How evergreen and deciduous trees coexist during secondary forest succession: Insights into forest restoration mechanisms in Chinese subtropical forest

Abstract

Deciduous broadleaved forests and evergreen broadleaved forests constitute the majority of global subtropical forests. Subtropical vegetation has faced strong anthropogenic pressures, and many natural forests destroyed throughout history have been restored to secondary forests due to various government conservation policies. In early forest restoration, deciduous and evergreen species with significantly different functional traits play different roles and jointly determine secondary forest successional trends, questioning how coexistence and competition between evergreen and deciduous trees profoundly impact community assembly and forest restoration. Although subtropical forest has been seriously threatened, scientific information on how these important ecological processes influence secondary forest regrowth is lacking. We established a 6-ha plot in a subtropical evergreen broadleaved forests in Guanshan National Nature Reserve, southeastern China. All trees individual were stem-mapped, identified to species and measured for diameter at breast height (DBH) and height. Pair-correlation and marked correlation functions indicated that deciduous species are comparatively more vulnerable to density-dependent regulation and size reduction in adjacent individuals during secondary forest succession. The interspecific association between deciduous and evergreen trees in the same DBH class gradually changes from significantly positive to negative as DBH increases, indicating seedling coexistence and competition between large late successional trees at a fine scale. Our results indicate that self-thinning caused by conspecific density-dependent regulation in deciduous trees and interspecific repulsion between evergreen and deciduous trees lead to forest gaps where clustered offspring naturally regenerate to coexist, embodying the important coexistence mechanisms facilitating secondary forest recovery to the zonal climax vegetation types dominated by evergreen species in this region. These findings improve our understanding of the self-recovery mechanisms of subtropical forests in secondary succession and provide important theoretical support to accelerate the future recovery of secondary forests.