Evergreen broad-leaved forests are common in the Asian humid subtropical zone and are characterized by coexistence of deciduous and evergreen broad-leaved tree species in the same climatic regime, raising questions as to the mechanisms involved. Leaf lifespan strongly influences whole-plant carbon- and nutrient-economies in relation to resource availability, often leading to partitioning of species with contrasting leaf habits into different environments, which could facilitate coexistence. Using leaf habit as a proxy for lifespan, we investigated the role of canopy gaps and topographic variation on the coexistence of evergreen and deciduous broad-leaved species in a subtropical evergreen broad-leaved forest in eastern China, focusing on the seedling stage. We used data on 8548 seedlings of 122 woody species, across 32 paired gap (higher light) and understory (lower light) sites in valley (higher soil nutrient supply) and ridge (lower soil nutrient supply) habitats in Gutianshan Forest Dynamics Plot. Using mixed models, we (1) tested the hypothesis that evergreen and deciduous broad-leaved species exhibit contrasting habitat preferences, (2) investigated demographic correlates of species’ habitat preference, and (3) tested if the variation in height- and density-dependent seedling growth and survival associated with contrasting habitats and leaf habits is a mechanism for species’ habitat preferences.
Results/Conclusions
While seedlings of evergreen species were abundant in all habitats, deciduous species were significantly more associated with habitats with greater resource availability, namely canopy gap and valley sites, compared to understory and ridge sites. Deciduous species achieved height-dependent growth advantage over evergreen species in canopy gap and valleys, compared with understory and ridge sites. Consistent with their long leaf lifespan, evergreen species had higher survival than deciduous species in all habitats. However, height-dependent survival was stronger for deciduous species, suggesting that these species may be more favored in habitats that enable them to achieve larger size quickly. Survival and growth depended strongly on both seedling height (positive effects) and conspecfic neighbors (negative effects) for both deciduous and evergreen species. In sum, we conclude that habitats with greater supply of above and belowground resources, such as fertile valleys and bright canopy gaps, play critical roles in promoting seedling regeneration of deciduous species, which may contribute to the local-scale coexistence of species with contrasting leaf habits in evergreen broad-leaved forests.