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Greater Yellowstone Ecosystem Report

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Regional models of global climate change for the northern Rocky Mountains predict warmer temperatures and western states have shown a recent trend towards increases in the fraction of precipitation falling as rain rather than snow. Thus, the amount of soil moisture available to plants during the growing season may already be varying significantly from historical conditions. In order to simulate drier summer soil conditions and test soil moisture and plant responses, we set up a pilot snow removal experiment in Grand Teton National Park, WY in 2008. Because most of the precipitation comes in the form of snow, and spring snowpack is about 50% water by volume, removing all snow present in early May can reduce the annual precipitation by approximately half. Snow was removed from three ~9x15 m experimental plots using a Thiokol in early May. These experimental sites were paired with adjacent control sites and the pairs were established along a hydrological gradient of meadow types representing hydric, mesic, and xeric sites. Dataloggers were installed in June and soil moisture was monitored at 5, 20, and 50 cm depths through August. Water potential was measured in Bromus and Carex during mid-July. Over the summer season, there were differences in soil water between the snow removal and control plots, as well as across the hydrological gradient. Decreases in soil water content and water potential of Carex showed strongest responses in the mesic site, whereas Bromus showed the strongest response in the hydric site. This is the first study to reveal that variability in spring snowpack can lead to measurable impacts on the vegetation community, even into mid-late summer when soil water is fairly scarce throughout the landscape. Furthermore, changes in snow depth are not likely to have equivalent impacts on the different community types within a region; the effects will vary predictably as a function of soil and plant community characteristics.