Patterns of tree regeneration and their implications for succession in Himalayan pine-oak forests, India

Factors affecting successional processes are critical determinants of the structural and compositional changes in a plant community. The present study examines the role of plant traits, such as seed size and dispersal mode, habitat factors such as canopy closure, leaf litter, soil moisture, disturbance, fire frequency, and landscape forest cover, in arrival and establishment of tree species in the oak-pine forests of the Western Himalayas. For this, we compared the regeneration, abundance, and composition of tree species in pine forests (pioneer stage) with that in oak-dominated hardwood forests (climax stage). The microsite factors and landscape forest cover varied significantly between the two stages. While species diversity was comparable across seedling, sapling, and pole stages, density of regenerating individuals was relatively lower in oak forest sites. The extent of overlap in species composition between oak and pine forest sites was also found to be low for both regeneration and adult stages. The amount of forest cover surrounding the study sites (landscape forest cover) did not significantly impact regeneration. Instead, factors related to establishment were determined to be more important. Regeneration density decreased with an increase in canopy closure (light environment) and litter depth, implying their crucial role in driving the successional trajectories. When plant traits were considered, all three regeneration stages of large-seeded and animal-dispersed species were of comparable densities in pine and oak forests, suggesting an absence of dispersal or arrival limitation. However, anthropogenic disturbance was found to negatively impact density of zoochorous species while fire frequency negatively influenced the diversity of large-seeded species. Control of extractive pressures, reducing incidence of fire, and retention of protected oak stands may be necessary to enable regeneration of zoochorous and of large-seeded species and thereby, natural succession to oak-dominated hardwood forest in this human-dominated landscape.