| Spatial patterns analysis of plants may provide significant insights into processes and mechanisms of species coexistence in forest communities. In order to understand community dynamics and mechanisms that control the distribution of species in tropical karst seasonal rainforests in Guangxi Nonggang Nature Reserve, we analyzed community characteristics, population structure, spatial pattern dynamics and spatial associations of dominant plant species of three1-ha tropical karst seasonal forest communities.We found that the three communities were species rich. The communities also had high taxonomic diversity with more than10%of species belonging to single genera and single family. The structure of DBH size class of all species in each of the three plots generally showed inverse "J" shape, indicating that all of the three communities were stable and successful regeneration. The frequency distributions of DBH size class of Excentrodendron hsienmu and Cephalomappa sinensis, the two constructive species, were discrete, which may be due to the middle layer species were easy damaged in a tropical karst seasonal rainforest. By comparison, the structure of DBH size class of Deutzianthus tonkinensis, another constructive species, was unimodal, which indicated that it would become a declining species in the future.By using the spatial point pattern correlation function g (r), we analyzed spatial distributions of dominant species in the three typical communities. The results showed that the distribution patters about80%of species were aggregative at scales of0-20m, and the percentage of species exhibiting a random or regular pattern increased with scale, mainly occurring at scales of>20m. Biological characteristics (e.g. seed dispersal limitation), and environmental heterogeneity may be the important factor to shape the species spatial distribution in this area.The spatial patterns of three constructive species changed with development of the communities. The patches distribution of Excentrodendron hsienmu at small scale was not continuous in horizontal direction and showing a mosaic distribution across the community, which indicated Excentrodendron hsienmu spatial patterns was not only influenced by their own regeneration type, but related to the heterogeneous distribution of karst rocky mountain soil. The scale of clustered distribution increased as the DBH size of Excentrodendron hsienmu grew. Larger DBH size class individuals of Excentrodendron hsienmu showed random distribution, and it gradually became aggregated distribution as the scale increased and finally tended to be random distribution. Form small to large diameter of Cephalomappa sinensis individuals, the scale of aggregate distribution is gradually reduced, and eventually showed random distribut. Deutzianthus tonkinensis were mainly randomly distributed at different growth stages. The variation of population distribution pattern resulted from the interaction between populations and environment, which reflected an adaptation mechanism of the population.The relationship between different age classes of individuals was a result of long-term interactions between populations and the environment, and hence it could provide an insight into population dynamics and stability. The spatial associations of different growth stages of Excentrodendron hsienmu, Cephalomappa sinensis and Deutzianthus tonkinensis could facilitate can distinguish our understanding of the mechanism that produced their spatial patterns. The results showed that positive interactions were to be found between small individuals and larger individuals of Excentrodendron hsienmu at small scale. On the whole, the relationships between the different growth stages of Excentrodendron hsienmu were basically positive interactions, and only slightly changed with the scale. Positive interactions were found between the different growth stages of Cephalomappa sinensis except at several certain scales. No significant spatial associations between the different growth stages of Deutzianthus tonkinensis were observed at all spatial scales. Therefore, to sum up, the results verified that dispersal limitation may play an important role in determining spatial patterns of Excentrodendron hsienmu and Cephalomappa sinensis.Exploring interspecific spatial associations are helpful in understanding the interactions between species within the community and the niche differentiation among species. We analyzed59species pairs, and the results showed that interspecific associations of some species pairs were unstable. Because of the high habitat heterogeneity, resources competition between species-pairs at one scale could lead to niche differentiation; at the other scales, species-pairs could coexist and even became companions. This area about80%of species-pairs showed aggregated distribution at scale of0-20m, but few species-pairs displayed significant associations. These results suggested that dispersal limitation may cause the interspecific individuals aggregated distribution, in contrast, habitat heterogeneity resulted in the population niche differentiation.We found that the proportions of positive interactions, negative interactions and spatial segregation were about30%, respectively, indicating that the spatial distribution were complex, and spatial segregation promoted successful regeneration in the karst seasonal rainforest.In summary, the three typical karst seasonal rainforest communities were now in stable status. Biological characteristics (e.g. seed dispersal limitation), and environmental heterogeneity may be the important mechanism to shape species spatial pattern in this area. However, different ecological processes and mechanisms can produce the same distribution pattern, how to distinguish the different mechanisms such as dispersal limitation, habitat heterogeneity and negative density dependence which contribute to maintain the biodiversity in the karst seasonal rainforeststill remains to be explored. |
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