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

Abstract

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).

Interactions between beech and oak seedlings can modify the effects of hotter droughts and the onset of hydraulic failure

“…Additional microclimate measurements would be needed to confirm this mechanism. Moreover, as previously observed (Bussotti et al ., 1995; Grossoni et al ., 1998; Cavender‐Bares et al ., 2007; Watson et al ., 2023), both species reduced their minimum stomatal conductance ( g min ), individual and whole‐tree leaf area (LA leaf and LA total ) (Fig. S5), and specific leaf area (SLA) under D and HD, thereby showing a long‐term acclimation strategy to limit water loss.…”

“…2; Table S1). These findings contradict LA leaf reductions in monocultures vs mixtures observed in grasslands (Watson et al ., 2023), which could be related to differences between grasses and trees or the relative young age of our seedlings. Nevertheless, these anatomical adjustments were insufficient for beech to overcome the higher competitiveness of oak.…”

“…Cohabiting tree species often exhibit distinct hydraulic strategies to deal with drought, enabling such beneficial interactions (Grossiord, 2020). For instance, mixing juvenile trees can improve the microclimate and diminish atmospheric drought impacts due to a cooler, more humid atmosphere reducing the vapor pressure deficit (VPD) (Wright et al ., 2021; Watson et al ., 2023). Many studies in various plant communities from herbaceous to forests have reported such facilitative processes (e.g.…”

Atmospheric drying and soil drying: Differential effects on grass community composition

“…Species labels are located on the ordination based on their species scores and rescaled for visualization with pot communities. (Watson et al, 2023). Also, P. secunda gains a benefit from growing in higher diversity plant communities during drought, likely due to microclimate amelioration in these communities (Aguirre et al, 2021).…”

“…(2021) manipulated atmospheric relative humidity in an outdoor mesocosm experiment and demonstrated that soil drought only limited aboveground biomass when atmospheric conditions were also dry. Another study conducted in this same experimental system demonstrated that reducing atmospheric relative humidity caused one grass species ( Poa secunda ) to allocate energy belowground at the expense of aboveground structures, but this also only occurred in combined soil drought and atmospheric drought conditions (Watson et al., 2023). A recent meta‐analysis identified reduced stomatal conductance, decreased leaf area, increased leaf abscisic acid, and increased root:shoot ratio as common responses of plants to increased VPD across a wide range of studies (López et al., 2021).…”