Spatio-Temporal Analysis of Ungulate Browsing on Willow (Salix spp.) Communities Within the Northern Range of Yellowstone (original) (raw)
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Estimating woody browse availability for ungulates at increasing snow depths
Forest Ecology and Management, 2006
In northern temperate environments, assessments of ungulate winter range in forested ecosystems commonly focus on measuring availability of browse because the dietary proportion of browse is typically high in winter. In many cases, these efforts ignore reductions in browse availability due to snow burial because this effect is difficult to measure. In this paper, we show how consideration of snow burial can alter the relative availability of browse in 6 forest communities common in the east slopes of the Rocky Mountains of the Alberta, Canada. We developed species-specific allometric relationships of stem diameter and woody twig biomass and height for the common shrub species and used these models to predict the decline in the proportion of total biomass/stem available to ungulates relative to stem height. Based on measures of stem densities, we then compared declines in browse availability for all browse species and for preferred browse species with increasing snow depths. Species compositional differences between forest types influenced the decline in browse availability as snow depth increased. Our stem-based approach is more flexible than previous plot-based approaches because it does not assume a distribution in stem heights but uses local information. Adjustments in browse availability are essential in habitat and food selection studies of ungulates where snow accumulates #
Willow on Yellowstone's Northern Range: Evidence for a Trophic Cascade?
Ecological Applications, 2007
Reintroduction of wolves (Canis lupus) to Yellowstone National Park in 1995-1996 has been argued to promote a trophic cascade by altering elk (Cervus elaphus) density, habitat-selection patterns, and behavior that, in turn, could lead to changes within the plant communities used by elk. We sampled two species of willow (Salix boothii and S. geyeriana) on the northern winter range to determine whether (1) there was quantitative evidence of increased willow growth following wolf reintroduction, (2) browsing by elk affected willow growth, and (3) any increase in growth observed was greater than that expected by climatic and hydrological factors alone, thereby indicating a trophic cascade caused by wolves. Using stem sectioning techniques to quantify historical growth patterns we found an approximately twofold increase in stem growth-ring area following wolf reintroduction for both species of willow. This increase could not be explained by climate and hydrological factors alone; the presence of wolves on the landscape was a significant predictor of stem growth above and beyond these abiotic factors. Growth-ring area was positively correlated with the previous year's ring area and negatively correlated with the percentage of twigs browsed from the stem during the winter preceding growth, indicating that elk browse impeded stem growth. Our results are consistent with the hypothesis of a behaviorally mediated trophic cascade on Yellowstone's northern winter range following wolf reintroduction. We suggest that the community-altering effects of wolf restoration are an endorsement of ecological-process management in Yellowstone National Park.
Landscape Ecology, 2002
We determined changes in willow (Salix spp.) cover in two valleys of the eastern slope of Rocky Mountain National Park, Colorado, USA, and related these changes to suspected causative factors. Changes in vegetation were inferred from digital maps generated from aerial photo-interpretation and field surveys conducted with a global positioning system. The decrease in riparian shrub cover was approximately 20% in both valleys over the period between 1937/46 and 1996, while the decline in tall willow (> 2 m tall) cover was estimated to be approximately 55% in both valleys. Suppressed willows (< 1.5 m tall) were predominantly located in areas affected by flooding and in areas where major river reductions were observed. Both valleys had sites that were being colonized by willows in wet meadows, and open areas created by flood disturbance. The potential causes of willow decline are many. Willow decline was associated with simplification of river spatial pattern, i.e., less complex branching and channelization, and a large flood disturbance. The causes of the reduction in river meanders were not determined, but are likely related to a decline in beavers, an increase in elk, and, possibly climate change. An increase in elk placed increased browsing pressure on willow during the period of the willow decline. Other factors such as climate changes and human activities could have also contributed to the willow decline. The persistence of these riparian ecosystems depends in large part on biotic interactions, particularly between willow, beaver, and elk.
