Community-level Seedling Regenerating Mechanism In Tiantong Evergreen Broad-leaved Forest

Understanding the mechanisms maintaining the species coexistence in the community remains a major challenge for ecologists. The seedling regeneration is a key process for maintaining the species coexistence in the community. Therefore, many hypotheses attempted to explain the process of seedling regeneration in the community. The dispersal limitation hypothesis, Janzen-Connell hypothesis (hereafter called J-C hypothesis) and habitat preference hypothesis were three of the most discussed hypotheses. However, each of three hypotheses could not explain the whole process of seedling regeneration in the community perfectly. Therefore, we hope to combine three hypotheses through their temporal differentiation. For this purpose, we investigated the seedlings in 561 seedling plots (1 m2) in the 20-ha stem-mapped forest dynamics plot in a subtropical evergreen broad-leaved forest with rich species diversity in the Tiantong Forest Park, Zhejiang province during the three years (seven census), and designed the mixed regression models which can comprehensively analyze the data with classified variables (species, age, census time, station and plot). We analyze the dynamic changes of these three hypotheses in the different stage of seedling regeneration (newly germinated seedling occurrence, seedling establishment and seedling development). The major results of this study are as follows:(1) In the stage of newly germinated seedling occurrence (in cotyledon expansion stage), the spatial distribution pattern of newly germinated seedlings was mainly limited by the distribution of conspecific reproductive individuals. The newly germinated seedlings preferred to aggregate in their parent tree. Therefore the dispersal limitation was the major mechanism for the spatial distribution of newly germinated seedlings in the community. Meanwhile, a few of micro-habitat factors also had significant effect on spatial distribution of newly germinated seedlings. In particular, the canopy openness had a significant, negative effect on individual distribution of newly germinated seedlings in the community. The newly germinated seedlings preferred to occur in the shade. The habitat preference was the assistant mechanism for the spatial distribution of newly germinated seedlings in the community. J-C hypothesis did not have a significant effect on the spatial distribution of newly germinated seedlings in the community.(2) In the seedling establishment stage (from germinated to established), the J-C hypothesis was the major mechanism for seedling survival in the community. The seedlings preferred to establish away from the conspecific adult neighbors with low conspecific density. Meanwhile, a few of micro-habitat factors also had significant effect on seeding establishment in the community. The canopy openness, as the most important external factor on seedling survival, had a significant, positive effect on seedling survival. The seedlings preferred to establish in the understory with better light environment in the community. The habitat preference was also the assistant mechanism for seedling survival in the establishment stage. However, the effects of J-C hypothesis and habitat preference on seedling survival in the establishment stage were decreased with the seedling age.(3) In the seedling development stage (from established to be a new reproductive individuals), the relative growth rate (RGR) was the major factor which influenced the seedling survival in the short-term temporal scale in the community. This result suggested the RGR was the best indices to represent the survival rate of seedling in the seedling development stage. In addition, only the canopy openness had a significant effect on RGR of seedlings in development stage. The seedlings grew faster in the understory with better light environment in the community. The seedling survival rate would also increase in that environment indirectly. Therefore, the habitat preference was the major mechanism for seedling regeneration in the development stage in the community.Synthesis:In the stage of newly germinated seedlings occurrence, the major mechanism for the spatial distribution of newly germinated seedlings is dispersal limitation which causes the aggregative pattern between the newly germinated seedlings and their parent tree. In the stage of seedling establishment, the J-C hypothesis was the major mechanism for the seedling establishment. The J-C hypothesis would make the seedlings establish away from their parent tree and with low density of conspecific seedling neighbors. This pattern could homogenize the aggregative pattern of newly germinated seedlings and provide the space for heterospecific seedlings regeneration.In the stage of seedling development, the habitat preference was the major mechanism for seedling regeneration. In addition, in the whole stage of seedling regeneration in the community, light habitat preference of seedlings has temporal differentiation. In the stage of newly germinated seedlings occurrence, the newly germinated seedlings prefer to germinate in the low light environment. However in the stages of seedling establishment and development, the seedlings prefer to survival and grow in the understory with better light environment. These results above suggested the three seedling regenerating hypotheses (dispersal limitation hypothesis, J-C hypothesis and habitat preference hypothesis) mainly influenced one of stage of seedling regeneration. They have temporal differentiation obviously. We could combine three hypotheses through their temporal differentiation and improve their predictive ability for the whole process of seedling regeneration in the community.