enhancement may also disappear within only one to two years postfire (Dills 1970, Lloyd 1971, Lyon and Stickney 1976, Wood 1988).

The purpose of our investigation was to document the effects of the fires on elk habitat, survival, productivity, and dietary responses. We predicted that elk cow:calf ratios, recruitment, and elk population size would increase following the fires (Leege 1969, Christensen and others 1989, Boyce and Merrill 1991, Singer and others 1989, Chapters 13, 14) although increased mortality might occur immediately after the fires. We also predicted forage quality (in other words, concentrations of nitrogen, macronutrients, and forage digestibility) would increase in burned nonwoody plants (DeWitt and Derby 1955, Daubenmire 1968, Old 1969, Dills 1970, Lloyd 1971, Grelen and Whitaker 1973, Wilms and others 1981, Umoh and others 1982, Rowland and others 1983, Ohr and Bragg 1985, Chapter 5). We predicted that the 1988 fires would alter elk distribution and habitat preferences, due to these changes in forages and forest cover. For example, elk should avoid burned forests in the winter because these burned forests accumulate more snow than unburned forests and the snow will be more crusted from wind and diurnal thawing (Chapter 13). Thermal cover in burned forests will be reduced resulting in greater heat loss by elk (Meiman 1968, Jones 1974, Davis 1977). Burned grasslands may green up earlier in the spring than unburned grasslands due to blackened surfaces (Daubenmire 1968, Hobbs and Spowart 1984).

Methods

Herbaceous Biomass Responses to the Fires

We estimated herbaceous biomass (in other words, all nonwoody growth) in paired burned and unburned key winter range plant communities on the northern (p.119) range of YNP (Fig. 1.2). The winter range communities sampled included big sagebrush, Douglas fir, and dry bunchgrass. We separated our sampling of burned Douglas fir into those stands burned by cool fires (mostly understory) and those stands burned by hotter soil heating (canopy fires, residual burning of larger down fuels), following the soil heating mapping of Despain and others (1989). Two summer range communities were also sampled: lodgepole forest and wet meadow. In the grassland and shrub communities, we sampled fifteen randomly located 0.5 m2 (circular) plots in both burned and unburned sites. Plots were sampled for peak herbaceous biomass in 1990 and 1991 using the point intercept method and regressions of biomass to hits established for the area by Frank and McNaughton (1990). We separated live from dead material. In forested communities, we clipped all herbaceous vegetation in 0.5 m2 plots to ground level, then dried and weighed it. We clipped vegetation in the forest since the point intercept method was not extensively tested there (Frank and McNaughton 1990). At each random point, the placement of the plot frame was chosen by randomly selecting a direction and number of steps from the point. The sampling design included a two-stage stratified random sampling with four replicate burned/unburned sites for each vegetation type (primary units) and fifteen replicate biomass plots (secondary units in each burned treatment). Plots at each study site and burn treatment were compared with a blocked ANOVA (blocked by site).

Effects of the Fire on Plant Nutrient Concentration

Nutrient concentrations of samples of the live biomass of the three most common winter range grasses(Pseudoroegnaria spicata, Festuca idahoensis, and Koeleria cristata [bluebunch wheatgrass, Idaho fescue, and Junegrass, respectively]) were sampled in unburned and burned grassland (n = 12 bunchgrass clumps/species/year) in the fall season just prior to elk arriving onto the winter range in 1989 and 1990. Nitrogen concentration, fat fiber, Van Soest fiber, total ash, gross energy, Ca, P, K, Mn, and Mg concentrations were analyzed (Rangeland Ecosystem Science Department, Nutritional Analysis Laboratory, Colorado State University) according to the Association Office of Analytical Chemists (AOAC 1970). In-vitro dry matter digestibility (DMD, hereafter referred to as digestibility) was determined by the method of Tilley and Terry (1963) as modified by Pearson (1970). We additionally collected midwinter samples for forage quality (percent N and digestibility) in elk feeding craters (see also Chapter 13) in burned and unburned sites (Norland and others 1996) to document forage (p.120) quality of what animals were actually feeding on. We also sampled summer range forages from the Central Plateau of the park (lodgepole, wet meadow types) in midsummer to coincide with the time elk were using these forages.

