FIRE, ELK, AND ASPEN POPULATION DYNAMICS IN BANDELIER NATIONAL MONUMENT: RECOMMENDATIONS FOR POST-FIRE ASPEN SUSTAINABILITY STEPHEN M. FETTIG,* MATTHEW C. BOSSLER, KEVIN J. BUFFINGTON, M. ADAM ULLOM Resource Management Division, Bandelier National Monument, 15 Entrance Road, Los Alamos, NM 87544. Department of Biology, University of Wisconsin,- Eau Claire, Eau Claire, WI 54702 (KJB) Department of Conservation Biology, College of Santa Fe, Santa Fe, NM 87505 (MAU) *Correspondent: Stephen_fettig@nps.gov ABSTRACT—The influence of browsing by elk (Cervus elaphus) on aspen (Populus tremuloides) and other woody shrubs in Bandelier National Monument has been a controversial topic throughout the last decade. Using two seasons of observational and statistical data we examined the hypothesis that physical barriers such as fallen trees help to protect aspen and other woody shrubs from ungulate browse. In 2005, we explored randomly selected sites in our search for refuged trees. In 2006, we traversed the entire study area searching for definitively refuged trees. Our findings suggest that refuges of fallen trees with a height greater than 80 cm allowed aspen to escape ungulate browse and attain heights above 2 m. RESUMEN—Utilizando dos temporadas de datos empíricos y estadísticos, pusimos a prueva la hipótesis que barreras físicas tales como árboles caídos ayudan a proteger a Populus tremuloides y otros arbustos de la herbivoría de ungulados. Atravesamos los sitios de alta elevación de Cerro Grande en el Monumento Nacional Bandelier en busca de árboles protegidos por barreras. Nuestros hallazgos sugieren que árboles caídos con un diámetro mayor a 80 cm permiten que el Populus alcance alturas mayores a 2 m . Following the 1988 Yellowstone National Park fires, researchers observed that fallen conifer, which had been killed in the fire, provided “refugia” for regenerating aspen sprouts (Ripple and Larson, 2001). These “safe zones” of woody debris or “jackstraw” offered aspen ramets (individual members of a clone) protection from ungulate browse (Turner et al., 2003). The Ripple and Larsen study provoked us to question to if this phenomenon was present in Bandelier National Monument, where a similar regime of fire and heavy ungulate browse has been well documented (S.M. Fettig et al., 2005, unpublished report, Bandelier National Monument, Los Alamos, NM). The elk (Cervus elaphus) population in the Jemez Mountains of New Mexico has fluctuated throughout the last decade. Due to over hunting, elk were extirpated from the region by 1900. In 1948 and again from 1964 to 1965 elk were reintroduced to the area and their population grew exponentially (Allen, 1994). Data from the New Mexico Department of Game and Fish suggests that by the late 1980s or early 1990s, the Jemez Mountains population of reintroduced elk had grown to levels greater than were known since the year 1200 (Allen, 1994). A survey in 2002 estimated the Jemez Mountains elk population to be approximately 5,200 individuals (New Mexico Department of Game and
Fish, 2006). Elk numbers in and near Bandelier National Monument have also dramatically increased during the last 35 years. Heavy browsing by elk is apparent on several plant species throughout the park. (Wolters, 1996). Fieldwork since 2000 has examined the influence of browsing by elk on aspen (Populus tremuloides) and other woody shrubs within the park’s mixed conifer forests (S.M. Fettig et al., 2003, unpublished report, Bandelier National Monument, Los Alamos, NM). These studies have shown widespread browsing of regenerating aspen sprouts. In 2002, fieldwork on randomly selected high intensity burn plots, from the 2000 Cerro Grande fire, documented a clear decline in the density of regenerating aspen sprouts from an estimated 6,000 to 8,000 stem/ha to less than 300 stems/ha with the mean stem height less than 0.5 m. Such a low sprout density in burned areas, coupled with high levels of browse, implies aspen sprout survival is questionable. However, the full extent of the spatial impact on aspen and woody shrubs by elk browse is unknown (S.M. Fettig et al., 2005, in litt.). Our study explored the physical characteristics and landscape features required to protect aspen (Populus tremuloides) and other woody shrubs from ungulate browse. METHODS--Study Area--Our study took place in Bandelier National Monument located in the southeast portion of the Pajarito Plateau in the Jemez Mountains on the southern edge of the Rocky Mountains in north central New Mexico (Figure 1). Data Collection and Analysis--Field work extended over two summers. In our first year from June to August 2005, we investigated the influences of browsing by elk on