Aspen, Elk, and Fire in the Rocky Mountain National Parks of North America Clifford A. White; Charles E. Olmsted; Charles E. Kay Wildlife Society Bulletin, Vol. 26, No. 3. (Autumn, 1998), pp. 449-462. Stable URL: http://links.jstor.org/sici?sici=0091-7648%28199823%2926%3A3%3C449%3AAEAFIT%3E2.0.CO%3B2-G Wildlife Society Bulletin is currently published by Allen Press.
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ASPEN CONDlTlON
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Aspen, ell., and fire in the Rocky Mountain national parks of North America Clifiord A. White, Charles E . Olmsted, and Charles E. Kay Abstract Trembling aspen (Populus tremuloides) forests in the Rocky Mountains are an indicator of ecological conditions because they have maintained their presence for thousands of years through vegetative reproduction, and these communities have high biodiversity. Aspen can be tied to ecosystem condition through a 4-level trophic model that links humans, wolves (Canis lupus), elk (Cervus elaphus), and aspen through the processes of predation, herbivory, burning, and differential wildlife responses to humans. We used a comparative study of research literature and historic photographs to evaluate aspen change over time in 6 Rocky Mountain national parks in Canada and the United States: Jasper, Banff, Yoho, Kootenay, Yellowstone, and Rocky Mountain. Across all parks, aspen has consistent responses to increased browsing by ungulates and decreased frequency of fire. Although aspen was historically vigorous in all parks, today most stands are in decline. Trees are intermediate to mature in age (60-120 years old), and elk browse off new suckers before they reach 2 m in height. Fire, combined with browsing, has hindered regeneration of aspen. The exception to this pattern is northern Jasper National Park where elk densities appear to have been reduced by wolves in the 1970s, and aspen regenerated. W e found a gradient of increasing human-caused ecosystem changes in Rocky Mountain national parks: (1) historic conditions with frequent fires and low elk density regulated by humans, wolves, and other predators (i.e., all parks); (2) current conditions of patches of high elk density, where wolves are displaced by human land use, within a matrix of moderate elk density, where wolves and other predators are present (i.e., Banff and Jasper national parks); (3) recent conditions inside parks, where wolves are absent, and very high elk density is regulated by competition for food (i.e., Yellowstone before 1995); and (4) potential future conditions, where increasing human land use around parks displaces carnivores and reduces hunting, and very high elk density occurs throughout landscapes. Aspen stands regenerate well in areas of low elk density and in some areas of moderate elk density; however, in areas of high and very high elk density, aspen does not regenerate to heights >2 m, and burning accelerates clone deterioration. Our recommendations to national park managers are to restore carnivores, use fire in areas of low elk density, and control human uses that displace carnivores.
Key words
Cervus elaphus, ecological integrity, elk, indicator species, national park management, Populus tremuloides, trembling aspen
Trembling aspen (Populus tremuloides) forests are an important community type in the Rocky Mountain national parks of Canada and the United States. In Banff and Jasper (Alta.: Can.) and Yoho and Kootenay @.C., Can.) national parks, aspen covers 2 m (C. E. Kay, unpubl. data)
Many trees >2 m killed by fire, prolific post-fire suckers browsed off before reaching >2 m tall, some clones killed completely (Bartos and Mueggler 1979, 1981; Bartos eta!. 1994; Kay and Wagner 1996; Romme et al. 1995; C. E. Kay, unpubl. data) Willow browsing by elk heavy, beaver numbers down substantially from historic levels (Kay 1990, 1997a; Kay and Wagner 1994; Chadde and Kay 1991; Singer eta!. 1994) No data available