Samples of burned bark from aspen and lodgepole pine and burned needles from Engelmann spruce (Picea engelmannii), lodgepole pine, and Douglas fir (Pseudotsuga menziezii) were collected from trees where elk were observed feeding on them. Similar samples from unburned trees were collected for comparison from as near as possible to the burned trees. Mean nutrients were compared with the t-test (Proc t-test, SAS), except for nutrient concentrations of spruce tree needles which were not normally distributed. Percent N was transformed with x/log(x), and digestibility was transformed with log(log(x)) for spruce.

Elk Diet Composition Before and After the Fires

Winter elk diets were determined by fecal analysis. Fresh fecal samples were collected at the same approximate fifteen locations from within the park during each of the six consecutive winters from 1985–1986 through 1990–1991, including three prefire winters and three postfire winters. Each sample was a composite of 5 g of fresh fecal material from 10–12-pellet groups. Botanical composition of each sample was expressed as a percentage of relative cover of identifiable plant fragments in two hundred random microscope fields (Washington State Univ., Wildlife Habitat Lab, Pullman). Elk diet samples were compared among years with a nonparametric test (SAS 1990) because neither the raw data nor a log transformation was normal. Only forage species that comprised 〉 5 percent of the elk diets in any single year were included in the analysis. Each plant species was analyzed separately. Tukey's procedure was used for all pairwise comparisons (p ≤ 0.05).

Elk Population After the Fires

Aerial counts of elk were made from fixed-wing aircraft (Super Cub) and helicopter surveys during the winters of 1986–1993. During each fixed-wing survey, the northern elk range (about 1,340 km2) was surveyed both inside and outside the park boundary. To facilitate counting, the entire winter range area was divided into sixty-six count units of 20.3 km2 ± 1.3 km2 (mean ± SE, range = 4−45km2). The boundaries of the count units consisted of rivers, creeks, and other readily recognizable topographic features. Four different aircraft teams (pilot and observer) surveyed about one-quarter each of the northern range during (p.121) each survey (approx. seventeen count units per team). Aircraft teams always surveyed the entire area in one day. Each count unit was surveyed in elevation contours approximately 0.5 km apart.

To evaluate the efficiency of the fixed-wing surveys, we fitted forty-seven elk (thirty-four adult cows, twelve adult bulls, and one yearling bull) with white radiocollars (Samuel and others 1987). Radiocollared elk missed during the survey were located by a fifth aircraft, and the sighting conditions (pilot/observer experience, elk density strata [high, medium, low], location in and out of the park, snow and forest cover variables) were recorded.

The dichotomous classification of elk groups seen or missed was treated as the dependent variable in a logistic regression analysis (Samuel and others 1987, Singer and others 1989, Singer and Garton 1994). A logistic regression model following Samuel and others (1987) was used for predicting visibility y, where visibility probability was:

(1)
y=eu1+eu

where u was the equation describing sightability and e was the base of the natural log. Elk population estimates were then corrected for each count unit by logistic regression of visibility factors found to be significant (density strata, location in or out of the park, snow and forest cover variables). Stepwise logistic regression suggested that visibility for elk groups was best explained by group size, vegetation cover, and activity according to:

(2)
u=0.97+0.37G0.54C1.70A

where G = group size, C = percent conifer canopy cover, and A = elk activity (Singer and Garton 1994, Unsworth and others 1994). Percent conifer canopy cover was defined as the percentage of cover that could obscure an elk over the area the group observed as described in the technique manual (Unsworth and others 1994).

Habitat Selection by Elk

We sampled elk habitat selection during ten fixed-wing surveys of the entire northern winter range conducted between December 1986 and March 1992 (one to three surveys per winter). The locations of all elk groups observed were plotted on a 1:62,500 topographic map, and group size, percent forest canopy, percent snow cover, and elk behavior (standing, bedded, moving) were recorded. Forest canopy that would obscure an elk from observation and snow cover were (p.122) estimated visually for the area connecting the outermost elk in the group following Samuel and others (1987) and Unsworth and others (1994).