aspen sprouts and woody shrub species. We randomly selected sites using the Alaska Pack extension to ArcView 3.3, Alaska Region 2005 (Environmental Systems Research Institute, Redlands, California). We located sites using a Garmin (Olathe, Kansas) GPS 12. Each located North American 1927 datum (NAD 27) coordinate pair (Easting and Northing) was used as the northeast corner of a 10 m x 10 m plot. For each plot, we recorded general site information including Easting, Northing, elevation, slope, aspect, dominant over-story species, percent canopy cover, terrain characteristics, and whether or not the site had burned in the recent past. We also counted the number of aspen trees and aspen sprouts in the plot. In addition to aspen (Populus tremuloides) sprouts, we searched for woody shrub species within each plot. These species included: wild raspberry (Rubus strigosus), chokecherry (Prunus virginiana), red elderberry (Sambucus microbotrys), serviceberry (Amelanchier utahensis), mountain maple (Acer glabrum), dogwood (Cornus stolonifera), thimbleberry (Rubus parviflorus), ocean spray (Holodiscus dumosus), mountain mahogany (Cerocarpus montanus), cliffbush (Jamesia Americana), ninebark (Physocarpus monogynus), barberry (Berberis fendleri), gambel’s oak (Quercus gambelii), currant (Ribes), wild rose (Rosa), whortleberry (Vaccinium myrtillus), mock orange (Philadelphus microphyllus), mountain lover (Paxistima nyrsinites), New Mexico locust (Robinia neomexicana), buckbrush (Ceanothus fendleri), snowberry (Synphoricarpos oreophilus), and shrubby cinquefoil (Potentilla fruticosa).
For each shrub species present within the plot, we recorded the maximum height of each species and noted whether it was unrefuged (unprotected from browsing) or refuged (physically protected from browsing, either completely or partly). Digital photographs of each plot documented overall site characteristics and specific situations such as clearly refuged plants. From June to August 2006 we continued investigating the influences of elk on aspen ramets and woody shrub species in Bandelier National Monument. We visually surveyed all areas above an elevation of 2000 m to identify refugia. Refugia consisted primarily of fallen trees, rocks, and other natural obstacles which inhibited ungulates from reaching the young trees. All measurements were conducted using a standard metric ruler, diameter at breast height (DBH) measuring tape, and a Hastings (Hastings, Michigan) telescoping measuring stick. We recorded the measurements of the refuged tree and its subsequent refuging agents. Multiple fallen trees acting as a single unit (one inside the other, or atop the other) were recorded as a single system of trees. Small sticks were defined as either branches of fallen trees or as sticks on the ground. We recorded the width and height of fallen trees, rocks, small sticks, system of trees, and vegetation creating the refuge. Standing trees were measured by diameter at breast height (DBH). Circumference of each refuging agent was estimated to the nearest percent. The mean height of nearby unrefuged plants of the same species as the refuged plant was measured for comparative analysis. The height of the refuged tree was recorded to the nearest decimeter. Physical and spatial characteristics were determined at each site. The Easting, Northing, and elevation were recorded. Slope and aspect were measured using a Silva (Finland) compass. Each site was visually characterized as burned, unburned, rocky, steep, thick with trees, open, grassy, many dead falls, or a combination thereof. Canopy cover was visually estimated. Digital photographs were taken of each site and its particular refuge features. The presence or absence of browse on the refuged tree was recorded. Refuged aspen sites were additionally characterized by individual ramet (<2.5 DBH) and tree (>=2.5 DBH) counts within a 10 m x 10 m plot surrounding the refuged tree. We used MS-Excel (Excel 2003) to run t-tests on mean heights of plants and refuging agents. XLISP-STAT (Cook and Weisberg 1994, Weisberg 1985) was used to run linear regression analysis and one way analysis of variance. In order to insure that the assumption of normality was not violated, we performed a square root transformation on the number of species per plot. A threshold level of α = 0.05 was used throughout our analysis. All tests with α < 0.05 were verified to not have violated the assumptions of normally disturbed residuals and constant variance. RESULTS—2005— Ninety-seven suitable sites were included in the analysis. Sites varied in elevation (2510 m to 3090 m, mean = 2784 m), slope (1o to 34o, mean = 16o), and aspect (35o to 350o, magnetic north = 0o).
