Ecological Dynamics on Yellowstone's Northern Range (2002) / Chapter Skim
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3 Present Conditions: Vegetation
3 Present Conditions: Vegetation
Pages 43-82
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From page 43... ...
UPLAND SHRUBLANDS AND GRASSLANDS OF THE NORTHERN RANGE Shrublands Big sagebrush-Idaho fescue is the most abundant sagebrush-grassland type. It occurs on sites with thin cobble soils to well-developed loams, gener43
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From page 44... ...
The habitat type is dominated by mountain big sagebrush (Artemisiatridentata ssp. vaseyana)
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From page 45... ...
At the highest elevation, this type is summer range for elk and deer. At middle elevations, it is used as spring and fall range by all ungulates and as winter range by bighorn sheep and mountain goats.
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From page 46... ...
sarothrae, and Opuntia poZycantha. Factors Influencing Present Conditions of Sagebrush and Grasslands Sagebrush: Ungulate Use Big sagebrush is a particularly important food plant for several Yellowstone ungulates, especially in winter.
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From page 47... ...
Ungulate browsing in low-elevation sagebrush sites near the park boundary has resultedin significant negative effects on big sagebrush (WamboIt 19961. In some cases, up to 91~o of the leaders were removed and unbrowse~plants had higher productivity (45 g per plans)
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From page 48... ...
He found that elk fed heavily on the highly palatable bluebunch wheatgrass but moved off before the grass flowered so that there was little difference between enclosures and the surrounding areas in the total amount of green biomass of all species at the end of the growing season. The grazing and migration pattern in the northern winter range results in modest spring and summer grazing on the Tower ranges that receive heavy winter pressure and more intense grazing at higher elevations as snow recedes and green-up occurs (Singer and Harter 19961.
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From page 49... ...
After fire, numerous seedlings may establish, although there is little evidence that these plants survive and reproduce, especially for mountain big sagebrush (Wambolt et al.
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From page 50... ...
Those sites are a critical winter range for a variety of other ungulates, especially pronghorn. It appears that, without extensive andintensive management to offset the damage done by elk browsing and grazing, the sites will continue to be degraded as resources for pronghorns and other ungulates ~VamboTt and Sherwood 1999)
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From page 51... ...
These photographic comparisons also show a decline of aspen stands throughout the area, a phenomenon also seen in remotely sensed data that show aspen changes inside and outside northern YNP (Ripple and Larsen 2000a)
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From page 52... ...
In most cases, the krummho~z is on exposed ridges or rocky outcrops. The elevation of these woody communities is well above the northem range especially the northern winter range.
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From page 53... ...
Ungulate Use Many forest stands in the northern winter range and in the upper GalIatin River drainage are heavily browsed and highlined (i.e., a browsing pattern on trees caused by ungulates removing foliage and live twigs as high as they can reach, thus creating a high line usually a few meters above the ground)
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From page 54... ...
Recently, invasion of nonnative white-pine blister rust into the Rocky Mountains has caused mortality in whitebark pine andI~mber pine (Kendall and Schirokauer 1997, Kendall end Asebrook 19981. Proximity of Ribes spp., the secondary host for the fungus, as well as appropriate environmental conditions are necessary for blister rust to invade pine stands.
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From page 55... ...
Aspen stands also are considered prime areas for livestock grazing and can be extensive enough to provide a quality watershed and attractive scenery (DeByle and Winokur 19851. However, many aspen stands throughout the west, including YNP, appear to have declined during the twentieth century as old trees died and little recruitment took place.
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From page 56... ...
However, aspen stands in YNP are not as robust as stands in the mid-intermountain regions of Utah and Colorado (Mueggler ~ 988~. In the Rocky Mountains, aspen is primarily a clonal species that reproduces almost exclusively by root sprouting and produces large stands composed of stems from one or a few genetic individuals (Barnes 1966, McDonough ~ 985, Tuskan et al.
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From page 57... ...
Elk eat the tips of aspen sprouts and the bark of mature trees, except where the smooth white bark has been replaced by thick, black, corky bark in response to prior injury. Aspen root sprouts and seedlings may be severely browsed by elk during winter or during spring and fall migrations between the summer and winter ranges.
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From page 58... ...
Except for aspen clones in enclosures, most aspen stands on the northern range contain aging or senescent trees with a relatively dense understory of root sprouts mixed with a herbaceous ground cover. About 85 % of the large aspen alive today on the northern range originated before ~920 (Romme et al.
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From page 59... ...
These taller individuals then die, returning the clone to aged trees and herb-height root sprouts. Multiaged or multiheight aspen clones are scarce in Yellowstone, especially in the northern parts of the park.
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From page 60... ...
60 Ecological Dynamics on Yellowstone's Northern Range
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From page 61... ...
Typically, fire removes competing overstory conifers, triggering profuse aspen root sprouting and potentially producing extensive clonal stands. Reduction in fire frequency consequently may reduce the number and extent of aspen clones.
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From page 62... ...