Suckers browsed off before reaching 2 m in height, stems barked, few trees 2 rn in height, or lasting increases in sucker production (C. E. Olmsted, unpubl. data)
No bark stripping visible (from photos in Whyte Museum of Canadian Rockies collection)
Aspen sucker condition at park establishment from historic photographs
Prolific suckering, no twig browsing visible (Kay et al.1994)
Aspen condition c. 19301970 in areas with high elk density during elk c~llling period
Suckers browsed off before reaching 2 m in height, stems barked (Cowan 1947)
Aspen cond~tionc. 19701990 in high vis~toruse areas with high elk density
Aspen condition c. 19701990 in wildlife exclosures
Condition of willows and beaver populations c. 1970-1 997
In areas with moderate or greater elk density, heavy willow browsing, beaver populations decreasing (Nietvelt and Bayley 1997)
Aspen and willow response in areas recolonized by wolves
Abundant suckers, stems near trails reaching >2 m height (Dekker 1985; Dekker et al. 1996; C. A. White, unpubl. data) .$--I0elWkmL in low wolf predation areas; 1-2 elbkm' in wolf predation areas (Llekker et al. 1996, 13aquetet al. 1996, Banff-Bow Valley Study 1996)
Elk density on winter ranges C. 1970-1 990
Yellowstone Ecosystem (Wyo. and Mont., USA) No bark stripping visible (Kay 1990)
Aspen bark stripping before c. 1850 fronr historic photographs
Aspen response to fire c. 1970-1 990
Yoho and Kootenay (B.C., Can.) No bark stripping visible (from photos in Whyte Museum of Canadian Rockies collection)
12-25 elk/krn2 (Houston 1982, Singer et al. 1994)
Most suckers browsed off before reaching 2 m in height, stems barked (Packard 1942, Olmsted 1979, Hess 1993)
Willow browsing by elk heavy, beaver numbers down substantially from historic levels (Hess 1993) No areas recolonized by wolves
15-25 elWkm2(calculated from data in Hess 1993: 21-26, Baker et al. 1997)
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Wildlife Society Bulletin 1998, 26(3):449-462
Fig. 3. Company D, Minnesota National Guard camp at Little Blacktail on Yellowstone National Park, Wyoming northern winter range in 1893 showing no elk-induced bark injury, multi-sized (aged) aspen, and little or no twig browsing by ungulates. Photo by F. J. Haynes, H-3070, courtesy of Haynes Foundation Collection, Montana Historical Society, Helena.
elk population is food-regulated (Coughenour and Singer 1996). Elk eat all palatability classes of forage, including normally unpalatable species such as spruce (Picea engelmanii; Kay 1997~). Prescribed fires conducted between 1974 and 1985 in Banff andJasper national parks and in Jackson Hole, Wyoming failed to regenerate aspen to heights >2 m (Kay 1990,Bartos et al. 1994,Kay et al. 1994,White et al. 1994, Kay 1997~). Fires killed mature trees, and elk browsed off new suckers before they reached tree height. Some bum areas were small and may have concentrated elk use (Bartos and Mueggler 1979, 1981). Subsequent large prescribed fires in Banff and the 1988wildfires in Yellowstone National Park stimulated abundant aspen suckering (Fig. 5A), but also failed to regenerate aspen to heights >2 m (Kay et al. 1994, White et al. 1994, Romme et al. 1995, Kay and Wagner 1996). In Yellowstone, the 1988wildfire, followed by intense elk browsing, killed some aspen clones (C. E. Kay, unpubl. data; Fig. 5B). Researchers in United States and Canadian parks report sharp declines in beaver colonies and surrounding areas of willow habitat. In Banff National Park, the number of beaver lodges in Vermilion Lakes declined >90%since the 1970s because elk herbivory on willows has significantly reduced the amount of food and dam-building material available for beaver (Nietvelt and Bayley 1997). Although beaver and tall-willow communitieswere once common along northern Yellowstone streams, beaver are now "ecologically extinct," and willow cover is sharply reduced, except in wildlife exclosures 1997a; Chadde and Kay 1991, Kay and Wagner 1994, Singer et al. 1994).
