Root flexibility and functional convergence: how drought reshapes plant trait space

Understanding how plant functional traits vary within and among species is central to predicting ecosystem responses to climate change. Functional diversity arises from the variation of traits within and among species, influencing productivity, stability, and ecosystem functioning. While aboveground traits have been extensively characterized, fine-root traits and their intraspecific variability (ITV) remain less explored, particularly in response to environmental stress such as drought. Because traits jointly determine plant performance and community dynamics, assessing how drought affects both the magnitude of trait variation and the structure of functional space is key to understanding plant strategies to withstand water limitation.

Here, we combined analyses of above- and belowground traits across 52 European herbaceous species grown under drought and control conditions to examine how drought alters the partitioning of trait variation and the occupation of functional space. Using linear mixed models, PERMANOVA, and trait probability density functions, we decomposed variance across hierarchical levels and mapped ITV and drought resistance in functional space.

We show that, although among-species differences explain most of the total trait variance, fine-root traits display markedly higher ITV (up to 50%) than analogous leaf traits. Drought did not change the relative contribution of ITV but led to a reduction in overall functional space occupancy, indicating convergence of species toward similar trait combinations. Species investing more in dense root tissues and maintaining smaller size were both more resistant to drought and more variable within species, highlighting the role of ITV in drought resistance.

These findings reveal that drought acts as a strong trait filter leading to functional convergence among species, while substantial intraspecific flexibility—particularly in root traits—supports species persistence and helps maintain community resilience and ecosystem functioning under increasing water deficits. This work underscores the importance of incorporating ITV into biodiversity monitoring and modeling to better anticipate shifts in ecosystem functioning in a changing climate.