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

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THE NORTHERN RANGE 48 Figure 4.4. In 1893, tall willows were abundant along watercourses in Yancey's Hole, near Tower Junction, and have since largely disappeared. Kay (1990,199l)has proposed that elk were solely responsible for this disappearance, but historical evidence indicates that as early as 1883, 5,000 elk were wintering on the northern range, and probably more by the 1890s. In the 1960s, fewer than 5,000 elk were able to suppress willow growth on the northern range (Barmore 1980), leading to the question of why elk were unable to suppress tall willows at locations like Yancey's Hole in 1890s. Changes in climate since the 1890s may have reduced the willows' ability to defend themselves from browsing. NPSphoto. determining the fate of the willows. During the elk reductions of the 1960s, when the northern elk herd was suppressed to less than 5,000 for at least five years (and less than 4,000 for at least two years), willows were still heavily browsed (Barmore 1980, Singer et a1. 1994, 1996b). This episode of elk reduction in the 1960s provides a useful window on earlier times in the park's history as well. Kay (1990) has published historic photos (such as Figure 4.4), which show robust willow growth on the northern range in the period 1893-1897, and has suggested that unnatural increases in elk on the northern range were the cause of declines or disappearance of these willow stands. It is known that elk numbers were suppressed during the industrial-scale hide-hunting era in Yellowstone in the 1870s, but it is unclear from the historical record precisely how extreme this suppression was, or how long it lasted (Schullery and Whittlesey 1992, 1995; Schullery in press). However, as early as 1883, local observers reported a minimum of 5,000 elk wintering on the northern range in the park, and by the 1890s it appears that the number of wintering elk may have been considerably higher than that (Houston 1982, Schullery and Whittlesey 1992). During the period 1883-1896, then, elk were more abundant on the northern winter range in the park than they were during the period 1963-1969, but as historical photographs such as Figure 4.4 clearly show, willow growth in the 1880s and 1890s continued to be robust and tall in the presence of large numbers of wintering elk. This should raise the question of what had changed between 1896 and 1960: why did willows thrive between 1883 and 1896, in the presence of at least 5,000 elk, when willows were suppressed by even fewer elk between 1963 and 1969? As discussed below, changes in climate, with consequent changes in the willows' ability to defend themselves from browsing, are now considered by some investigators to be a likely answer to this question. Because elk and bison, the most important ungulates on a biomass scale, eat mostly grasses and sedges (80 to 97 percent of the diet) and because the condition of northern range grasslands has been the primary concern of managers and investigators since the 1920s, willow research was not initially as high a priority as grassland research. Only two willow studies were conducted between 1986 and 1990. Singer et a1. (1994, 1996b) reported that suppressed-height willows were found in roughly one-third of the northern range willow stands. Intermediate and tall willow stands appeared to be healthy and vigorous; most management concerns focus on willows whose height is suppressed by browsing. Investigations suggested that suppressed willows were more common at lower elevations that are warmer and drier. Suppressed willows were more sought out as forage by ungulates and had lower levels of secondary defense compounds, lower levels of protein, and less water stress than taller willows. Suppressed willows produced only 12 percent the current annual growth of tall willows; even after 29 years of protection from all ungulate grazing, suppressed willows still produced only 33 percent as much growth as tall willows. Suppressed willows apparently grew on sub-optimal sites. As mentioned earlier, willow production was positively correlated with higher precipitation and elevation on the northern range. Most tall stands
WOODY VEGETATION 49 grow at higher elevations. Intermediate height stands are found at a wide variety of locations. Although less water stress was detected in suppressed willows, some of the willow communities on marginal sites may already have died, or may have failed from too much browsing. Shootroot ratios of the remaining suppressed stands may have adjusted to a more favorable relationship for water balance. An interaction between climate, water tables, and willow herbivory was suspected. Por example, death of many willow plants in two intermediate height stands was documented following the drought of 1988. Only about half of marked willow communities responded positively to burning in the fires of 1988. This suggested that some willow stands are on poor sites, because willows are reported to be stimulated by burning in nearly all other studies. There is a possibility that a warmer (1.6°P [16.2°C] higher average temperature) and drier (2.4 inches less precipitation) climate this century has contributed to the decline in abundance and stature of willows (Balling et aJ. 1992b, Singer