Density dependence has long been considered an important mechanism for species coexistence in forests. Density-dependent processes can be important mechanisms driving differences in species diversity across latitudes. Here we examined the decline in strength of density dependence with increasing latitude, and particularly how density dependence affected both conspecifics and heterospecifics.
Conspecific individuals within a species were predominantly aggregated at the three different latitudes of the three study sites in China. The percentage of aggregated species declined with increasing spatial scale and growth stages, which confirmed the overall importance of density dependence. Compared with a latitudinal gradient, the intensity of aggregation in the most northerly temperate (Changbaishan) plot was significantly higher than that in the tropical (Bawangling) or subtropical (Heishiding) plots. This showed that the strength of density dependence among conspecific individuals at low latitudes was stronger than that for high latitudes. We found that the more closely related species were more spatially adjacent in the temperate plot, while the opposite was true in the tropical and subtropical plots at most scales. After calculating the recruitment probability of all species of mature trees, we found that 19 of the 32 species in the tropical plot and 7 of the 12 species in the subtropical plot were less likely to recruit near closely related species. In the northern temperate plot, only one species demonstrated this phenomenon. These results therefore suggest that latitudinal variation in the intensity of negative density-dependent recruitment resulting from specialist natural enemies (the Janzen–Connell hypothesis) may contribute to the latitudinal gradient of diversity in trees.
The strength of density dependence at low latitudes was stronger than at high latitudes, regardless of whether this dependence was measured between only conspecific individuals or between individuals of closely related species.