Common‐garden experimental evaluation of phylogenetic and functional diversity as drivers of plant performance

Biodiversity is known to influence the performance of coexisting species and has been proposed as a management tool to enhance the resilience and functioning of plant communities under drought. However, experimental studies to disentangle the roles of phylogenetic and functional diversity as drivers of plant performance remain extremely scarce. We conducted a common‐garden trial creating eleven experimental annual plant assemblages spanning broad phylogenetic (phylogenetic species variability ranging from 0.23 to 0.93) and functional diversity, capturing major axes of plant strategy variation (specific leaf area, seed mass and maximum height and a multi‐trait index) and combined with two levels of water availability treatments. We quantified species performance by measuring the proportion of fruiting individuals and the average seed production per species and pot. To our knowledge, this experimental design is entirely novel within community ecology and enabled us to evaluate how combined phylogenetic and functional diversity influence species performance and fitness under average precipitation versus severe drought. The study provides experimental evidence that phylogenetic and functional diversity jointly shape the performance and reproductive success of coexisting species, with effects strongly mediated by water‑stress conditions. High phylogenetic diversity enhanced plant performance, particularly under severe drought conditions, likely due to niche differentiation in water‑use strategies and variation in drought tolerance among species. In contrast, greater functional diversity in maximum plant height and specific leaf area showed a unimodal response on plant performance, probably reflecting stronger competitive asymmetries. Our results suggest that, high phylogenetic diversity enhanced the positive effects of functional differentiation on plant performance by buffering competitive asymmetry and promoting resource partitioning. However, increase in functional diversity alone may reduce performance in closely related species assemblages. Thus, the balance between the effects of functional and phylogenetic diversity should be considered to properly identify the mechanisms that promote species coexistence.