Growth and physiological responses of subalpine forbs to nitrogen and soil moisture: investigating the potential roles of plant functional traits

Anthropogenic inputs of biologically available nitrogen (N) and climate change are simultaneously altering N and soil moisture availability in terrestrial ecosystems. Yet, plant responses to concurrent changes in both N and soil moisture in non-grassland ecosystems remain poorly understood. Our objective was to investigate how rooting depth and N-fixing ability—two functional traits we expected to mediate soil moisture and N limitations—influence forb responses to N and soil moisture availability in the Rocky Mountains USA. We assessed the growth and physiological responses (i.e., chlorophyll fluorescence, transpiration rate, and floral display) of four subalpine forb species to N additions across a naturally-occurring soil moisture gradient during one growing season. Soil moisture had a stronger positive effect on growth in shallow-rooted species and N additions had a stronger positive effect on photosynthetic capacity in species without N-fixing abilities. Transpiration rates were not consistent with soil moisture limitations expected for shallow-rooted species, and soil moisture and N had a neutral or negative influence on maximum floral displays across species. Nitrogen and soil moisture appeared to each limit separate response variables in some cases and we did not observe any N?×?soil moisture interactions. These findings suggest that shallow-rooted species may be more vulnerable to increased drought severity and that increased N availability may disproportionately benefit species without N-fixing abilities. However, mixed support for our hypotheses suggests that environmental conditions and functional traits not evaluated here likely influence subalpine plant responses to soil moisture and N availability.