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.

The interspecific variations of plant functional traits can characterize the niche positions of species within communities, while the intraspecific variations can accurately display the species' niche breadth. Revealing relative contributions of intra- and interspecific variations to plant functional community structure is crucial in understanding how the species coexist together, especially in species-diverse ecosystems. To explore how the intra- and interspecific variations of plant functional traits change along the successional pathway in heterogeneous conditions, we established a series of plots and measured main plant functional traits along the natural regeneration in karst forest ecosystems. By quantifying the intra- and interspecific variations of plant functional traits, we found that the changes in intraspecific variations were relatively lower compared to changes in interspecific variations throughout the natural regeneration. Further analysis showed that the community spatial structure contributed more to the intraspecific variations of plant functional traits, while the soil physicochemical properties contributed more to interspecific variations. Our study suggested that tree species might tend to narrow their niche and change the positions to release the niche overlap when faced with heterogeneous habitat conditions.