The Spatial-temporal Variation Of Species Diversity In The Natural Tropical Forests Of Jianfengling On Hainan Island, South China

Species diversity is the basis and core of biodiversity research. Because of the spatial-temporal heterogeneity of ecological factors and the forest disturbance, species diversity on heterogeneous landscapes changes with the spatial environmental gradient and the temporal community successional gradient in the tropical forest. Study on the spatial-temporal dynamics of species diversity is not only important to understand the formation and maintenance mechamisms of biodiversity, but also useful to carry out biodiversity conservation and management activities. Tropical forest is a terrestrial ecosystem with the most abundant species and complicated structures. which plays an important role in the regional and global biodiversity conservation and ecological function maintenance. Thus, it is an ideal object to study the spatial-temporal variation of species diversity in the tropical forest at local or small scales. Previously, most researches on the spatial-temporal heterogeneity of species richness were carried out at large regional or global scales, while seldom researches were carried out at small or local scales. In order to explore the spatial-temporal variation of species diversity in the tropical forest at local or small regional scale, this thesis analyzed the variation of species diversity with the changes of vertical and horizontal distances, environmental gradients and recovery process after disturbance based on field investigations of 164 grid sample plots (each with a size of 625 m2) and two permanent plots in the most typical tropical forest area in Jianfengling, Hainan Island, China. The main results are as follows.(A) The basic characteristics and spatial heterogeneity of species diversity in the tropical forest(1) The tropical forest has a rich array of species composition. 65144 individuals with DBH (diameter at breast height)≥1.0 cm were recorded in the 164 grid plots, which belong to 83 families, 256 genera and 617 species. Lauraceae and Rubiaceae were the most dominant families with more than 14% ofthe total number of individuals. There were 41 species with importance values higher than 0.67. Among them, importance values of Livistona saribus, Castanopsis fissa and Gironniera subaequalis were higher than 2.0. There are 286 rare species with numbers of individuals less than 10, which accounted for 46.35% of the total species richness but only 1.44% of the total number of individuals. Psychotria asaitica was the most abundant, accounting for 4.43% of the total number of individuals. There were 842 species in the herbage and seedlings layer, including 57 Hainan Island endemic species. The tree height class with the most rich species and abundant stems was from 2.5 m to 5.0 m. The DBH distribution of all individuals took an inverse"J"shape.(2) Relationship between species richness and elevation in Jianfengling showed a bimodal pattern, which was different from those found in other regions. S?rensen similarities between pairs of plots decreased with increasing geographical distance in some range and along the progressive forest recovery process. Dispersal limitation might play an important role in this patterning.(3) Spatial-temporal patterns of species richness were closely related to the heterogeneity of multiple environmental variables. The principle component analysis and multiple regression analysis showed that elevation and disturbance type were the two most important factors in defining the patterns of species diversity. Moreover, relationship between species richness and multiple environmental variables was influenced by different disturbance types. Species richness was significantly correlated with elevation, slope position, soil water storage capacity and soil capillary water holding capacity in the old-growth forest. While in the secondary forests, it was only correlated with elevation.(B) Influences of different disturbance types on the species diversity in the tropical forest(4) Typhoon is an important natural disturbance which frequently influences the maintainence of species richness in the tropical rain forests on Hainan Island. Many wind-blown trees, broken branches and fallen leaves appeared after Typhoon Damrey in 2005, which directly made the decrease of crown density, and the increase of penetrating light and many canopy gaps. Community composition and structure changed significantly after the typhoon. The damages could be classified into two types: 1) Direct damage which mainly influenced the arbor layer. Large DBH individuals were damaged severely. 2) Indirect damage which mainly influenced the sapling layer and the understorey layer by the wind-blown trees, broken branches and fallen leaves overwhelming other individuals. Individuals died in small area and some species even disappeared. Moreover, at least 10.42% biomass returned to the forest floor after Typhoon Damrey, which greatly influenced the carbon cycle of the forest ecosystem.(5) Forest logging is an important factor that influences the species diversity. However, it was controversial on whether forest logging would indeed reduce the species diversity. The different conclusions were probably caused by different sampling regimes. For both the old-growth and the secondary forests, the species diversity on the 625 m2 plot level varied greatly, which was related to the forest logging regimes and length of recovery time. However, the logged forest had higher species diversity than the old-growth forest on the 625 m2 plot level.(C) Variation of species diversity along the tropical forest recovery gradient(6) Both the slope of species-area curve and rate of species richness accumulation increased with the progressive recovery process and the increasing sampling scales after forest logging. This phenomenon supports the resilience of forest recovery in Jianfengling. Moreover, it validates the biotic relaxation hypothesis and its temporal and spatial scale dependence.(7) The forest communities became much more similar after forest logging in Jianfengling, which disclosed the existence of biotic homogenization phenomenon at the local scale. Especially, biotic homogenization effects were more stronger for the individuals with DBH smaller than 5 cm than those with DBH larger than 5 cm. Moreover, the species abundance distribution also disclosed those forests with longer recovery time have higher species richness and evenness than those with shorter recovery time after forest logging.(D) Estimating the species richness in the tropical natural forest and characteristics of Hainan Island endemic species in Jianfengling(8) Regional species diversity is the source of local species diversity, while local species diversity is the basis of regional species diversity. This chapter estimated the species richness based on MaxEnt method. The spatial-temporal variation of species diversity has significant influence on the MaxEnt method to estimate species richness. MaxEnt was not always effective and was inclined to give an upper limit of species richness estimated in a single area. MaxEnt should be used with caution because of some restrictions, including sampling area at the anchor scale, sample plot number, historical disturbance factors, evenness of species distribution and heterogeneity of sample unit. The smaller the sampling area at the anchor scale is, the higher the estimated species richness is. Sampling heterogeneous plots and sufficient sample plots will improve the accuracy of species richness estimation and reduce the impacts of arbitrary sampling strategies. Finally, it is also found that the logistic model for species area relationship (SAR) is better than the power-law model for SAR to fit the species richness estimated by MaxEnt method. Most important of all, multiple estimation methods should be combined in order to get reliable species richness estimated.(9) Jianfengling forest is a typical tropical forest distribution district on Hainan Island. It has abundant Hainan Island endemic species, up to 158 species and 39.8% of the total Hainan Island endemic species in Hainan Island. The dominants of these endemic species belong to Lauraceae, Fagaceae and Rubiaceae. The endemic species richness increased with total species richness but it didn't show obvious elevational distribution patterns. Endemic species richness increased obviously after forest logging, especially after selective logging, which supported the hypothesis that isolation would make endemic species richness increase.