Seedling density in the regeneration layer is an important property for successful regeneration. Our results demonstrate that the average regeneration density of CBNP was 3,674 ± 1,602 trees per ha (see results section). This mean density is considerably higher than that of sub-tropical forests [4], but comparable with other forest locations in Vietnam, such as the Highlands forest (around 3400 tree per ha) [46] and limestone forests in Quangninh Province, Vietnam (3814 tree per ha) [54]. However, in Vietnam even higher regeneration densities have been reported. For example, in the Cat Tien National Park, tree regeneration density ranges from 2850 to 8150 trees per ha [45]; in other broadleaf evergreen forests of Vietnam (Xuan Son National Park) densities reaching around 35000 trees per ha have even been reported [55]. Since we could not identify any specific environmental factor explaining variation in regeneration density, we can only speculate about the most important drivers. It is known from studies in various biomes around the world that light availability plays a crucial role in regeneration abundance and distribution [3, 6, 56]. It is likely that the narrow range of light availability (from 8.21 (± 2.75) to 10.37 (± 11.68), e.g. for ISF see Table 5) in our study prevented us from confirming its importance in our case. However, even if significant differences in light availability only partially explain regeneration density [56], it is known from other studies that disturbances due to logging [46], livestock browsing, and microsite characteristics [17] are additional explanatory factors in seedling density variation. However, in our study, environmental factors and human disturbances did not appear to affect tree regeneration density (Table 1, Table 2). Our results suggest that competition within the regeneration layer may also play a role, indicating the importance of dominant tree species [57]. Thus, the eight most dominant tree species in the regeneration layer accounted for 54% of all seedlings and the 16 most dominant tree species, representing 72% of total seedling abundance (see Appendix A). Our inconclusive results underscore the need for additional research to explain regeneration density more mechanistically. Approaches should focus more on species traits, such as how fruit coat requires specific environmental conditions to allow successful germination and establishment [58].
Many studies have used seedling, sapling, and mature tree species densities as criteria for forest regeneration evaluation status [4, 7, 59]. Forests are classified as having good regeneration potential when number of seedlings > number of saplings > number of trees; the potential is poor if the numbers of seedlings and saplings are fewer than the present mature tree species [4, 7, 59]. We question the suitability of this approach for some forest types since it does not take developmental stages into account; for example, where mature tree density is so high that regeneration is inhibited due to low light availability. These forests should not rate as poor since their potential for regeneration may still be high. We modified this approach, focusing on species richness and diversity indices of the tree regeneration and overstory layer rather than on tree density. Even though this approach is also quite simplistic and may not consider different recruitment events over time that may have shaped the regeneration as well as the overstory [60], relating overstory and regeneration richness and diversity can give insights to potential trajectories of tree species richness. We found that tree species richness and diversity in the regeneration layer was lower than in the overstory layer (see Fig. 1, Fig. 2, Fig. 3). The 97 tree species that were found in the regeneration layer accounted for 71% of the overstory tree species (136 tree species) (see results section, Appendix D, Appendix E). After extrapolation for completeness of sample coverage, species richness in the overstory was 1.43 times higher than species richness in the regeneration layer (see results section, Fig. 3). Our results are comparable to the other studies conducted in Vietnam. Tran, et al. [46] found 107 tree species in the sapling stratum and 90 tree species in the seedling stratum compared to 144 tree species in the overstory layer in an evergreen broadleaf forest. Blanc, et al. [45] reported tree species numbers of 92, 83, 53, 1, and 43 respectively in five one ha sample plots in the overstory layer of Cat Tien National Park, whereas the number of regeneration tree species were 50, 52, 20, 1, 24, respectively.
The found poor status of species richness in the regeneration layer in our study was verified by the various ratios (Fig. 4, Table 3). In addition, separating the regeneration into height classes indicates that the gap between overstory and regeneration richness and diversity is even increasing with time, as the ratios were highest for the largest height class representing the oldest regeneration (Appendix F). Our results may therefore hint towards potential community alterations in the future that have been observed in other tropical forests [61, 62]. Decreasing species dispersal by large vertebrates is mentioned as an important factor for such community alterations [61]. In our study, only 38% of the regenerating tree species came from overstory tree species (same species ratio), 30% came from outside the plots (newly occurring species ratio) (Table 3). The trend was also observed for the threatened tree species, which had an equally poor regeneration species rate (36%) (Fig. 2, Table 3). Interestingly, the threatened tree species were mainly found around the parent trees in our study area. According to Janzen [63], seed density of a given tree species decreases with distance from the parent tree but also varies with seed size and seed dispersal processes, and is affected by plant parasites and seed-eating animals. However, more detailed research is needed to determine whether low seed production, low germination rates, low survival rates or insufficient dispersal can explain the observed low representation of mature tree richness in the regeneration layer.