Similarly, beaver and willows are in sharp decline in Rocky Mountain National Park mess 1993). Dekker (1985, Dekker et al. 1996) was the first to record a deviation from the general pattern of aspen and willow decline in the Rocky Mountain parks. In areas of northern Jasper frequented by wolves during the 1970s (Carbyn 1974a, 1974b), Dekker (1985) noted prolific aspen and willow regrowth. C. A. White (unpubl. data) verified those observations. A collection of 1950s photographs shows intensive elk browsing on aspen with no young age classes visible. Carbyn (1974~)documented high kill rates of elk by recolonizing wolves during the 1970s. He calculated that wolves could eliminate elk in the area within a few years if ol, served predation rates continued. At approximately the same time, tree-ring dates indicate that a new c o hort of aspen suckers regenerated in many stands, particularly those close to wolf-frequented trails (C. A. White, unpubl. data). By 1997 many of the new stems had reached 3-5 m tall (Fig. 6),and although elk use of aspen appears to be increasing, many stems will likely survive to mature tree age. Similar conditions may be occurring in areas recently recolonized by wolves in Banff National Park (C. A. White, unpubl. data).
Discussion Evaluating alternative hypotheses for aspen decline We use information on trembling aspen condition and trend in several Rocky Mountain national parks to test hypotheses on the decline of aspen. Alternate explanations for aspen decline are based upon whether, over the long-term, elk population levels were generally high and regulated by competition for food (1 principal hypothesis with 2 variants), or generally low at a predator-regulated equilibrium.
Fig. 4. Ungulate-proof wildlife exclosure in Upper Beaver Meadows, Rocky Mountain National Park, Colorado, in September 1997.
Aspen condition
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Fig. 5. Aspen burn plot 27A in Yellowstone National Park, Wyoming: (A) August 1989 after the clone was top-killed by the 1988 fire, and abundant new suckers sprouted from the live root system; and (€3) August 1996 after several years of heavy elk browsing when no live suckers remained and the root system appeared to be dead.
Food-regulation ("bottom-up") hypothesis. This paradigm predicts that a long-term, stable state of montane areas occurs when aspen is browsed extensively by abundant, food-regulated elk. This is termed "natural regulation" in Yellowstone National Park (Cole 1971, Boyce 1991, Coughenour and Singer 1996). Heavily browsed aspen persist in the ecosystem due to regeneration by fire (Gruel1 1979, Houston 1982, Boyce 1989), variable chemical-defence levels that protect aspen suckers against herbivory (Despain 1991), or a complex interaction of factors such as fire, elk starvation, winter severity, and climate change (Houston 1982, Boyce 1991, Romme et al. 1995). The current degeneration of aspen is due to an increase in the number of elk back to a normal food-regulated equilibrium following intense human hunting during the late 1800s. Wolf and other predation on elk is considered a "non-necessary adjunct" (Cole 1971), which removes animals that will die anyway due to starvation, and thus, cannot substantially lower elk populations below food-regulated levels (Boyce 1996). Existing data do not s u p port this viewpoint in any of the Rocky Mountain parks. Photographs of mature aspen taken at the time of park establishment show no bark-stripping (Table 2), indicating long-term low elk density (Kay and Wagner 1994). Furthermore, archaeological-site data and early explorer journals for the Rocky Mountains in the United States (Kay 1990, 1994, 1997b) and Canada (Kay et al. 1994, Kay and White 1995) repeatedly show a pattern of low elk abundance. Fire-suppressionhypothesis. A variant on the food-regulation hypothesis is based upon fire effects. (lyg2) proposed that (1980) and widespread fire suppression was the primary cause
of aspen decline. According to this view, fires caused extensive aspen suckering that could theoretically "swampnherbivory effects in areas with moderate or higher elk density. This would allow at least some aspen suckers to grow into sapling or tree form (Bartos and Mueggler 1979). However, monitoring of numerous burn areas in and near Rocky Mountain national parks (Table 2) shows that this has not happened (Bartos et al. 1994, Kay et al. 1994, Kay 1997b). Instead, the combination of fire and high-intensity elk browsing actually accelerated the demise of aspen (Kay and Wagner 1996). Given the longterm historical regime of frequent fires (