The aerial observations also sampled winter feeding habitats. Nearly all elk groups were feeding or bedded near feeding sites at the time of the surveys. We observed little tendency for elk within the park to move from feeding sites into tree cover until afternoon. Therefore, most surveys were conducted in the morning (before noon local time). We later generated elevation, slope, aspect, vegetative cover type, habitat type, and burn category (burned, unburned, mosaic burn) (Mattson and Despain 1984, Despain and others 1989) using the park's GIS. Availability of each habitat category was also generated from the GIS. We also analyzed habitat use from the helicopter surveys in order to document any differential habitat and burn area preference by elk sex and age class.

Confidence intervals on proportional elk use of all habitat and burn categories were compared to availability using Bonferroni confidence intervals (Neu and others 1974, Miller 1981, Byers and others 1984). The terms selected, avoided, and expected imply elk habitat use greater than (+), less than (−), or equal to (0) availability at p 〈 0.10 level, respectively. We conducted habitat preference tests on groups because individuals within a group were not independent, which would favor Type I errors (White and Garrott 1990). Ten habitat types recognized on the northern range were aggregated into four types for purposes of the preference test because each type must contain less than 5 percent of the total elk groups (Neu and others 1974). The five categories were: lodgepole pine forests, Douglas fir forests, other forests, sagebrush/grasslands, and wet grasslands dominated by Deschampsia and Carex spp. Differences in selection of habitat and burn categories amongst years and between sexes of elk were tested with a log likelihood chi-square procedure (SAS 1990). Fisher exact tests are reported for comparisons with small sample sizes (〈 10).

Results

Fire Effects on Herbaceous Biomass

In winter range communities, burning increased forb biomass in sagebrush the first growing season postfire (1989) and in Douglas fir forest and bunchgrass communities the second growing season (1990) after the fire (Fig. 6.1). Grass biomass was reduced by burning both years in big sagebrush communities, but grass biomass increased in the bunchgrass type. Considerably less biomass was (p.123)

Elk Biology and Ecology Before and After the Yellowstone Fires of 1988

Figure 6.1. Aboveground dry matter plant biomass in unburned and burned (a) winter and (b) summer range plant communities in 1989 and 1990 on Yellowstone's northern elk winter range. Lodgepole 3 = a mature lodgepole forest and other communities as described in Mattson and Despain (1984). Differences between burned and unburned sites were tested with ANOVA * = p 〈 0.05.

produced in burned lodgepole pine forests on summer range the first and second postfire growing season, except for biomass of grasses and one forb, fireweed (Epilobium augustifolium), which were increased by burning (p 〈 0.05, Fig. 6.1a). Grass biomass production increased about 20 percent on burned wet meadows on summer range both the first and second postfire growing seasons. Other than fireweed, forb biomass in wet meadows was not influenced by burning (Fig. 6.1b).

(p.124) Effects of the Fires on Snow Depth and Density

Snow depths did not differ between burned and unburned sagebrush or Douglas fir stands in either early or late winter (p 〉 0.05). But snow was 5 percent denser in burned compared with unburned sagebrush communities late during the second postfire winter (1989–1990), while snow was 6–7 percent denser early in both the second and third postfire winters in burned versus unburned (p 〈 0.05, Norland and others 1996). These minor differences in snow density did not affect elk feeding, since the density of elk feeding craters did not differ between either burned and unburned sagebrush or Douglas fir stands (p 〈 0.05, Norland and others 1996). But the differences in snowpack in burned lodgepole pine stands on the northern range where canopy burns were more typical in 1988 were apparently substantial enough to cause an avoidance of burned compared with unburned lodgepole pine stands (Singer and Harter 1996).

