Print PageMany grassland ecosystems worldwide are experiencing woody plant encroachment, including high-elevation meadows in the Sierra Nevada, California. Hypothesized causes of encroachment are numerous and include changes in land management and climate. Conversion of these meadows to forest may result in loss of important ecosystem services, such as water storage and release, which they provide. This ongoing study aims to determine the extent and severity of conifer encroachment in subalpine meadows, to identify factors limiting and/or facilitating encroachment, and to examine effects of encroachment on subalpine meadows’ ecohydrology, with the goal of informing meadow management and restoration. Here we present results regarding the extent of conifer encroachment, determined from observational surveys, as well as demographic and dendroecological data on encroaching lodgepole pines which allow us to examine the timing of encroachment. We conducted detailed field surveys of 32 subalpine (above 9000 ft) meadows in wilderness areas of Yosemite National Park and rapid assessments of 36 meadows on the western side of the central Sierra Nevada. Encroaching conifer ages were estimated both in the field, using node counts, and in the lab, using tree ring analysis; tree age correlated with height (R2 = 0.708), allowing us to estimate initial establishment dates.
Results/Conclusions
2009 and 2010 field surveys in Yosemite National Park indicate that encroachment is ubiquitous up to the krumholtz vegetation zone, and rapid assessments outside Yosemite indicate that this pattern occurs throughout the central Sierra Nevada range. Within meadows, encroaching conifers display various spatial patterns and a range of spatial heterogeneity. Patterns include islands of encroachment, systematic encroachment from one end, dense encroachment from the edges, and limited scattered encroachment. In regards to timing, trees appear to be encroaching in synchronous waves, with a strong increase in encroachment around 1940-1975, a decrease in the late 1970s, and an increase since the early 1980s. Maximum potential germination appears to be strongly dependent on moisture regime, being highest in areas of moderate water availability. Seed availability supplies a further filter on first-year seedling abundance. Re-visitation of young (<10 years) encroachers indicates that the establishment phase (i.e. survival to 2-3 years) is a key stage of encroachment, and that continuing survival rates are high once a seedling passes the critical 2-3 year threshold. This data regarding extent/severity and timing of encroachment gives us a better understanding of key processes related to vegetation change at high elevations, in a system important to the region’s hydrology.
