Study On The Maintenance Mechanisms Of Species Diversity In The Natural Old Growth Tropical Forests On Hainan Island,China

Understanding the maintenance mechanisms of species diversity in communities is a central goal in community ecology.Two major theories have been put forth to explain the diversity maintenance of communities.Niche theory proposes that deterministic processes govern the species diversity.In contrast,neutral theory emphasizes the importance of stochastic processes.After a series of empirical and theoretical explorations and debates,more and more ecologists are recognizing the importance of combining niche theory with neutral theory in explaining the maintenance of diversity in species rich communities.As one of the most complex and species-diverse terrestrial ecosystems,tropical forests play a significant role in maintaining biodiversity and regulating global climate.The maintenance mechanisms of species diversity in tropical forests is one of the most important concerns of ecology in recent years.Hainan Island is one of the main distribution regions of tropical forests in China,and there are six major natural vegetation types on the Island,including tropical montane rain forest(TMRF),tropical lowland rain forest(TLRF),tropical coniferous forest(TCF),tropical deciduous monsoon rain forest(TDMRF),tropical montane evergreen forest(TMEF),and tropical montane dwarf forest(TMDF).TMRF and TLRF are the zonal vegetation types in the high(>800m)and low(<800m)elevational areas respectively.Up to the present,most studies on species diversity maintenance of communities on Hainan Island just focus on single forest vegetation types and systematic analysis across vegetation types is lacking.In this study,based on the investigations on 17 1-ha forest dynamics plots as well as measurements on environmental factors and plant functional traits across six old-growth tropical forest types in the Bawangling Nature Reserve on Hainan Island of China,by utilizing methods like quantitative regression,model fitting and null model test,we compared and analyzed the roles of different mechanisms in maintaining species diversity in different vegetation types from the following aspects: species abundance distribution curve fitting,environmental and spatial variables,spatial patterns,functional traits,functional diversity,functional redundancy and functional over-redundancy.The major results from this study are as follows:1.The maintenance mechanisms of species diversity in the natural old growth tropical forests based on species abundance distribution models: we define the minimum stem size to tag trees in a community as the community size(community size,DBHmintag).The species-abundance accumulation curve and the rank-abundance curves showed that the species richness in the two zonal vegetation types were higher than those of the four azonal vegetation types.The species abundance distributions(SAD)of communities with different community sizes(DBHmintag?1cm,5cm and 10 cm,respectively)in the two zonal vegetation types were all best fitted by neutral models.In contrast,the best models of the four azonal vegetation types varied with tree DBH size.The neutral model performed better than the niche preemption model in the communities with DBHmintag?1cm and 5cm.The communities with DBHmintag?10 cm were all best fitted by the niche models.This result suggested that the best-fit models of SAD varied with vegetation types as well as tree size.Therefore,the species diversity was maintained simultaneously by a combination of several mechanisms in the same community.Habitat heterogeneity might be the major constraint in determining the neutral or niche process in diversity maintenance.2.The maintenance mechanisms of species diversity in the natural old growth tropical forests based on environmental and spatial variables: there were significant differences in species composition for the six vegetation types.Environmental variables and spatial variables jointly explained 29.7%~49.9% of the variation in species composition.For the zonal vegetation(TMRF and TLRF),the pure spatial variables had more explanatory power than the environmental variables.In contrast,for the other four vegetation types,environmental variables explained more variations in species composition.Therefore,the species diversity of communities was probably simultaneously driven by dispersal limitation and environmental filtering.However,zonal vegetation was mainly determined by dispersal limitation and azonal vegetation was affected by environmental filtering.3.The maintenance mechanisms of species diversity in the natural old growth tropical forests based on spatial patterns: the four models of spatial point process were performed to evaluate the potential contribution of random processes,habitat heterogeneity,dispersal limitation and the joint effects of dispersal and habitat heterogeneity,respectively.The result showed that the homogeneous Thomas process and inhomogeneous Thomas process were the best models and simulated spatial patterns of more than 85% of tree species.This suggested that dispersal limitation and the joint effects of dispersal and habitat heterogeneity were the dominant abiotic processes in controlling spatial patterning of trees in tropical forests.The null model combined with size-distance regression analysis detected that 37% to 44% tree species were affected by competition and 9% to 24% tree species were influenced by facilitation across the six tropical vegetation.The species proportion of being affected by competition were higher in two zonal vegetation where abiotic environmental was suitable.In the harsh TMDF,however,there were more species affected by facilitation.4.The maintenance mechanisms of species diversity in the natural old growth tropical forests based on distribution of functional traits: the distribution range of most functional trait in the two zonal vegetation were higher than those of azonal vegetation.The range of specific leaf area(SLA),leaf dry matter content(LDMC)and leaf phosphorus concentration(LPC)were larger than null model in the two zonal vegetation types.Except wood density(WD),the range of SLA,LDMC,LPC,leaf nitrogen concentration(LNC),and maximum height were lower in four azonal vegetation types.The standard deviation of nearest-neighbor distances(SDNND)of functional traits in the four azonal vegetation types was larger than the two zonal vegetation types except LPC.In the zonal vegetation,SLA,LDMC,LNC and MH were lower than the null expectations of zero.In azonal vegetation,however,SLA,WD and Hmax were larger than random.The kurtosis of functional trait showed similar patterns to SDNND.Our results demonstrated that limiting similarity is important in the assembly of the zonal vegetation but environmental filtering might be the dominant process in azonal vegetation.5.The maintenance mechanisms of species diversity in the natural old growth tropical forests based on functional diversity: functional richness(FRic)was the greatest in TMRF among six vegetation types.Functional evenness(FEve)in the TMRF and TLRF was significantly higher than the four azonal vegetation types.TDMRF had the highest value of functional divergence(FDiv)and functional dispersion(FDis)and Rao's quadratic entropy(Rao)in the TDMRF and TMDF were significantly higher than other vegetation types.Three functional diversity indexes(FRic,FEve,and FDiv)were not significantly different but FDis and Rao were significantly lower from random.These results suggested that the stochastic process might be the main process in the two zonal vegetation types.In contrast,functional diversity indexes were significantly lower than expectation in the TCF,TMEF and TMDF,which indicated that environmental filtering might be the dominant process in community assemblage.In the TDMRF,the FRic and FEve were significantly lower and FDiv was significantly higher than expected by chance,respectively.But FDis and Rao did not vary from a null expectation of zero.These results suggested that the TDMRF was simultaneously determined by random process,environmental filtering and limiting similarity.In addition,phylogenetic structure all showed the clustering trend,indicating that the effect of environmental filtering were predominant across six old-growth tropical forests,and the specific reasons for the phylogenetic clustering will be analyzed in the further research.6.The maintenance mechanisms of species diversity in the natural old growth tropical forests based on functional redundancy and functional over-redundancy: there was only 19.1% variation in relative three-dimensional functional richness across species richness gradients of six old-growth tropical forests.The functional redundancy and functional over-redundancy were significantly positively but functional vulnerability was negatively related to the species richness.This indicated that species richness could increase community function and stability.Most species in all vegetation types were packed into few functional entities,which formed a pattern of functional over-redundancy.It was also found that the six old-growth tropical forests had a certain degree of functional vulnerability.