Mature aspen stands that were burned in 1988 produced hundreds of thousands of root sprouts per hectare, but the ~ 988 fires resulted in a reduction in the number of aspen within shrub-aspen communities on the northern range (Kay end Wagner 1996~. The density of shrub-aspen before fire (19~36)
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From page 63... ...
(1997) concluded that the aspen population in the elk winter range in Rocky Mountain National Park is declining, largely in response to elk browsing.
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From page 64... ...
. It is unlikely that woody plants, including aspen stands with little highlining and extensive tall willow communities, could have existed if large numbers of ungulates used the northern range in winter in the late ~ SOOs.
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From page 65... ...
Intensive ungulate browsing on aspen may result from factors that concentrate the animals near the aspen clones, and/or from the ungulate herds being sufficiently large that nearly all aspen is utilized. If ungulates are "prevented" from migrating to portions of their historical winter range, the density of ungulates on the remaining available winter range will be unnaturally high.
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From page 66... ...
However, this cannot explain the lack of regeneration, particularly given that the ~ 988 fires resulted in profuse root sprouting that was intensively browsed. Ungulate browsing clearly contributes to the lack of regeneration, although the Tong-term dynamics of aspen in northern YNP are not well understood.
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From page 67... ...
or through behavioral changes, as might be induced by the presence of predators such as wolves, unless much more extensive lower-elevation winter range becomes available. RIPARIAN COMMUNITIES OF THE NORTHERN RANGE Riparian ecosystems are the transition from stream to upland.
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From page 68... ...
For example, beaver activity may elevate the water table and create suitable sites for willow community expansion, whereas stream incising may Tower the water table and cause stress to willows. In the Rockies, valley geomorphology directly influences the extent and type of riparian communities (Patten 1998~.
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From page 69... ...
Riparian communities in the northern range exist, or have existed, because conditions were suitable ~ ~ ~ for recruitment of the characteristic riparian species, (2) for maintenance of established species because the alluvial water table was sufficiently shallow to maintain plant growth and survival, and (3)
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From page 70... ...
Willows are also less tolerant of drought conditions and lower water tables than cottonwoods (Stromberg et al.
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From page 71... ...
An analysis of peak and annual flows in the Yellowstone River near Corwin Springs shows that both peak flows and annual discharge volumes were generally below average in the earlier part of the twentieth century (Figures 3-l and 321. Peek flows are analyzedin addition to annual discharge because they often closely represent snowpack conditions, the conditions that recharge and maintain elevated water tables in the watershed.
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From page 72... ...
Increased precipitation at high elevations has been used to explain the vigor of higher-elevation willow communities (YNP 19971; however, willows occur along stream courses where they have access to groundwater and soils wetted from the stream or from capillary rise of water from the water table (Dawson and EhIeringer ~ 99 I, Busch et al. ~ 992, Flanagan et al.
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From page 73... ...
Snow accumulation may play a more important role in riparian vegetation structure through protection of riparian shrubs as pointed out by Singer (19961. Not only does deep snow accumulation around riparian woody plants reduce browsing use, it also may prevent use of higher-elevation riparian vegetation by ungulates.
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From page 74... ...
do not indicate any unusual reduction or increase in stream flows over the past century beyond normal variation. Consequently, surface hydrological changes do not appear to be sufficient to have directly affected riparian availability of groundwater, although if some streams incised their channels during the past several decades, that could have lowered the surface flow enough to lower the alluvial water table to levels that could stress riparian vegetation.
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From page 75... ...
, which are beaverfoodandbuildingmatenal (Bailey 1930, Wright and Thompson 1935, Jonas 19551. In the absence of beavers, water tables in areas elevated by beaver dams may decline so that riparian vegetation cannot survive.
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From page 76... ...
76 Ecological Dynamics on Yellowstone's Northern Range
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From page 77... ...
No seed-producing catkins were found outside, but they averaged over 300,000 per m2 inside. Ungulate browsing also removes pollen-producing catkins.
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From page 78... ...
Concentrations of defensive or secondary chemicals in riparian vegetation may influence the magnitude of ungulate use. Several studies were designed to determine whether Tow-stature willows have lower concentrations of defensive chemicals and thus are more palatable and more heavily used by herbivores than teller willows (Singer et al.
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From page 79... ...
The committee concludes that some npanan losses may be due to changed hydrological conditions in addition to responses of vegetation to ungulate use. Flooding events and water tables are still suitable for recruitment of the dominant riparian vegetation.
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From page 80... ...
Types with aspen, as well as some graminoid communities, were saturated for part of the summer; community types with spruce, and cinquefoil, as well as other graminoid-dominated communities, were dry most of the summer. Water tables were variable among study sites.
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From page 81... ...
Several sites with declining water tables were near glacial ponds. Water chemistry was also variable with most pHs near or slightly below neutral.
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From page 82... ...
There may be drying of wetland depressions but there are no long-term data on shallow groundwater levels in northern range locations where drying may be occurring to explain changes in these depressions. Some of the depressions may also be filling in, reducing the amount of area available for wetland species (committee observation, Yellowstone National Park northern range, June 19991.
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