Effects of the Fires on Forage Quality in Elk Winter Range

Forage quality was generally higher on burned sites, in both the first and second postfire winter. Nitrogen concentrations of the three most common grass species averaged 32 percent higher in burned versus unburned habitats both winters (p 〈 0.05). Dry matter digestibility (DMD) was higher for all three species the first postfire growing season (1989), but only Idaho fescue possessed higher DMD during the second postfire growing season (1990).

Macronutrients were not higher on burned sites the first and second growing seasons following the fires. Percent K was reduced in Idaho fescue during the first growing season and percent Mn was reduced in Idaho fescue and Junegrass during the second growing season. We found no fire effects on Ca, P, or Mg. However, fibrous constituents (percent cellulose, lignin, fiber, and ash) were higher in forages on burned sites by the second growing season.

Effects of the Fires on Forage Quality in Elk Summer Range

Nitrogen concentration was higher on burned sites for all forages in lodgepole pine forests on summer range the first summer postfire (p 〈 0.05). No differences in nitrogen concentration were noted for forages in burned and unburned wet meadow communities. Differences in nitrogen concentration had completely (p.125) disappeared by the third summer postfire. Very few differences were noted in digestibility with increases found in burned fireweed and other forbs in the lodgepole pine forests the first summer postfire. Again, all differences had disappeared by the third postfire summer (1991).

Effects of the Fires on Burned Bark Forages

Burned aspen bark had a lower nitrogen concentration (p = 0.05) and digestibility (p = 0.07) than unburned bark. Nitrogen concentration was not higher in burned Douglas fir needles (p = 0.98), but digestibility was marginally higher (p = 0.07) in the first postfire winter. Digestibility was lower in burned aspen bark (p = 0.04).

Elk Mortality During the Fires

A total of 243 elk were found dead from the fires within the park and another 84 elk within burned portions of the remainder of the Greater Yellowstone Area outside of the park. This represented about 0.7 percent of the elk present in the area at the time of the fires, based upon population estimates for eight elk herds that used the park each summer (Singer 1991).

Elk Mortality the First Winter Following the Fires

A total of 1,004 elk carcasses were counted during April of 1989 on the random sample (22 of 66) count units. Based on this sample, we estimated 3,021–5,757 elk (95 percent confidence interval) died over the entire winter range during the winter of 1988–1989. These elk died primarily from malnutrition (DelGiudice and Singer 1996, DelGiudice and others 1991). Another 2,773 elk were harvested outside of the park. The elk population was estimated to be 23, 237 ± 834 at the beginning of the winter. Thus an estimated 24–37 percent of the northern Yellowstone elk population was lost during the first postfire winter due to the combination of winter malnutrition and harvest (Fig. 6.2).

Survival of adult cows and bulls averaged 0.91-1.00 prior to the fires (Fig. 6.2). Fifty-seven percent of all adult cow mortalities and 88 percent of all bull mortalities that were recorded during four years of monitoring occurred during the winter of 1988–1989. Adult bull survival was only half (0.46) the survival of adult cows (0.84) during the first postfire year. Adult cow survival rate did not vary statistically among years (p 〉 0.05), although numerically cow survival dropped to 0.87 during the first postfire winter (1988–1989). Adult bull survival (p.126)

Elk Biology and Ecology Before and After the Yellowstone Fires of 1988

Figure 6.2. Annual survival rates (±SE) for adult cow (n = 34), adult bull (n = 13), and calf elk (n = 127) before and after the fires of 1988 in Yellowstone National Park. An asterisk denotes statistically significant difference, p 〈 0.05, between years within a sex/age class using Chi-square tests.

(p.127) was significantly lower over the winter of 1988–1989 (0.46) compared with the three other winters (p 〈 0.05). Calf survival was also lowest the first postfire winter (0.16) compared with the other three winters (0.51) (p 〈 0.05). Winter undernutrition or closely related causes (DelGiudice and others 1991) accounted for most of the adult elk mortalities during four years (87 percent of all adult mortalities). Winter malnutrition was the second leading cause of mortality for radiocollared calves during four years (22.7 percent of all calf mortalities). The fires and climatic events of 1988–1989 reduced survival of elk calves. Calves were born lighter and later in 1989 the first spring following the fires (Singer and others 1997).

