Changes in the space-trait occupation of species in response to drought

Background/Question/Methods

Trait-based approaches are increasingly used to evaluate how plant species respond to environmental changes and their effects on ecosystem processes. Some studies have shed light on plant strategies based on their functional traits in the face of drought, but the change in the species’ occupation of the functional space under limiting and non‐limiting water conditions remains poorly understood. As drought is expected to increase under climate change scenarios, it is important to better know the plant functional responses to cope with water shortage evaluating their new possible strategies as well as their ability to resist resource limitation. It could be expected that species that are more resistant to drought (i.e. those experiencing a smaller decline in biomass compared to non-drought conditions) will be those that are more plastic in response to water stress. To explore how the trait space occupation of species changes in response to drought, we grew 52 herbaceous species in monoculture under drought and well-watered treatments. We measured traits relating to response to drought and characterized the functional trait space using a Principal Component Analysis (PCA). Then, we estimated how each species occupied this space in both treatments via Trait Probability density (TPD) functions. We then estimated the amount of functional space occupied by each species in each treatment, as well as the dissimilarity in trait values between the two treatments (trait plasticity). We used log-ratio biomass and dissimilarity values to map drought resistance in the functional space through generalized additive models (GAM).

Results/Conclusions

We found that species that invested more in safe root tissues but conserved small size were more resistant to drought, being able to even increase their biomass compared to well-watered conditions. Similarly, small species with thicker roots and big leaves are more plastic and experienced trait displacement towards less conservative strategies under water stress. As a consequence, the total variability of traits in the species pool was much smaller in drought than in well-watered conditions, suggesting strong trait filtering acting on conservative species. However, this change was not reproduced at the species level. Our results underline the importance of studying the distribution of species within the functional space in the face of environmental fluctuations and how such changes affect the overall volume of the functional spectra. Knowing these effects of drought is key to better understand plant strategies to withstand drought and anticipate shifts in ecosystem functioning facing climate change.