et aJ. 1996a). Willow status on the northern range was relatively stable or even improving during the past three decades (Chadde and Kay 1988; Singer et aJ. 1994, 1996b). Heights, stem numbers, and total cover of browsed willows on permanently marked transects were stable or increased slightly (Chadde and Kay 1988; Singer et aJ. 1994, 1996b). Nearby protected willows increased two to five times in cover since 1958, but the number of stems from protected willows declined and heights declined since 1986 (Singer et al. 1994, 1996b). Willows inside exc10sures provide a dramatic example of the consequences of complete protection from ungulate browsing. but exc10sures represent only one extreme point in the spectrum of ungulate herbivory. As noted earlier, one study showed significantly higher plant diversity outside a willow exclosure than inside it (Chadde and Kay 1988). In fact, the willows inside the exclosures have suggested to us that they are growing in what might be termed "currently suboptimal sites." Even after 29 to 33 years of large herbivore protection, there were no new willow recruits or individuals, and no expansion on the edges of the willow stands inside the exclosures (Singer et al. 1994). Ungulate-use rates on suppressed willows were not lower during the 19608, when elk populations were reduced to 25 percent those of the late 1980s (Barmore 1980, Singer at al. 1994, 1996b). Drastically reducing elk numbers during the 1960s did not achieve the goal of reducing browsing on willows (Barmore 1980; Singer et al. 1994, 1996b). An interesting part of local folklore about northern range willows is that their condition can best be judged by comparing them with other willows outside the park. At first glance, this might seem a useful test, and in some respects it does allow for a comparison of differing management approaches. However, to point to willows in some drainage outside the park as somehow "proving" that something is "wrong" with park willows is rarely a safe comparison. For example, many willows on upper Slough Creek, outside the park to the north, are typically taller than those on the northern winter range, but this is no surprise; these willows are at higher elevations in cooler and more shaded locations, receive more annual precipitation, and are not on the elk winter range. In fact, studies have shown that these willows are subjected to more intense herbivory from local concentrations of moose that, unlike elk, browse willows for most of the year (Singer and Cates 1995). These willows, growing in a cooler, wetter area, are tall and vigorous, better defended by chemical defense compounds, and they recruit new individuals regularly, especially on new alluvial sites. These willows, like the very large willow communities south of Yellowstone Lake ill the park, are on summer range and are less heavily browsed by elk but browsed hard by moose; in winter, snow depths exclude elk from these willows but not moose. Conversely, tall willows are abundant along the Yellowstone River 20 to 40 miles north of the park but these willows are on ranches that experience little or no elk herbivory. Domestic livestock not only do not consume willows to any extent, but they also graze grasses near the willows, thus increasing the competitive advantage of the willows. Most willows outside the park, even those on
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YELLOW"STONE'S NORTHERN RANGE COMPL
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Page 32 and 33: THE NORTHERN RANGE 16 Northern rang
Page 34 and 35: Figure 2.3. Sum ot rotal annual riv
Page 36 and 37: THE NORTHERN RANGE 20 Figure 2.4 Tr
Page 38 and 39: THE NORTHERN RANGE 22 dendrochronol
Page 40 and 41: THE NORTHERN RANGE 24 Figure 2.7. A
Page 42 and 43: THE NORTHERN RANGE 26 et al. (1991)
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Page 60 and 61: Figure 4.1. Northern range, showing
Page 62 and 63: Figure 4.2. Willow distribution in
Page 66 and 67: THE NORTHERN RANGE 50 public lands,
Page 68 and 69: THE NORTIlERN RANGE 52 Figure 4.5.
Page 70 and 71: THE NORTHERN RANGE 54 able uncertai
Page 72 and 73: THE NORTHERN RANGE 56 to confirm an
Page 74: THE NORTHERN RANGE 58 RESEARCH RECO
Page 77 and 78: SOIL, EROSION, SEDIMENTS, AND WATER
Page 79 and 80: SOIL, EROSION, SEDIMENTS, AND WATER
Page 81 and 82: ated with ungulate winter ranges. T
Page 83 and 84: ELK POPULATION ISSUES IS YELLOWSTON
Page 85 and 86: ange grazing issue, Yellowstone man
Page 87 and 88: ELK POPULATION ISSUES 71 ... Other
Page 89 and 90: ELK POPULATION ISSUES 73 1996d), so
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Page 98 and 99: THE NORTHERN RANGE 82 Competition t
Page 100 and 101: Figure 7.3. Bison winter range. Sin
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Page 106 and 107: THE NORTHERN RANGE 90 tion size, an
Page 108 and 109: THE NORTHERN RANGE 92 activity (95
Page 110 and 111: THE NORTHERN RANGE 94 pursue the su
Page 112 and 113: THE NORTHERN RANGE 96 factors were
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THE NORTHERN RANGE Figure 7.13. Num
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THE NORTHERN RANGE 102 complex set
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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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