Elk Diets Before and after the Fires

Elk consumption of all grasses other than Calamagrostis rubescens declined dramatically the first winter following the fires, but steadily increased the second and third winters (Fig. 6.3, p 〈 0.0001). Consumption of Carex and Juncus in-creased the first and second postfire winters (p = 0.001).

Winter severity was high the first winter after the fire. Snow densities were higher and extensively crusted due to high winds and severe cold (the winter was rated severe at −2.5 on a scale of +4 = mildest and −4 = most severe (Farnes 1996). The crusted snows and burned herbaceous forages forced elk to eat forage which protruded above the snow, such as the tall big sagebrush, tree bark, tree needles, aspen bark, and mosses growing on trees. Consumption of Artemesia tridentata increased the winter after the fires (p = 0.006), but decreased to prefire levels in the second postfire winter (Fig. 6.3). Consumption ofpine bark and needles from both lodgepole pine and Douglas fir continued into the second winter (Fig. 6.3).

Elk Population Responses to the Fires

The northern Yellowstone elk population grew steadily following cessation of artificial controls from 1968 to 1988. In 1988, peak numbers Of 23, 240 ± 830 elk were estimated for the northern Yellowstone winter range (16–17 elk/km−2). High elk numbers prefire in early 1988 were associated with a series of mild winters (nine of the ten winters prior to 1988 had less than long-term-average precipitation [ Chapter 10]). During this time period, elk expanded their range and increased their use of areas north of the park. Both the mild winters and the range expansion (see also Coughenour and Singer 1995, Lemke and others 1998) resulted in a higher potential carrying capacity for elk, primarily due to the larger effective foraging area during the winter for elk (Chapter 13). During the first (p.128)

Elk Biology and Ecology Before and After the Yellowstone Fires of 1988

Figure 6.3 (facing page). Elk diets on Yellowstone's northern winter range three winters before and three winters after the fires of 1988. Different letters denote statistical differences in mean percent composition of diets between years using ANOVA for the species in each individual graph.

(p.129) postfire winter, we estimated that the elk herd declined by 24–37 percent. By 1994, the elk herd had recovered to prefire levels.

Elk movement patterns were also significantly altered by the fires and associated events of the 1988–1989 season. Elk migrated in large numbers from the park during the winter of 1988–1989 due to the combined effects of drought, burning of 22 percent of the winter range in the park, and deep winter snows in the park. Fifty-four percent of the elk population migrated and were counted north of the park by late winter, an area that comprised only 18 percent of the winter range area but where snows were not as deep and no fires had occurred. This was the third-largest migration out of the park since 1916 (Houston 1982). Some young elk even remained outside of the park during the following summer, as shown by three radiocollared yearling elk observed north of the park boundaries during the summer, a phenomenon never observed with marked elk prior to the fires.

Elk Habitat Selection

Radiocollared elk cows were quite mobile, migrating out of the park and away from burned areas. Bull elk were less migratory that winter, staying in the park and staying in burned areas where they consumed (1) the few sedges that sprouted on burned wet meadows following the fires (no fall green-up occurred in any other plant communities), (2) tree bark from felled, burned aspen trees, and (3) lodgepole pine bark and needles from the mosaic of burned, unburned, and partially burned trees. Apparently these were all submaintenance foods because bull elk died at a higher rate than cows the first winter.

Bull and cow groups differed not only in their use of burned areas (p = 0.013, but also in the aspect on which they were sighted (p 〈 0.001) and in the plant communities they frequented (p = 0.019). But there was no difference between the sexes in their use of slopes (p = 0.790). Bulls used northeast aspects more and northwest aspects less than cows (Fisher exact test, p = 0.036). Both sexes preferred moderate slopes (〈 21 percent), both pre- and postfire. Cows used dry grass and shrub habitats more and moist grass and unburned shrubs less than bull groups (Fisher exact test, p = 0.019, 0.015, respectively). Differential use of burned areas between genders was found only immediately postfire.

