Spatial and temporal patterns of conifer encroachment into subalpine meadows in the central Sierra Nevada, California

Background/Question/Methods

Many 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 Sierran 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, timing and severity of conifer encroachment in subalpine meadows, and to identify factors limiting and/or facilitating encroachment, with the goal of informing meadow management and restoration. We conducted detailed field surveys of 30 subalpine (above 9000 ft) meadows in wilderness areas of Yosemite National Park and rapid assessments of 48 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. Environmental factors were derived from PRISM data and the USGS National Elevation Dataset. 

Results/Conclusions

Four years of surveys (2009-2012) in Yosemite National Park indicate encroachment is ubiquitous up to the krumholtz vegetation zone near treeline, and rapid assessments throughout the central Sierra Nevada range show similar patterns. Meadows are more likely to be heavily encroached (> 0.2 trees/m²) if they have a shallow slope (p=0.034) and high mean annual temperature (p=0.005). 

Meadows show three temporal patterns, with encroachment starting approximately 1960, 1970, or 2000, and thus have either many seedlings (<6 years) or many adult trees (>10 years). When comparing our observations to studies from the 1970s-1980s, we see similar timing of encroachment up to the 1960s-1970s. However, previous studies report higher densities of young trees than we currently observe for the corresponding cohorts. This indicates that trees ≤10 years have high mortality with older trees mostly surviving to 2012. Our repeat observations similarly identify establishment to 6 years as a key phase of encroachment while seed traps indicate seed is plentiful. This data regarding severity and timing of encroachment gives insight into which meadows are most at risk of heavy encroachment, when encroachment has occurred, and which lifestages exert the greatest control on encroaching conifers in a system important to the region’s hydrology.