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.