| Tree mortality is an important part of forest dynamics and community assembly, which is affected by many ecological processes. Detecting spatial pattern of snags and driving factors of tree mortality is critical to improve our understanding of tree mortality, population dynamics and the mechanisms of species coexistence.In 2014～2015, we conducted a census of snags (dbh≥10 cm) of the woody plants in the 25 ha Badagongshan (BDGS) forest dynamics plot. Based on this survey data, we used the univariate and bivariate pair correlation function, g(r), to analyze the spatial distribution pattern of snags and the association between snags and live trees. The tree mortality-habitat associations in the plot was tested by Torus-translation method and the relative importance of neighborhood-density and topographical factors on tree mortality were examined by generalized linear mixed model (GLMM). The results showed that:(1) There were 2569 snag individuals, belonging to 34 families,54 genera and 79 species (27 only identified to genus,13 unknown) in the plot. The density of snags in the plot was 102.76 plant·ha-1.The mean dbh was 16.78 cm with a maximum of 83.50 cm. Tree size structure of snags was inverse "J" type.(2) At community-level, snags showed aggregated distribution in most scales of the 0-50 m scale; snags and live trees showed significant negative correlation at 0-3 m scale. At species-level,12 of the 18 common species with snags≥25 were detected significant negative association between live trees and snags at different scale in the 0-50 m range.(3) The number of the species whose mortality were positively associated with valley, slope, ridge was 1,1 and 3 respectively; and the number of species with a negative association was 22,11 and 27 respectively; However, the mortality of most species in the plot had no association with any of the three types of habitats. Among the top 20 species, only 6 species were found to have an association with habitat, while the other 14 species did not show any significant association.(4) Basal area of conspecific neighborhoods showed a significant positive or negative effect on tree mortality at different research levels (community, life form and species); However, basal area of heterospecific neighborhoods showed a constant significant positive effect at all levels; Tree size was significantly negatively correlated with adult and large tree mortality, but non-significantly positively correlated with old trees; Topographic factors (except elevation) showed no significant correlation with tree mortality.In summary, the distribution of snags in the BDGS forest dynamics plot was aggregated, indicating that tree mortality was nonrandom. We found that tree mortality was mainly driven by interspecific competition and intraspecific negative density dependence, and topography conditions had little significant effect on tree mortality. Our study demonstrates that interactions of species tended to be more important in affecting tree mortality than habitat variables in this subtropical evergreen and deciduous broad-leaved mixed forest. The ecological interactions or processes of negative density dependent effects and interspecific competition play a very important role in species coexistence and the maintenance of biodiversity in the subtropical forest. |
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