Yellowstone's Northern Range - Greater Yellowstone Science ...

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THE NORTHERN RANGE 54 able uncertainty about the extent to which elk were reduced on the northern range in the 1 870s and l880s (Schullery and Whittlesey 1992). As already noted, the market-hunting harvest was quite high in the mid-1870s, but the slaughter seemed to have dropped off by the end of that decade (Schullery and Whittlesey 1992). It also appears that if elk were markedly reduced during the 1870s, they began to recover by the early 1880s. Houston (1982) and Coughenour and Singer (1996a) document the quickness with which the northern elk herd recovered following the reductions of the 1960s, and there is no reason to believe elk could not do the same in the late 1800s. As already mentioned, Schullery and Whittlesey (1992) published contemporary accounts of 5,000 elk reported in mid-winter (February) on the northern range between Mammoth Hot Springs and Cooke City as early as 1883 and continuing after that year. During the most recent period of extreme elk population reduction (1963-1969), when there were usually fewer than 5,000 elk on the northern range, Barmore (1975) observed that they were still numerous enough to browse almost all aspen suckers (75 percent leader use). Some set of circumstances, almost certainly involving climate, enabled the aspen of the 1870s and l880s to grow to tree height despite browsing, something they were unable to do during the 1960s. Houston (1982) reviewed climatic reconstructions based on tree-ring analyses, concluding that "the highest winter precipitation for the entire 161-year (1750- 1910) period occurred from about 1877 to 1890." It seems most likely that a variety of factors influenced aspen success in the 1870s and 1880s. Besides climate, other factors that may have played a part include fire history, commercial trapping of beaver in the 1870s and 1880s, and reduction of elk numbers or displacement of elk from some areas by human activities, especially hunting. It may be that all aspen clones that developed tree-size growth in that period were not successful in doing so for exactly the same reasons. Whatever causes are eventually identified for the success of aspen in the 1870s and 1880s, it is clear now that the traditional viewpoint-that an abrupt change in elk numbers is solely responsible for the aspen decline-is incorrect. Elk prevent aspen from reaching tree size today because elk eat the young trees, but that does not mean that elk are the sole cause. Research has resulted in "new hypotheses that are being investigated: 1) under proper growing conditions, aspen suckers can grow large enough in a single season and can produce enough defensive chemicals to keep elk from completely debarking and preventing young plants from attaining tree size. If proper conditions do not exist, the clone is kept in a shrub or perennial forb stage by browsing, but the clone itself-its root system-is not eliminated; and 2) aspen show an opportunistic sexual reproductive strategy following burning. The fires of 1988 provided an excellent opportunity to test fire's role in the reproduction of northern range aspen. In the year following the fires, an unprecedented amount of aspen growth from seedlings was documented (Kay 1993, Renkin et al. 1994, Renkin and Despain 1996b, Romme et al. in press). However, on the northern range few if any of these seedlings have been able to escape browsing and grow to adult height, despite their tremendous numbers. Large fires alone were unable to enable aspen to escape elk and grow to tree height on the northern range (Figure 4.8). Seedlings have been much more successful on Yellowstone's summer ranges at higher elevations. The fires of 1988 led to the widespread establishment of aspen Hin extensive portions of YNP where there was no aspen before the fires" (Romme et al. in press). Establishment sites were primarily in burned lodgepole-pine forests, especially in the west-central part of the park; this event may have important implications for understanding the long-term persistence of aspen in the park. Genetic studies revealed that these new seedlings derived from some distant seed source presumably to the west of the park, not from existing aspen clones in the park. The fires of 1988 thus resulted in an apparently substantial increase in genetic diversity in the aspen populations of Yellowstone National Park. These new seedlings are persisting: "They are elongating slightly and increasing in density in at least some places despite
WOODY VEGETATION 55 • Canopy Burn Mixed Burn • Non-forested Burn Undilierentiated Burn Figure 4.8. Bum map from the fires of 1988. Map by Yellowstone Spatial Analysis Celltel: moderately heavy browsing pressure, and ... they are establishing new clonal population structures" (Romme et aJ. in press). It may take several decades to determine whether these new clones can thrive in lodgepole-pine forests when the lodgepole grow tall enough to shade them out. The fate of the 1989 aspen seedlings in wet meadows may share the same long-term wait to determine if they survive, and survival may depend on whether aspen are in shrub or tree form. It is important to note that aspen do not have to grow into trees to establish and spread by root suckers. If they do persist, they may provide an instmctive example of how aspen have maintained themselves in the park area's relatively inhospitable environment for thousands of years, and whether they persist or not, they are currently offering an important opportunity to further study this popular and controversial species in Yellowstone. The ecological circumstances facing aspen on the northern range continue to change, and in recent years may have done so in favor of aspen. The initiation of wolf recovety in 1995, and a recent trend toward much wetter winters with deeper snows, may contribute to a return to the circumstances that prevailed the last time that aspen escaped browsing and grew to tree height on the northern range. Until that set of circumstances is duplicated, however, it is probable that aspen will continue to occupy the northern range at the lower levels now present in many shrub-height clones. It seems likely that other species of equally precarious status, such as cottonwoods and water birch, may share this same fate. RESEARCH RECOMMENDATIONS: QUAKING ASPEN It seems important to continue studying aspen on the northern range, even if it is somehow determined that their decline is entirely within the realm of natural ecological processes and is not due to elk numbers alone. Age-structure studies on the northern range should continue, so that sufficient data is obtained
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YELLOW"STONE'S NORTHERN RANGE COMPL
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Page 42 and 43: THE NORTHERN RANGE 26 et al. (1991)
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Page 68 and 69: THE NORTIlERN RANGE 52 Figure 4.5.
Page 72 and 73: THE NORTHERN RANGE 56 to confirm an
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Page 77 and 78: SOIL, EROSION, SEDIMENTS, AND WATER
Page 79 and 80: SOIL, EROSION, SEDIMENTS, AND WATER
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Page 83 and 84: ELK POPULATION ISSUES IS YELLOWSTON
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Page 87 and 88: ELK POPULATION ISSUES 71 ... Other
Page 89 and 90: ELK POPULATION ISSUES 73 1996d), so
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Page 106 and 107: THE NORTHERN RANGE 90 tion size, an
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THE NORTHERN RANGE 104 practically
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THE NORTHERN RANGE 106 appear suppr
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AppendixA. Conferences, Meetings, a
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lJO Year Estimate Comment Source 19
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lJ2 , Estimated # of Elk Year Date
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lJ4 Winter Total Count 1967-68 397
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116 Year Date of Count Parkwide Par
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118 Counts and estimates of mule de
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120 Northern Yellowstone winter ran
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122 Population estimates and reduct
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124 Actual count Inside Outside Est
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Pronghorn counted during spring sur
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128 Year 1939 1940 1941 1942 1943 1
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Ground and aerial counts of bighorn
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REFERENCE LIST NOTES ON THIS REFERE
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Pages 199-210 in Carbyn, L.N., S.H.
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Yellowstone National Park. Technica
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mammal mortality in the 1988 Yellow
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National Park. International Journa
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Mehus, C.A. 1995. Influences ofbraw
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__ , and __ . 1996a. Preburn root b
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Pages 127-138 in F.J. Singer, ed. E
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some northern Yellowstone elk. M.S.
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I I I I I
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