How topography and neighbor shape the fate of trees in subtropical forest restoration: Environmental filtering and resource competition drive natural regeneration

The structure of plant communities at local scales depends on both the spatial heterogeneity of abiotic environmental factors and the biotic interactions within the community. However, although environmental filtering due to microtopographic heterogeneity and resource competition among plants caused by spatial variation in tree density and size are considered to be very important in explaining the mechanisms of community assembly, their effects on the processes of individual mortality and recruitment in natural forest regeneration, as well as their relative contributions, are still poorly understood. To address this, we established a 12-ha permanent plot in a subtropical evergreen broad-leaved forest area and measured microtopographic variables such as elevation, slope, aspect, and terrain position index (TPI) using a total station. We monitored the individual mortality and recruitment in forest natural regeneration through repeated surveys at 5-year intervals. We fitted spatial covariance models to jointly use multiple factors from three groups of variables (microtopographic effect, neighborhood density effects, neighborhood size effects) as explanatory variables to analyze their roles in driving the mortality and recruitment of all individual and 12 dominant species in forest natural regeneration at the neighborhood scale. Our results show that: (1) In the crucial early stages of secondary forest restoration, natural regeneration is influenced by a synergy of environmental filtering, due to microtopographic heterogeneity, and resource competition among plants. (2) For distinct species responses, evergreen dominant species’ mortality is largely explained by neighborhood effects, while deciduous species are more affected by topographic factors. Furthermore, the adverse effects of larger conspecific trees on younger trees indicate a pattern of competitive pressure leading to mortality among regenerating trees, such pattern emphasis the influence of parent trees on natural regeneration. (3) As trees grow, their interaction with these stressors evolves, suggesting a shift in their resource acquisition strategies and response to neighborhood effects and environmental factors. Despite these changes, the relative importance of topographic factors in determining survival and recruitment success remains constant. This research highlights the importance of considering both environmental and neighborhood effects in forest management, particularly in early secondary forest restoration.