Many previous studies have found that a single environmental factor fails to explain forest regeneration characteristics [1, 3, 4, 6, 7, 9, 11, 15–17, 19, 24, 50, 57, 64–68]. These results are confirmed by our study, since we found that PC2, which represented a fertility, rough terrain, and moisture gradient (see Appendix G, Table 4, Fig. 5), explained the pattern of tree species regeneration better than single environmental variables. However, the marginal R2 values of each model (Table 4) were very small, so although we can confirm a link between species richness ratios and environmental factors, we did not observe a strong relationship. We assume that other unidentified factors or factors functioning on a larger scale must be considered such as rainfall seasonality [69], water erosion [70, 71], and flooding period [72, 73]. In particular, increasing extreme events can have major impacts on seedling establishment effective over extensive areas. In general, tropical forests are considered as very sensitive to changing climatic conditions and interannual climate variability as the forests display for example strong coevolutionary interactions and specializations that can be decoupled by global change. In addition, changing environmental conditions may eliminate the narrow niches in tropical forests and by this species diversity [74, 75].
As previously mentioned, one important factor affecting tree regeneration patterns at the local scale may be light availability. However, we did not find an influence of light-related factors (represented by PC1) on the tree species richness and diversity ratios (Table 4); we assume that our gradient in light availability was too small (Table 5). Therefore, we can only speculate as to whether higher light availability would have resulted in more balanced ratios between overstory and regeneration tree species richness.
Previous studies have also demonstrated variability in tree species composition along topographic gradients [18, 76–82], because topography affects soil formation (including soil fertility, moisture, and depth) and creates microhabitats [80, 81, 83, 84]. Microhabitats contribute to regeneration niches which in turn are strongly linked to species coexistence [23, 65]. In our research, topography was represented by the percentage of rock surface, slope, and elevation. We assume that a combination of rock surface, slope, and limestone ridges strongly affect soil characteristics (soil nutrient status, humus, soil moisture, and depth), which may have implications for seed storage ability [6, 59]. With increasing percentage of rock surface, soil cover and soil depth decrease (Table 4, Fig. 5, Appendix G). Furthermore, with increasing slope, soils become shallower, store fewer nutrients, and are more prone to erosion. Therefore, factors indicating rough terrain may have created unfavorable conditions for seed storage and germination [6, 80].
Besides topography and light, soil factors are considered as most important for natural forest regeneration [2, 3, 16, 17, 65, 67, 77, 85]. In our study, soil moisture as well as base saturation and CEC were represented by PC2 and affected the species richness ratios negatively. However, this unexpected result may be a methodological artifact, since soil moisture and soil chemical properties were determined for the upper 20 cm of the soil only. It is likely that these 20 cm do not sufficiently represent the real status of soil moisture and soil fertility. This view is supported by the finding that soil depth was negatively correlated to PC2, and thus influenced the species richness ratio positively.
Forest regeneration of tree species depends on both natural disturbances and anthropogenic activities. Natural disturbances can increase the variability in light conditions, influence seed arrival, and contribute to the diversity of seeds by providing regeneration niches [23, 86, 87]. In addition, natural disturbances also affect recruitment patterns of colonizing species, influence soil resource levels, and determine longer term community development [88]. Human activities may have similar effects but they can additionally affect seed bank composition, for example by removing dominant tree species [67, 88]. However, we did not find a strong effect of human disturbances on species richness and diversity ratios. Only the number of footpaths was related to PC2 (r=-0.21) (see Appendix G, Fig. 5). But this relationship was negative; therefore, the number of footpaths had a positive effect on the ratios, lending support to the idea that disturbances can promote the regeneration process. This is supported by Tran, et al. [46] who found a higher similarity between the regeneration and overstorey richness in forests with high intensity selective logging compared to forests with a lower management intensity or unlogged forests after 30 years because of sufficient sunlight reaching the forest floor to facilitate seed germination and seedling growth. Although we do not have records of natural disturbances or historic human impact, long-term effects of former disturbances may still be reflected in the richness and composition of the regeneration layer or even more so of the overstorey layer and can explain current richness differences between layers [60, 89, 90]. Thus, both natural disturbance and historical human influence should be taken into account when investigating regeneration patterns of tree species including threatened species.