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 are 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 affect the functional traits of a focal grass species (Poa secunda) growing in monoculture and 8-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).

We measured soil moisture at three depths on three dates throughout the experiment: February 7, February 27, and March 9. We measured 6mm volumetric water content (VWC %) using the ML3 ThetaProbe soil moisture sensor (Dynamax, Houston, TX, USA). We measured 60mm and 150mm VWC using a PR2 Profile Probe and HH2 Moisture Meter with access tubes installed in the center of each pot (Dynamax, Houston, TX, USA). We averaged soil moisture for the three dates at each depth to have a single soil moisture value (VWC %) per pot per soil depth.

For our model species, P. secunda, we measured leaf area, specific leaf area (SLA) and stomatal density in both monoculture and higher diversity mixtures. For belowground traits (total root biomass, root:shoot ratio, and fine root:coarse root ratio at two depths) we could not differentiate between plant species and thus we examined whole community traits. We harvested aboveground biomass on May 14, 2020. Leaves were selected from harvested biomass. Because leaves were chosen from harvested biomass (not rooted plants) we could not sample the most recently expanded leaf of each plant. Instead, we systematically selected the two largest P. secunda individuals and measured all parts from base to stem. We included the largest leaf per each of these two individuals per pot in our analysis (N=2 leaves per pot). 

We calculated leaf area using a flatbed scanner, then outlined and measured the leaf (ImageJ, Bethesda, Maryland).  Leaf samples were dried to a constant weight in an 80° C oven. We created stomatal peels of each individual leaf using clear nail polish (Sinful Colors Professional Clear Coat, Beltsville, Maryland) to count stomata (Hilu & Randall 1984).

Belowground biomass was harvested in September 2020 due to delays imposed by the COVID-19 pandemic. Given that the species in this study are perennial species and had entered dormancy by the second harvest, we do not expect the delayed harvest would have any effect on belowground biomass production. For root collection, we took one soil core per pot (N=32) separating the sample into two depths: 0-16.5 cm, and 16.5 cm-33cm. Soil cores were rinsed through mesh sieves to separate roots. Roots were categorized into fine (<2mm) and coarse (>2mm) roots using a digital caliper. Roots were then dried to a constant weight in an 80° C oven.