Our data suggests that refuged (protected from browsing) heights of aspens sprouts were significantly taller than unrefuged (unprotected from browsing) heights (P = 0.0285, df = 12). We found no significant difference for unrefuged aspen-sprout heights between burned and unburned areas on shallow slopes (<15o) (P = 0.1693, df = 21). The difference in unrefuged height was not significant between burned and unburned areas on steep slopes (>= 15o) (P = 0.6567, df = 30). In unburned areas the mean unrefuged aspen-sprout height was 61.4 cm on steep slopes and 63.3 cm on shallow slopes. In burned areas the mean unrefuged aspen-sprout height for steep slopes was 68.2 cm while it was 40.6 cm for shallow slopes. Regression analysis suggested that species richness had a positive relationship with slope in burned areas (P = 0.0007, df = 12). In unburned areas, slope was also positively correlated with species richness (P = 0.0001, df = 81). RESULTSâ&#x20AC;&#x201D;2006â&#x20AC;&#x201D; Of the 178 refuged sites visited, 89 were host to shrubs which showed no evidence of ungulate browse. There was no significant difference in the heights of browsed and unbrowsed refuged trees (P = 0.29, df = 176). Refuged trees had a mean height of 226.3 cm (SD = 98.6 cm) while unrefuged trees had a mean height of 72.8 cm (SD = 28.3 cm) (Fig. 2). In addition, this difference in height was observed within each individual species (mountain maple (P < 0.0001), chokecherry (P = 0.005), aspen (P < 0.0001), oak (P < 0.0001)) (Fig. 3). A paired t-test showed that refuged trees were significantly taller than unrefuged trees (P = 0.0001, df = 94). Refuged trees over 2 m were protected by fallen trees with an average height of 80.0 cm (SD = 32.5 cm), while those under 2 m were protected by fallen trees 68.7 cm (SD = 24.1 cm) high. The difference in refuged tree heights were significant (P = 0.04, df = 109). Thirty-four percent of the refuges visited had at least one fallen tree present. When the dominant refuging agent was a fallen tree, its mean height was 76.9 cm (SD = 28.7 cm) with a mean width of 71.9 cm (SD = 42.7 cm). The refuged trees at these sites were 201.6 cm high (SD = 86.3 cm) (Table 1). The tallest aspen sprouts were found protected by rock. The dimensions of the other dominant refuging agents are listed in Table 1. We found 86 refuged aspen with a mean height of 231.5 cm (SD = 90.3 cm) while unrefuged aspen had a mean height of 73.2 cm (SD = 18.9 cm). A paired t-test showed a significant difference in height between refuged and unrefuged aspen (P < 0.0001, df = 58). Areas that showed signs of burn had a higher number of aspen sprouts (P = 0.0007, df = 108) than unburned sites. There was not, however, any significant difference between burned and unburned areas in respect to unrefuged height or refuged height (P = 0.60, df = 57 and P = 0.93, df = 84 respectively). An increase in canopy cover had a negative affect on the mean height of refuged aspen (P < 0.0001, df = 81) (Figure 3). The mean heights of chokecherry, mountain maple, oak and elderberry had no significant relationship with canopy cover. Regression analysis of refuging agent height and refuged tree height resulted in a positive, non-significant relationship (P = 0.097, df = 142). A similar non-significant,
positive relationship was found for refuging agent width and refuged tree height (P = 0.083, df = 133). One-way analysis of variance revealed Northing and elevation had significant negative relationships with refuged tree height (P < 0.0001, df = 173 and P = 0.0069, df = 173 respectively). Slope had no relationship with refuged tree height. DISCUSSION--The effects of both heavy browsing by elk and fire have had a strong influence on aspen regeneration over the past 100 years (Hessl, 2002). These variables on their own (browse and fire) have been confirmed to stimulate aspen ramet growth (Bartos et al., 1994). Apparently when coupled these interactions have a detrimental effect on aspen regeneration leading to a pattern of suppressed ramets of low stature (Ripple and Beschta, 2003) or complete aspen reproduction failure (Muegler, 1989) The relationship between ungulate browse, fire, and aspen regeneration is by no means an issue specific to Bandelier National Monument. These interactions have been well documented in National Parks throughout the Rocky Mountains (Huston, 1982; Keigley, 1997; Meagher and Huston, 1998; Singer and Zeigenfuss, 1998; White, Olmsted, and Kay, 1998; Ripple and Larsen, 2001). The combination of fire and high-intensity elk browse has been proven to accelerate the demise of aspen (White et al,. 