4.2 Exploring key factors affecting regeneration dynamics along various environmental axes
By analyzing the relationships between environmental factors and the density and species richness of regenerative individuals, we found that altitude, TN, AP, canopy density, and tree density explained the regeneration dynamics of the community more effectively than other environmental factors. These results indicate that at a community level, habitat preferences related to topography, soil nutrients, and light play a key role in regeneration success and that the J-C effect represented by adult density also affects the regeneration pattern. Previous studies have also shown that species distributions and seedling recruitment in tropical forests are related to topography, soil conditions, and light availability. For example, gaps are important in maintaining species diversity and regeneration in ancient or tropical and subtropical forests (Hubbell et al. 1999; Sapkota et al. 2009). In tropical forests, soil type and topography play an important role in shaping diversity, as both of these factors influence water and nutrient availability (Miyamoto et al. 2003; Phillips et al. 2003).
At the regional scale, elevation usually represents the variation in topography and habitat, which may influence soil fertility and forest gaps by affecting the slope, slope aspect, and slope position (Kraft et al. 2008; Brown et al. 2013). In addition, altitude has also been described as being the most important environmental variable by which habitat heterogeneity affects species distribution patterns (Beck and Chey 2008; Rowe and Lidgard 2009). Tree species are particularly sensitive to topographic habitat combinations during the seedling and sapling stages. For example, in a new tropical forest in Ecuador, more than 80% of new species show one or more topographic habitat associations (Metz 2012). Many studies have confirmed that N is an important element necessary for plant growth, and its absence may limit the synthesis of chlorophyll and thus weaken photosynthesis in plants (Fortunel et al. 2014; Mueller et al. 2013). The widespread presence of carbon-fixing (Leguminosae ) plants in tropical forests may also be another reason for the close relationship between TN regeneration (Diaz-Gustavo et al. 2015). Previous studies have shown that phosphorus is generally lacking in tropical forest soils, and the total phosphorus content was mostly below 0.8 g kg-1 (Cleveland et al. 2011). Phosphorus is also a major element contributing to plant photosynthesis, affecting the survival and causing a uniform distribution among regenerating individuals (Long et al. 2011). Forest gaps have a direct effect on the distribution patterns of seedlings and young trees and help the plants in the lower layers of the forest obtain sunlight (Silva et al. 2012; Sharma et al. 2016). Therefore, the canopy density of forests and the density of regenerating individuals tend to show a close relationship in terms of promoting diversity in the later growth stages of seedlings (Suárez-Esteban et al. 2016). In terms of seed dispersal and germination, the density of parent trees tends to have a positive effect on the density of nearby conspecific seedlings (Lowe and McPeek 2014). The composition of the recruited seedlings is highly dependent on the species composition of the surrounding trees, which promote the successful regeneration of seedlings (Winkler 2011). These results indicate that biological mechanisms (negative density-limiting effects) are also important in promoting the recruitment of tree seedlings.