Intraspecific trait changes in response to drought lead to trait convergence between- but not within species

Drought is expected to increase in future climate scenarios. Although
responses to drought of individual functional traits are relatively
well-known, simultaneous changes across multiple traits in response to
water scarcity remain poorly understood despite its importance to
understand alternative strategies to resist drought. We grew 52 herbaceous
species in monocultures under drought and control treatments and
characterized the functional space using seven measured above- and
belowground traits: plant height, leaf area, specific leaf area, leaf dry
matter content, specific root length, average root diameter, and root dry
matter content. Then, we estimated how each species occupied this space
and the amount of functional space occupied in both treatments using trait
probability density functions. We also estimated intraspecific trait
variability (ITV) for each species as the dissimilarity in trait values
between the individuals of each treatment. We then mapped drought
resistance and ITV in the functional space using generalized additive
models. The response of species to drought strongly depended on their
traits, with species that invested more in root tissues and conserved
small size being both more resistant to drought and having higher ITV. We
also observed a significant trend of trait displacement towards less
conservative strategies. However, these changes depended strongly on the
trait values of species in the control treatment, with species with
different traits having opposing responses to drought. These contrasting
responses resulted in lower trait variability in the species pool in
drought compared to control conditions. Our results suggest strong trait
filtering acting on conservative species as well as the existence of an
optimal part in the functional space to which species converge under
drought. Our results show that changes in species trait-space occupancy
are key to understand plant strategies to withstand drought, highlighting
the importance of individual variation in response to environmental
changes, and suggest that community-wide functional diversity and biomass
productivity could decrease in a drier future. Knowing these shifts will
help to anticipate changes in ecosystem functioning facing climate change.