Pinus massoniana population dynamics: Driving species diversity during the pioneer stage of ecological restoration

As a dominant and active component across ecological restoration processes, the population dynamics of pioneer tree species significantly affect species composition and community assemblage. To assess the effect of pioneer populations on species diversity, this study investigated species diversity and population characteristics in five secondary forests dominated by Pinus massoniana in southeastern China. For 40 years, all anthropogenic activities (e.g., logging, farming, etc.) have been banned on the hillsides of these five secondary forests to promote afforestation, for which these forests have thereafter been left to grow naturally. We found that: (1) the basal area (BA) of P. massoniana drove changes in species diversity across forest plots, explaining 31.6% of total variation. The species richness of the herb layer was negatively correlated to BA, while the diversity and evenness of the tree and shrub layers were both positively correlated to the BA. (2) As the size of individual P. massoniana specimens increased, the trend in species richness of the shrub layer and the diversity indexes of both the tree and shrub layer was first to increase and then to decrease once again, while the species richness of the tree layer stabilized after reaching a certain threshold. (3) Species diversity of the forest plots did not improve over time along with an increase in the density of the pioneer species. When the P. massoniana population density was low, the diversity indexes of both the tree and shrub layer increased with an increase in population density. Conversely, when the P. massoniana population density reached 2800 plants/ha, species diversity began to decline. However, when the P. massoniana population density increased to 3733 plants/ha, species diversity of the plant community exhibited an upward trend. Our results indicated that forest species diversity was extremely sensitive to P. massoniana population dynamics during the pioneer stage of ecological succession. Moreover, the pioneer population was the driving factor for both development and change in species diversity, and it did so through habitat adjustment of its ecological relationships.