The experimental manipulation of atmospheric drought: Teasing out the role of microclimate in biodiversity experiments

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..., 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 le...,