Seedling density dependence regulated by population density and habitat filtering: Evidence from a mixed primary broad-leaved Korean pine forest in Northeastern China

Key message The effects of distance dependence, negative density dependence (NDD), phylogenetic density dependence, and habitat filtering were integrated to provide additional evidence in temperate forest tree seedling survival. The main focus of this study was to explore how population density and habitat filtering regulate NDD. An approach involving four classes of population density and three classes of soil moisture was tested, including the effect of habitat variables to more accurately evaluate the underlying ecological processes affecting the density dependence of seedlings. Context NDD is an important mechanism for the maintenance of species diversity across multiple life stages, particularly during seedling recruitment. By regulating specific population structures to maintain species diversity, the effects of density dependence and distance dependence are sometimes difficult to distinguish. Nevertheless, the contribution of NDD to community assembly, relative to other processes such as habitat filtering, remains a subject of debate. Recently, it has been reported that seedling survivals are also negatively correlated with phylogenetic relatedness between neighbors and focal individuals. This effect is known as phylogenetic negative density dependence (PNDD). However, another opposite effect known as phylogenetic positive density dependence (PPDD) has also been reported to exist. Aims The objectives of this study are to examine the following: (i) how population density affects negative density dependence (NDD); (ii) how habitat filtering regulates the NDD; (iii) whether more evidence can be found for PNDD or PPDD and why; and (iv) whether the intensity of negative density dependence is affected by the distance between parent trees and seedlings. Methods The study was conducted in a 20-ha primary mixed broad-leaved Korean pine forest in Changbai Mountain of China. We used generalized linear mixed models to analyze how the seedling survival of 23 woody plant species relates to neighborhoods and habitat variables. Four models were established with and without habitat variables, and two of the four models were used to test how different population densities of focal seedlings and different gradients of habitat variable regulated negative density dependence. Results The following results were obtained: (1) the strongest conspecific negative density dependence (CNDD) was found within a radius of 15 m; (2) seedling survival were most strongly impacted by the density of conspecific seedling and adult neighbors in habitats with relatively low soil moisture; (3) the effect of seedling-seedling CNDD was especially significant, when densities ranged from 20 to 40 seedlings/4 m(2), and (4) there were some evidences of phylogenetic positive density dependence (PPDD), and the effect of seedling-seedling PPDD was increasing with an increase in soil moisture. Conclusion Our results demonstrate that conspecific negative density dependence played an important role in seedling survival, which is closely related to habitat filtering and population density. However, we found some evidences of phylogenetic positive density dependence. We suggest that future studies of neighborhood density dependence should increase awareness of evolutionary relationships.