Elk use of burned areas was influenced by slope (p = 0.011), aspect (p = (p.130) 0.016), and elevation (p 〈 0.001). Elk were more likely to use burned areas with gentle slopes (0–6 percent), with southwest aspects, and on middle elevation areas of the winter range. Groups of elk seen during aerial counts (n = 3,831 groups) used southwest slopes more than expected, but no other consistent selection for aspects was demonstrated. Elk utilized forests (either burned or unburned) less than expected during all surveys (Table 6.1). Elk groups consistently used burned dry grassland/dry shrubs (the burned type was used 52 percent more than the unburned patches; selection index of observed/expected = 1.858 ± 0.107 versus 1.22 ± 0.120), and burned moist grasslands/moist shrublands greater than expected following the fires (Table 6.1).

Selection for burned moist grasslands/moist shrubland (selection index = 1.407 ± 0.052) was 18 percent greater than for unburned patches of the same type (index = 1.195 ± 0.138). Elk group sizes were larger in grassland than in forests, and elk groups were larger in unburned than in burned grasslands (p 〈 0.05).

Discussion

Effects of the Fires on Forage Production and Quality

The modest increase (20 percent) in forage production in dry grasslands and wet meadows that was attributable to the fires of 1988 was biologically important because these are two very important elk habitats. Small to no increases in production were observed in sagebrush, Douglas fir, and lodgepole pine forests. Part of the variation among and within habitats could be attributable to the different fire intensities experienced. Hotter fires and higher soil heating were typical in habitats that exhibited short-term declines in forage production (Despain and others 1989). Hotter fires in forest or tall shrub communities with high fuel loadings killed seeds and many plants (Despain and others 1989, Knight and Wallace 1989, Bartos and others 1994). Plant recovery in these types required several years. For example, recovery of plant biomass in burned summer range lodgepole forest required three growing seasons, similar to the three- to five-year span noted for forested communities in other areas (Leege 1969, Lyon 1971, Peek and others 1979, Merrill and others 1994, Brown and DeByle 1989, Bartos and others 1994, Chapters 4, 14).

The Yellowstone grasslands did not experience the large increase in biomass in response to fire seen in tallgrass prairie (Knapp and Seastedt 1986, Seastedt and others 1991) (p.131)

Table 6.1. Winter habitat preferences by elk in burned and unburned habitats before and after the fires of 1988, northern Yellowstone National Park, Wyoming

Use: Availability Ratioa

1 Cell

9 Cells

Habitat Category

% of Area in Habitat Category

1st Winter After Fires

2d and 3d Postfire Winters

1st Winter After Fires

2d and 3d Postfire Winters

Unburned in 1988 Lodgepole Pine/Subalpine Fir

22.63

0.438

0.389

0.451

0.367

Whitebark Pine/Douglas Fir

16.08

0.888

0.720

0.845

0.696

Sagebrush/Grassland

37.82

1.833+

1.463+

1.796+

1.492+

Wetlands

2.54

0.520

1.920+

0.560

1.800+

Burned in 1988 Lodgepole Pine/Subalpine Fir

6.30

0.048

0.079

0.063

0.111

Whitebark Pine/Douglas Fir

4.10

0.341

0.488

0.390

0.512

Sagebrush/Grassland

9.30

0.516

1.785+

0.503

1.710+

Wetlands

0.50

0.400

0.800

0.400

1.200+

Notes: Percentage of northern winter range covered by habitat types (availability), and the ratio of observed herd fraction to availability fraction on: (a) single 50 × 50-m or, (b) nine contiguous 50 × 50-m pixels sampled: the winter just after the fires (1989), and subsequent postfire winters (1990, 1991). Minus signs indicate that the pixel or pixel group was used less than expected by chance. Plus signs indicate usage more than expected by chance.

