UC Merced

Sierra Nevada meadow plant communities are influenced by multiple multi-scale environmental processes ranging from elevation and climate to local soil moisture regimes. In recent years, land managers have faced concerns over potential impacts of recreational pack stock use to these meadow communities. Detecting the effects of such a stressor amongst a large amount of inherent natural variability requires taking into account variation due to environmental processes across multiple temporal and spatial scales. I evaluated the influence of pack stock use within Sequoia and Yosemite National Parks on three meadow plant community responses: 1) total vegetation cover and bare ground, 2) multivariate species composition and abundance, and 3) local-scale spatial variability in plant community structure. The study design accounted for inherent natural variability across multiple scales by: 1) controlling for among-meadow variability by using remote sensing data to select non-stock (`control') sites, 2) accounting for within-meadow variation in the local hydrology using in-situ soil moisture readings and 3) controlling for variation in stock use patterns by sampling across the entire available gradient of stock use. Increased cover of bare ground was detected only within "dry" meadow areas at the two most heavily used stock meadows. No difference in species composition or abundance was detected for any level of soil moisture or stock use. Increased local-scale spatial variability in plant community structure was detected in "wet" meadow areas at the two most heavily used meadows. These results suggest that at the meadow scale plant communities are generally resistant to the contemporary levels of recreational pack stock use studied. However, within-meadow responses such as increased bare ground can be context-dependent as a function of local-scale hydrological conditions and the ability to detect such effects may be dependent on short term (1-2 years) antecedent site conditions and use levels.