Soil versus atmospheric drought: A test case of plant functional trait responses.

Climate change alters mean global surface temperatures, precipitation regimes, and atmospheric moisture. Resultant drought affects the composition and diversity of terrestrial ecosystems worldwide. To date, there have been no assessments of the combined impacts of reduced precipitation and atmospheric drying on functional trait distributions of any species in an outdoor experiment. Here, we examined whether soil and atmospheric drought affected the functional traits of a focal grass species (Poa secunda) growing in monoculture and eight-species grass communities in outdoor mesocosms. We focused on specific leaf area (SLA), leaf area, stomatal density, root:shoot ratio, and fine root:coarse root ratio responses. Leaf area and overall growth were reduced with soil drying. Root:shoot ratio only increased for P. secunda growing in monoculture under combined atmospheric and soil drought. Plant energy allocation strategy (measured using principal components) differed when P. secunda was grown in combined soil and atmospheric drought conditions compared with soil drought alone. Given a lack of outdoor manipulations of this kind, our results emphasize the importance of atmospheric drying on functional trait responses more broadly. We suggest that drought methods focused purely on soil water inputs may be imprecisely predicting drought effects on other terrestrial organisms as well (other plants, arthropods, and higher trophic levels).