(a) The ratio of the percent of all group observations of elk (n = 3831 groups) to the percent of the total area in that habitat category. A “+” implies use greater than, “−” less than, no sign equal to availability of the habitat category with Bonferroni confidence intervals (Neu and others 1974, Miller 1981, Byers and others 1984).

and other more productive grassland systems (Moomaw 1957, Ribinski 1968, Wright and others 1979, West and Hassan 1985). The fires in Yellowstone's grasslands were cool and spotty because there was not the heavy litter and fuel accumulation seen in more productive grasslands. Typically, Yellowstone's grasslands have only 85 g/m−2 annual standing crop biomass compared (p.132) to several hundred grams per m−2 in more productive grassland ecosystems.

In contrast to our predictions and to other reports, spring regrowth did not occur earlier, nor was senescence later on burned grasslands (Old 1969, Peet and others 1975, Skovlin and others 1983, Hobbs and Spowart 1984, Seip and Bunnell 1985; however, see Chapter 5).

Percent nitrogen and digestibility were enhanced in four of the six winter forages that we sampled from the winter range. On the summer range, there were few differences in forage quality attributable to the fires. Percent nitrogen was enhanced in only five of the eighteen species studied (see Chapter 5). Forage quality improvement was most notable in YNP in those communities with higher fuel loadings (big sagebrush, lodgepole forest, Douglas fir forest), where fires burned hotter and longer, thus releasing more nutrients. A number of other authors have also found more release of nutrients where fuels are higher and fires are hotter (DeWitt and Derby 1955, Daubenmire 1968, Old 1969, DeBano 1991, Seastedt and others 1991, Chapters 4, 5).

Elk foraging efficiency and elk condition were likely enhanced following the fires:

  1. 1. Biomass and quality were increased in two important elk habitats and several key forage species.

  2. 2. Leaf sizes of forbs and willows doubled and shoots were approximately 1.5 times larger on burned sites (Norland and others 1996).

  3. 3. Elk fed more on sedges and rushes following the fires possibly because accumulated dead litter in these types was removed by the fires. Foraging was probably far more efficient in sedge and rush communities once the dead litter was removed by burning.

  4. 4. Bunchgrasses were taller and more erect on the northern range (Singer and Harter 1996), also likely enhancing foraging efficiency in winter.

Higher urea nitrogen:creatinine ratios (DelGiudice and Singer 1996) and higher elk: calf ratios indicated northern Yellowstone elk were in better condition by the second postfire winter. Elk foraging efficiency (Canon and others 1987) and improvements in elk body weight and condition were observed due to burning (Rowland and others 1983).

We observed increased fiber, cellulose, and lignin concentrations in dry grasslands, rather than decreased levels as shown by Wilms and others (1981) and Smith (1960). We also observed little difference in macronutrient concentrations (p.133) in these same forages in contrast to most studies that reported increased concentrations in regrowth following fires (Old 1969, Lloyd 1971, Wilms and others 1981, Umoh and others 1982, Ohr and Bragg 1985).

As predicted, elk consumed previously unpalatable forage species following burning. Burned bark and needles were consumed in substantial amounts during the first postfire winter. Burned Douglas fir had higher nitrogen and digestibility values, but nitrogen and digestibility in aspen were lower than in unburned forages, and these items were likely inadequate forages for elk. Radiocollared elk that were observed consuming burned aspen and conifer bark and conifer needles died that first winter. In contrast to our findings, Jakubas and others (1) found higher forage quality in burned lodgepole pine bark in the Madison-Firehole winter range of YNP. They reported doubled digestibility and protein contents in burned lodgepole bark. Their analyses were conducted four years following the fires of 1988 and in a deep snow environment where elk continued to consume bark. They looked only at lodgepole pine bark, a species we did not sample. In our study area, consumption of burned bark by elk, which included Douglas fir and spruce, was restricted entirely to the first winter after the fires. We did not observe consumption of burned bark in the following winters. Regrowth of sedges and rushes following burning was also consumed at higher rates postfire.

