In California, mountain meadows provide an invaluable service by absorbing meltwater from winter snowpack and slowly releasing it downstream throughout the summer season. Plant communities that make up these meadows play an important role by contributing annually large amounts of organic material to meadow soils, reinforcing a meadow’s “sponge like” attributes. Droughts, such as the recent 2012-2015 California drought, can shift plant communities away from moist meadow species that produce high biomass towards drier non-meadow species. Added to this is growing concern that management activities, such as grazing, may exacerbate climate impacts by altering soil hydrologic conditions, further reducing water availability for meadow plants. Isotopic carbon (δ13C) composition in plants is sensitive to water availability and relates to physiological response to water stress; nitrogen (δ15N) composition relates to ecosystem N cycling and can be influenced by soil water and enrichment from grazing animals. We assessed the physiological stress of two perennial and dominant moist meadow indicator species (Calamagrostis breweri and Oreostemma alpigenus) by producing isoscapes of measured leaf (δ13C) and nitrogen (δ15N) compositions sampled in grids across pack stock grazed (N=3) and ungrazed (N=3) meadows during a major drought year (2012) in Sequoia National Park, CA.
Leaf δ13C and δ15N composition varied heterogeneously across meadows and differed among species. Average δ13C and δ15N values were -27.94 ‰ and -3.41 ‰ for O. alpigenus, and -27.12 ‰ and -3.51 ‰ for C. breweri, respectively. Permutation tests showed no overall differences (p < 0.001) between grazed and ungrazed meadows in δ13C and δ15N composition for either species. Rather, δ13C and δ15N values in both plant species were closely related to soil water contents. For example, δ13C exhibited a negative, non-linear relationship with soil water content. Spatially interpolated isoscapes of meadow surfaces showed areas with the highest δ13C values (i.e., most water stressed) were often near meadow edges. In grazed meadows, lower δ13C values for C. breweri was associated with higher overall community richness, suggesting that grazing disturbance could be promoting co-occurrence and facilitation among species. Plant δ15N values exhibited a linear negative relationship with soil water content across grazed and ungrazed meadows for both species. In the driest sections of meadows, δ15N approached near atmospheric concentrations. The use of isoscapes sheds light on how plant physiology can be spatially influenced by drought and allows us to place local anthropogenic disturbances in the context of larger scale climatic pressures.