Herbivores Influence the Growth, Reproduction, and Morphology of a Widespread Arctic Willow
PLoS ONE, 2014
Shrubs have expanded in Arctic ecosystems over the past century, resulting in significant changes to albedo, ecosystem function, and plant community composition. Willow and rock ptarmigan (Lagopus lagopus, L. muta) and moose (Alces alces) extensively browse Arctic shrubs, and may influence their architecture, growth, and reproduction. Furthermore, these herbivores may alter forage plants in such a way as to increase the quantity and accessibility of their own food source. We estimated the effect of winter browsing by ptarmigan and moose on an abundant, early-successional willow (Salix alaxensis) in northern Alaska by comparing browsed to unbrowsed branches. Ptarmigan browsed 82-89% of willows and removed 30-39% of buds, depending on study area and year. Moose browsed 17-44% of willows and browsed 39-55% of shoots. Browsing inhibited apical dominance and activated axillary and adventitious buds to produce new vegetative shoots. Ptarmigan-and moose-browsed willow branches produced twice the volume of shoot growth but significantly fewer catkins the following summer compared with unbrowsed willow branches. Shoots on browsed willows were larger and produced 40-60% more buds compared to unbrowsed shoots. This process of shoot production at basal parts of the branch is the mechanism by which willows develop a highly complex ''broomed'' architecture after several years of browsing. Broomed willows were shorter and more likely to be re-browsed by ptarmigan, but not moose. Ptarmigan likely benefit from the greater quantity and accessibility of buds on previously browsed willows and may increase the carrying capacity of their own habitat. Despite the observed tolerance of willows to browsing, their vertical growth and reproduction were strongly inhibited by moose and ptarmigan. Browsing by these herbivores therefore needs to be considered in future models of shrub expansion in the Arctic.
Willow–Bird Relationships on Yellowstone’s Northern Range
I t's 5:30 a.m., mId-June. I'm wading in knee-deep water, pushing my way through a dense willow thicket in Willow Park along Obsidian Creek in Yellowstone national Park. the icy creek, dark as a result of the tannins leaching from the willow stems, has not yet found its way into my heavily patched hip-boots. a beaver slaps his tail, a clear command to stay away and a warning bell for the rest of the colony that an intruder is approaching. startled for a moment, I return to my path and finally reach the point to which I have navigated with my GPs (Geographic Positioning system) unit and compass. I'm measuring songbird diversity in the willows, a task that often absorbs my concentration, but that requires me to remain vigilant of moose or elk that may be nearby. each point I navigate to is at the center of a randomly placed 40 m radius circle in which birds observed and heard are recorded over a 10 minute period. this is known as the point count method and is a commonly us...
The importance of willow thickets for ptarmigan and hares in shrub tundra: the more the better?
In patchy habitats, the relationship between animal abundance and cover of a preferred habitat may change with the availability of that habitat, resulting in a functional response in habitat use. Here, we investigate the relationship of two specialized herbivores, willow ptarmigan (Lagopus lagopus) and mountain hare (Lepus timidus), to willows (Salix spp.) in three regions of the shrub tundra zone-northern Norway, northern European Russia and western Siberia. Shrub tundra is a naturally patchy habitat where willow thickets represent a major structural element and are important for herbivores both as food and shelter. Habitat use was quantiWed using feces counts in a hierarchical spatial design and related to several measures of willow thicket conWguration. We document a functional response in the use of willow thickets by ptarmigan, but not by hares. For hares, whose range extends into forested regions, occurrence increased overall with willow cover. The occurrence of willow ptarmigan showed a strong positive relationship to willow cover and a negative relationship to thicket fragmentation in the region with lowest willow cover at landscape scale, where willow growth may be limited by reindeer browsing. In regions with higher cover, in contrast, such relationships were not observed. DiVerences in predator communities among the regions may contribute to the observed pattern, enhancing the need for cover where willow thickets are scarce. Such region-speciWc relationships reXecting regional characteristics of the ecosystem highlight the importance of large-scale investigations to understand the relationships of habitat availability and use, which is a critical issue considering that habitat availability changes quickly with climate change and human impact.