Elk Distribution and Habitat Use

Elk distributions and habitat use were greatly altered by the fires of 1988. More elk migrated north of the park each winter following the fires. Prior to the fires (1971–1987), migrations north of the park averaged 15 percent of the population (Houston 1982, Singer 1991). Following the fires, more than 50 percent of the herd migrated north of the park most winters (Lemke and others 1998). After the fires, 3,000–4,500 elk consistently spent the winters at the northernmost extremity of the northern Yellowstone elk winter range on the Dome Mountain Wildlife Management Area, where, prior to the fires, only 700 elk spent the winter (Lemke and others 1998).

Elk groups demonstrated a strong preference for burned over unburned dry and moist bunchgrass/shrublands areas from within the park. Elk groups in our study selected burned grassland habitats 52 percent more often than unburned grasslands, similar to the 40 percent greater preference of burned habitats that Pearson and others (1995) reported for the same area using different methods. Davis (1), Roppe and Hem (1978), and Canon and others (1987) also reported (p.134) a high preference by elk for burned habitats in other study areas, although Skovlin and others (1983) observed no preference by elk for burned grasslands in Oregon. We observed a decline in use of burned lodgepole forest, where total canopies were burned by fires, but equivalent use by elk of burned and unburned Douglas fir forests where canopies were largely or mostly intact. Surface fires tended to occur only in these forests.Davis (1977) also reported that elk preferred burned forests with standing dead timber to burned forests where no snags were left.

In spite of the short-term forage increases due to the fires within the park, more elk continued to migrate north of the park all winters since the fires. This area is substantially lower in elevation, snowpacks are lighter, and elk survival rates, as suggested by higher calf:cow ratios, were also substantially higher each winter (Coughenour and Singer 1995).

In burned forests, snow is deeper, snow cover duration is longer, and there is more crusting due to a combination of increased albedo, lower humidity, and greater wind velocity on burned areas (Lyon and Stickney 1976). Elk prefer closed-canopy forests in late winter, when snows become deep and dense in open areas (Bergen 1971, Beall 1976). Norland and others (1996) observed slightly deeper snows in burned forests in northern YNP. We observed slightly higher snow density on burned forest sites. These values (11 percent increase in snow density and 2.5 cm deeper snow) were apparently too minor to influence elk feeding crater densities in burned versus unburned Douglas fir forests (see Chapter 13). However, Singer and Harter (1996) reported that elk avoided burned lodgepole pine forests compared with unburned lodgepole pine forests.

Elk Population Responses to the Fires

Following immediate postfire declines, elk population subsequently increased rapidly, recovering to prefire levels by 1995, only five years postfire (YNP 1997, Lemke and others 1998). Summer survival of elk calves was low both the first and second years postfire (1989 and 1990), thus slowing the initial herd recovery. But higher calf:cow ratios were observed by 1991–1994 (YNP 1997, Lemke and others 1998). Subsequent elk populations have not continued to increase due to a combination of doubling in hunter harvests north of the park, more severe winters (Chapter 13) and possibly also the effects of wolf restoration in 1995 (YNP 1997, Lemke and others 1998, J. Mack, personal communication). The calculations of Singer and Mack (1999) predict that wolves would limit the northern Yellowstone elk population in conjunction with continued levels of antlerless harvest north of the park. All model scenarios with wolf predation predicted (p.135) declining elk populations if antlerless elk harvests are not reduced (Singer and Mack 1999), even with the improved short-term habitat conditions for elk due to the fires. This has apparently proven to be the case.

Acknowledgments

We wish to thank staff of Yellowstone National Park, Montana Department of Fish, Wildlife and Parks, U.S. Forest Service, numerous Student Conservation Association Volunteers, Sagebrush Aero, Inc., Gallatin Flying Service, Yellowstone Air Service, Wyoming Game and Fish, Yellowstone Grizzly Foundation, and Montana Wildlife Diagnostic Lab. Nutrient and digestibility tests were conducted by the Rangeland Ecosystem Lab, Colorado State University, and diet analyses by the Wildlife Habitat Lab, Washington State University. Suggestions from two anonymous reviewers are also gratefully acknowledged.

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