DOI: 10.5061/dryad.j0zpc86kv
title: Data from: The experimental manipulation of atmospheric drought: Teasing out the role of microclimate in biodiversity experiments

description: 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).
description: 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, <em>P. secunda</em>, 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
 <em>P. secunda</em> 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 (&lt;2mm) and coarse
 (&gt;2mm) roots using a digital caliper. Roots were then dried to a
 constant weight in an 80° C oven.