History and vulnerability of meadows in the Sierra Nevada
Although meadows occupy only a small proportion of the Sierra Nevada landscape, they contribute disproportionately to watershed services, ecosystem productivity, species diversity, cultural and recreational functions, and have often been the focus of wilderness legislation. Threats to meadows posed by anthropogenic activities have been the focus of past land management efforts, but threats posed by climate change may now exceed them. Information is needed to classify meadows based on their relative vulnerability to climate change versus land use factors. As a first step in evaluating meadow vulnerability, we evaluated past and current processes that influence meadow creation and persistence. We then utilized existing vegetation maps for the National Parks of the central and southern Nevada to create a database of >9,000 meadows. Inter-annual and intra-seasonal summer wetness signatures of each meadow were derived from Landsat Thematic Mapper images, and those data were used to infer meadow hydrologic regimes. That information plus other biophysical data were used to distinguish meadows that are highly sensitive to inter-annual variation in climate from meadows with stable hydrology that is less sensitive to climate.
Meadow characteristics are dictated by hydrologic processes controlled by geomorphology which affects soil formation, the amount of surface and sub-surface water, and the duration of seasonal saturation or inundation. Although geomorphology changes at geologic timescales, hydrology can vary over shorter time periods. Beginning 10,000 ybp conditions became warmer and more favorable for plant growth, erosion rates declined and aggradation of mineral sediments and organic material increased, soils developed, and many previous meadows became mostly forested in areas of shallow ground water. Beginning approximately 2,500 to 1,200 ybp, neoglacial conditions included periods of high winter snowpack, late season melting, and high summer water tables which caused tree mortality and retreat of forest ecotones to drier and more upland soils. Thus, most Sierra Nevada meadows may have only existed for a few thousand years. Future climate warming and earlier snowmelt may recreate earlier Holocene conditions that favor woody species over herbaceous species in meadows. However, the relative vulnerability of meadows to these changes likely varies based on their specific geomorphologic and hydrologic characteristics which are currently being evaluated for the National Park meadows of the central and southern Sierra Nevada. Results from those analyses will be reported in this presentation.