Abstract:

The zonal vegetation of subtropical regions in China is recognized as the evergreen broad-leaved forest (EBLF), a forest that usually contains many deciduous tree species. Preliminary investigation of a 20-ha plot at Tiantong (East China) showed that the proportion of deciduous trees reached 52.6%. How do these evergreen and deciduous trees coexist? Up till now, this question lacks a sufficient answer. Evergreen and deciduous tree species often differ in ecological habits, and this niche differentiation may be manifested in distinct habitat preferences. These habitat differences may be an important point in explaining the mechanisms by which these two groups coexist. To test this hypothesis, on the basis of the aforementioned investigation at Tiantong 20 ha plot, 55 evergreen and 42 deciduous tree species with abundance more than 20 individuals were selected for analysis. Canonical Correspondence Analysis (CCA) was used to explore the influence of topographic factors on spatial distribution, and torus-translation tests were employed to test the significance of the species-topography (elevation, convexity, and slope) association. Results were as follows: (1) CCA analysis showed that the three topographic factors (elevation, convexity and slope) explained 19.2% of the distribution of evergreen trees and 7.0% of that for deciduous trees. (2) 16.4% of the 55 evergreen trees and 28.6% of the 42 deciduous trees showed a significantly positive association with the valley habitat, and the proportion of negative association with the valley habitat was 40% and 7% respectively; 41.8% evergreen trees and 4.8% deciduous trees showed a significantly positive association with the ridge habitat, and the proportion of negative association with the ridge habitat was 10.9% and 47.6% respectively; 16.4% evergreen trees and 42.9% deciduous trees were positively associated with the disturbed habitat. Our results showed that topographic factors significantly influence the distribution of evergreen and deciduous trees in the Tiantong plot, and this influence seems greater for evergreen trees. Evergreen and deciduous trees generally showed opposite trends in habitat preferences, especially in valley and ridge habitats. This indicates that habitat partitioning may be a major mechanism allowing the coexistence of evergreen and deciduous trees, and that the niche theory, to some extent, may be sufficient in explaining species diversity maintenance in subtropical evergreen forests.

Key words: topography, evergreen trees, deciduous trees, evergreen broad-leaved forest, niche theory, habitat association, coexistence mechanisms