Evergreen vs. deciduous species: Whose water use efficiency can influence the plant trait spectrum in the karst evergreen-deciduous mixed broad-leaved forests?

Unraveling the drivers of the plant trait spectrum is crucial for explaining species coexistence, especially in biodiverse ecosystems. Focusing on the Maolan Nature Reserve, a typical karst evergreen-deciduous mixed forest, this study examined the contributions of water use efficiency (WUE) differences between those species to their trait spectra in 30 plots by measuring the plant functional traits. WUE characterization based on stable carbon isotopes and modeling revealed significant WUE and plant trait spectrum differences between evergreen and deciduous trees. In addition, their WUE could significantly influence their functional traits, functional diversity, and leaf economic spectrum. Moreover, the WUE of deciduous species played a more important role in influencing the plant trait spectrum compared to evergreen species. Specifically, compared to evergreen trees, deciduous trees contributed more to the functional diversity, mainly by altering the niche overlap. The findings demonstrated the key role of deciduous species in ecosystem functioning and highlighted the importance of niche differentiation for species coexistence in karst forests.