Temporal variability in grassland leaf water content is strongly and similarly dependent on soil moisture among dominant grasses

Vegetation in semi-arid rangeland ecosystems is typically characterized by the presence of a few dominant grass species that control ecosystem processes and, by extension, ecosystem services such as forage production. Thus, understanding and comparing the responses of dominant forage species to climatic variability can lend insight into how functioning and services in these ecosystems will respond to climate change. Here, we designed a transect-based study to assess and compare the temporal relationship between soil moisture (volumetric water content, VWC) and a key functional trait (leaf water content, LWC) among two of the dominant forage grass species in Southwestern Montana. Sampling occurred within the context of natural drought; up to the last sampling date, water-year precipitation was 40% below the long-term average. Temporal variation in LWC was strongly and similarly dependent on temporal variation in VWC within the upper 12 cm of the soil profile among the two forage species. As such, we further observed that the time since precipitation was a key driver of VWC, resulting in a declining trend of leaf water status as the growing season drought progressed among the two forage grasses. Our results indicate a strong and nearly identical vulnerability of two key forage grasses to fine-scale fluctuations in soil water availability, suggesting high vulnerability of semi-arid rangeland ecosystems of the western US to climate-driven changes in water availability.