Elk winter foraging at fine scale in Yellowstone National Park
Oecologia, 2005
The link between landscape properties and foraging decisions by herbivores remains unclear, but such knowledge is central to the understanding of plantherbivore dynamics. Our goal was to determine whether fine-scale foraging paths of free-ranging elk (Cervus canadensis) respond to spatial structure of habitats in Yellowstone National Park. During winter 2002 we gathered elk-foraging information by following snow tracks in open habitats located on hillsides and flat terrain. The 21 snow paths surveyed were comprised on average of 15 discrete snow craters connected to each other by relatively straight-line movements. Our analyses revealed two levels of selection: elk chose where to dig, and how much search effort to allocate at digging sites based on habitat characteristics. On hillsides, elk preferentially dug in areas of greater biomass of grasses and forbs, and simply walked through poorer sites without digging. Individuals also searched more intensively, creating larger craters, where food biomass was higher. On flat terrain, crater size decreased with snow depth and increased with snow density. Correlated random walk models usually were adequate to characterize elk movement on flat terrain, but not on hillsides. First, as the number of movements between local foraging areas increased, elk displacements on hillsides became shorter than expected from random patterns. This trend on hillsides was strongly influenced by interindividual variation in movement behavior. Second, elk tended to forage perpendicularly to aspect, resulting in horizontal displacements. Our study demonstrates that free-ranging elk adjust their foraging to fine-scale habitat structure.
Effects of bison on willow and cottonwood 4 in northern Yellowstone
2012
On the northern ungulate winter range of Yellowstone Park, willow (Salix spp.) and cottonwood (Populus angustifolia and P. balsamifera) have increased in height and cover in some places since the reintroduction of wolves (Canis lupus) and the subsequent changes in elk (Cervus elaphus) behavior and population densities. However, in the Lamar Valley, an important part of this winter range, many plants are still intensively browsed and recruitment has been limited. As elk numbers have declined and their distribution has changed in recent years, bison (Bison bison) have increased on the northern range. To distinguish bison effects from those of elk, we measured browsing that occurred in summer. We found average summer browse rates of 84% for willow and 54% for cottonwood seedlings in the summer of 2010, demonstrating that bison have become significant browsers in the Lamar Valley. Plants were increasing in size except where intensively browsed by bison, suggesting that a release from elk browsing has occurred, and that a trophic cascade is occurring from wolves to plants, mediated by both elk and bison. Release of bison from competition with elk, low levels of predation on bison, and lack of opportunity for migration and range expansion may be factors contributing to a high concentration of bison, with resulting effects on plant communities and biodiversity.
Concentrated seasonal habitat use by ungulates may amplify their influence on ecosystem processes. For example, white-tailed deer (Odocoileus virginianus) depend on forests of coniferous trees, such as Tsuga canadensis (L.) Carr. (eastern hemlock), for winter cover and forage in the northern portion of their range. We hypothesized that winter habitat use influences summer herbaceous-layer plant communities through cascading indirect effects resulting from the deposition of nitrogenous wastes and browsing of woody plants. Deer use was monitored from 2006-2008 via fecal pellet group counts in 15 T. canadensis stands across the western Upper Peninsula of Michigan. Detailed vegetation and soil sampling was performed during the summer of 2008. Estimates derived from winter inputs of fecal pellets, suggest that overwintering white-tailed deer deposit 3.3 6 0.8 kg N ha À1 yr À1 . Results from nonmetric multidimensional scaling ordinations and permutational ANOVAs of distance to group centroids of ground-layer vegetation and soil attributes indicated that high levels of winter deer-use were significantly associated with greater heterogeneity in ground-layer community composition and relative soil nutrient variability within and between stands. Our results highlight potentially important feedbacks between habitat selection by ungulates, site productivity, and plant community structure in forest ecosystems.