1998). High population densities of ungulates not only limit the establishment of ramets, but contribute directly to mortality of adult trees through debarking; creating vectors for insects and disease, e.g. cytospora canker (Cytospora chrysosperma) (Muegler, 1989). During the course of our study we observed a variety of physical obstructions which inhibited elk from reaching aspen and woody shrubs. Among these obstructions, or refuging agents, were rocks, steep slope, standing trees, vegetation, and fallen trees. Our study found a significant difference between refuged tree height and the height of unrefuged trees. Refuged aspen were three times taller than unrefuged aspen. Our data coincides with a previous study conducted in Yellowstone National Park (Ripple and Larsen 2001) where refuged aspen were found to be two and a half times taller than unrefuged aspen. Aspen are known to be prolific after disturbances such as fire (Bartos, et al., 1994). The majority of burned sites we visited resulted from the Cerro Grande fire in 2000. Our analysis concluded that burned sites exhibited no difference in refuged aspen height compared with unburned sites, though we found a higher number of aspen sprouts in burned areas. Refuged aspen heights were correlated negatively with canopy cover, supporting the theory that aspen is a shade intolerant species (Jones and DeByle, 1985 in Mitton and Grant, 1996). We predict that refuged trees which reach a height of over 200 cm have a good chance of escaping ungulate herbivory, provided the refuge remains impenetrable for an extended period. Shepperd and Fairweather (1993), suggest this could be up to 15 years. Our analysis shows a refuging agent height of at least 80 cm could allow a refuged tree to survive to adult stature based on the protection of the apical meristem from browse.
Our study area was within the winter habitat of a large portion of the Jemez Mountain elk population. The northern most and highest elevations of our study area overlap the elk’s summer range. The negative relationship of refuged tree height with both Northing and elevation could be explained by the year-round presence of elk in this area. Our 2005 study found a positive relationship between slope and species richness. We believe elk browse is responsible for reduced species richness in areas with shallow slope. An alternative hypothesis may be an avoidance of steep slope by elk, due to predation risk. Another alternative is high soil quality and the existence of ecologicalniches on steep slopes. Lastly, years of domestic animal use in the area may have diminished species richness on shallow slopes. We found strong evidence in the literature (Ripple and Larsen, 2001) and from our own analysis in support of the refuge concept as a mechanism for sustaining aspen in the landscape. Obstructions, such as dead fall or “jackstraw” around areas with high sprout densities, have a refuging effect and can help to protect aspen and woody shrub sprouts from browse. Land managers should consider creating “jackstraw” in order to deal with the combined effects of fire and ungulate browse in areas where fuel reduction is a high priority. Human-made refugia could help to protect aspen sprouts and mitigate browse intensity. Our findings suggest these places of refuge should have a height of at least 80 cm to deter ungulates. The post-fire felling of trees in areas of heavy browse can assure the continuation and sustainability of aspen.
TABLE 1--Dominant refuging agent dimensions and refuged (protected from browse) tree heights for all species; mountain maple (Acer glabrum), chokecherry (Prunus virginiana), aspen (Populus tremuloides), oak (Quercus gambelii), red elderberry (Sambucus microbotrys), serviceberry (Amelanchier utahensis). Refuging agents without clear dimensions were denoted with not applicable (na). Sites that had an equal percentage of two refuging agents were omitted. Dominant Refuging Agent Fallen Trees Rock Plant Lacking Refuging Agent Standing Tree Slope Road Small Sticks
Mean Height (cm) 76.89 102.71 109.28
SD 28.70 56.14 33.24
Mean Width (cm) 71.89 114.06 190.83
SD 42.67 41.23 71.69
Refuged Tree Height (cm) 201.57 241.65 238.80
SD 86.25 72.40 64.95
n 68 31 20
na na na na 325
na na na na 35.36
na 10.97 na na 47.50
na 10.64 na na 31.82
204.62 239.00 294.78 257.00 211.33
144.99 102.15 175.28 84.85 104.64
13 10 9 6 3
FIGURE LEGEND FIG. 1—Map of study area in northern Bandelier National Monument, New Mexico. Symbols denote refuge locations found in 2006. FIG. 2—Refuged (protected from browse) tree height for 2006 and unrefuged (unprotected from browse) tree height for 2005 and 2006. Whiskers represent 95% confidence intervals. FIG. 3—Refuged (protected from browse) and unrefuged (unprotected from browse) mean height comparison by each species; mountain maple (Acer glabrum), chokecherry (Prunus virginiana), aspen (Populus tremuloides), oak (Quercus gambelii). Whiskers represent 95% confidence intervals. FIG. 4—Linear regression of refuged (protected from browse) aspen tree height and percent canopy cover.
Figure 1.
Mean of All Refuged and Unrefuged Tree Heights (95% Confidence Interval) 300 250
n=179
200 Height (cm) 150 100
n=91
n=68
Unrefuged 2006
Unrefuged 2005
50 0 Refuged 2006
Growth Context
Figure 2.
Refuged/Unrefuged Site Comparison With 95% Confidence Intervals 600 Refuged
500
Unrefuged
Mean 400 Height 300 (cm) 200 100 0
Mt. Maple
Chokecherry
Aspen
Species Figure 3.
Oak
Linear Regression of Percent Canopy Cover and Refuged Aspen Height 500 450 400 350 300 Height (cm) 250 200 150 100 50 0
n = 83 p < 0.0001 R² = 0.1928
0
20
40
60
Percent Canopy Cover
Figure 4.
80
100
LITERATURE CITED ALLEN, C.D., 1996. Elk response to the La Mesa fire and current status in the Jemez Mountains. Pages 179-195 in Craig D. Allen technical editor, Fire effects in southwestern forests: Proceedings of the Second La Mesa Fire Symposium; 1994 March 29-31; Los Alamos New Mexico. General Technical Report RM-GTR286, Fort Collins, CO: U.S. Department of Agriculture, Rocky Mountain Forest and Range Experiment Station. BARTOS, D.L., BROWN, J.K., BOOTH, G.D., 1994. Twelve years biomass response in aspen communities following fire. Journal of Range Management. 47:79-83. HESSL, A., 2002. Aspen, elk, and fire: The effects of human institutions on ecosystem processes. Bioscience. 52(11):1011-1022. HOUSTON, D.B., 1982. The northern Yellowstone elk: ecology and management. Macmillan, New York. KEIGLEY, R.B., 1997. An increase in herbivory of cottonwood in Yellowstone National Park. Northwest Science 71:127-136. MEAGHER, M.M., Houston, D.B., 1998. Yellowstone and the biology of time. Oklahoma State University Press, Norman, OK. MUEGGLER, W.F., 1989. Age distribution and reproduction of intermountain aspen stands. Western Journal of Applied Forestry 4:41-45. NEW MEXICO DEPARTMENT OF GAME AND FISH.,2006. Elk regional management information.http://wildlife.state.nm.us/publications/documents/elkregionalmagtdo cument.pdf. (accessed August 2006). RIPPLE, W.J., LARSEN, E.J., 2001. The role of postfire coarse woody debris in aspen regeneration. Western Journal of Applied Forestry 16:61-64. RIPPLE, W.J., BESCHTA, R.L., 2003. Wolf reintroduction, predation risk, and cottonwood recovery in Yellowstone National Park. Forest Ecology and Management. 184, 299-313. SHEPPERD, W.D., FAIRWEATHER, M.L., 1993. Impacts of large ungulates in restoration of aspen communities in a southwestern ponderosa pine forest. . USDA Forest Service General Technical Report. Rocky Mountain Forest and Range Experiment Station. Fort Collins, Colorado.
SINGER, F.J., ZEIGENFUSS, L.C., CATES, R.G., BARNETT, D.T., 1998. Elk,multiple factors, and persistence of willows in National Parks. Wildlife Society Bulletin. 26, 419-428. TOWEII, D.E., THOMAS, J.W., 2002. North American elk: ecology and management. Smithsonian Institution Press, Washington D.C... TURNER, M.G., ROMME, W.H., AND TINKER, D.B., 2003. Surprises and lessons from the 1988 Yellowstone fires. Frontiers in Ecology and the Environment. 1:351358. WEISBERG, S. 1985. Applied linear regression, Second Edition. John Wiley & Songs, Inc, New York. WHITE, C.A., OLMSTEAD, C.E., AND KAY, C.E. 1998, Wildlife Society Bulletin, 26:449462. WOLTERS, G.L., 1996. Elk effects on Bandelier National Monument meadows and grasslands. USDA Forest Service General Technical Report. Rocky Mountain Forest and Range Experiment Station. Fort Collins, Colorado.