Spatial Heterogeneity Within A Tropical Montane Rainforest Dynamics Plot On Hainan Island,China

Spatial heterogeneity is defined as the spatial variability and complexity of certain attributes of a system.The spatial heterogeneity of plant communities generally refers to the variations in community features among spatial patches and along spatial gradients.Studying spatial heterogeneity in plant community could well promote our understanding of how plants adapt to the varied environmental conditions.In addition,preforming spatial heterogeneity analysis considering scale-dependent effects could also promote our understanding of biodiversity pattern and its potential maintenance mechanisms.Tropical rainforests have rich species composition and complex community structure,and play a dominant role in global biodiversity conservation and earth ecosystem services functioning.The tropical forest vegetation on Hainan Island is one of the most concerned area of biodiversity hotspots in China and across the world.Among the forest types on Hainan Island,tropical montane rainforest has the largest area of distribution and with a high proportion of well protected old-growth forest stands.Studies on the population structure,biodiversity maintenance and community assembly of tropical rainforests based on spatial heterogeneity analysis could well advance the development of plant community ecology.The forest dynamics plot?FDP?has been considered as an important platform for forest ecology,which plays a fundamental role in studying the structure,functioning and dynamics of forest ecosystems.In fact,most ecological processes have been proved to be strongly scale-dependent.Since FDP has a larger area than a common forest plot,both the biotic and abiotic factors in a FDP often change with the variation in spatial attributes.We established a 60-ha FDP?up to now,it is the largest FDP all over the world?in the old-growth tropical montane rainforest in Jianfengling nature reserve on Hainan Island,China in 2013.A systematic spatial heterogeneity analysis within this world's largest FDP should well promote our understanding of diversity maintenance and community assembly mechanisms in complex forest ecosystems.We used multi-variate statistical analysis to elucidate the spatial heterogeneity of tropical montane rainforest within this FDP.We systematically analyzed the spatial variation in site conditions,population patterns,community structure,plant functional traits and different facets of biodiversity along spatial scale gradients and among habitat types in the FDP.We explored how the community features,diversity pattern and their correlations with the abiotic conditions vary with spatial scales and site differentiations within this FDP.Finally,we discussed the potential diversity maintenance and community assembly mechanisms of the tropical montane rainforest.The main results were listed as follows:1.By performing spatial heterogeneity analysis on soil variables within the FDP,we found that all the soil factors exhibited a positive correlation at small spatial scales and the similarities of each soil property revealed an aggregated pattern in neighboring areas.However,the spatial structures of different soil variables varied significantly within the FDP.Among the9 measured soil variables,the range of semi-variance?81 m,31 m?and the lag distance of Moran's I?60 m,30 m?of soil available phosphorus content and soil pH value were apparently smaller than those of the other soil factors,suggesting that the spatial structures of these two soil factors were more complicated and varied more dramatically.On the other hand,the above mentioned values for soil total nitrogen content and soil available nitrogen content were significantly larger than those of the other 7 soil factors,suggesting these two soil factors had a simpler spatial structure and varied less in the FDP.Results of principle component analysis showed that soil total nitrogen content and soil available phosphorus content had a great proportion of contribution to the overall spatial variation of soil conditions within the FDP.In addition,the topographic factors had an important impact on the spatial variation in soil conditions.Based on the fuzzy C-mean cluster analysis of the topographic factors,the FDP could be divided into four site types:flat ground,gentle slope,steep slope and ridge.Results of ANOVA showed that the soil conditions varied with the habitat types,especially in soil pH value,soil total nitrogen content and soil available phosphorus content had a more dramatic variation.In summary,abiotic conditions had an obvious spatial heterogeneity within the FDP.And,topographic factors have strong constraints on edaphic conditions.Among the10 measured soil variables,soil available phosphorus content and soil pH contributed disproportionately to the spatial heterogeneity of abiotic conditions within the FDP.2.By performing Torus-translation examination on all species with abundance?60,we found that the spatial distribution of 61%of the examined species were significantly associated with at least one habitat type.In addition,results of direct species-environment gradient association analysis showed that 94.4%of the examined species'distribution were associated with at least one environmental factor.These results indicated that the spatial distributions of species were significantly influenced by habitat heterogeneity and most species had obvious habitat preference rather than randomly distributed.All these evidences supported in part the importance of niche process in species distributions.Comparisions among species-habitat associations of different growth stages showed that the percentage of species had significant habitat or environment gradients associations with a decreasing trend as trees grew.In addition,the preferred habitat or environment of a given species also varied with life history stages,supporting the regeneration niche theory,which claimed that the niche partition between species might occur at different life stages.In summary,our study emphasized the significant impacts of spatial habitat heterogeneity on species distribution.In addition,the habitat preference varied significantly among different species or at different life stages of a given species.3.By performing ANOVA on community structure among habitat types within the FDP,we found that individual density,mean diameter at breast height,mean height and dominant species varied significantly among habitat types.Differences were also found in dbh distributions,species-area relationships,species-individual relationships and species-abundance distributions.The fitting results of species-abundance distribution suggested that neutral process dominated the community assembly in the FDP.However,there was little difference in community assembly rule among habitat types with best fitting neutral process in flat ground.In summary,our results suggested that spatial heterogeneity in habitat factors had a significant influence on community structure.Both the structure and assembly of communities within this FDP were spatially heterogeneous.4.By performing ANOVA on biodiversity among habitat types within the FDP,we found that species diversity,functional diversity and phylogenetic diversity all varied significantly among habitat types.All the diveristy indices increased from flat ground through gentle slope and steep slope to ridge.Species diversity could well explain the variations in functional diversity and phylogenetic diversity,which suggested the existence of niche conservatism.The variations in relationships among them under different site conditions indicated that different ecological processes dominanted in different habitat types.The biodiversity patterns changed along spatial scale gradients within the FDP,which emphasized the importance of considering spatial scale-dependent effects when exploring the underlying ecological mechanisms in FDP.The species diversity increased,while functional diversity increased first and then decreased with increasing spatial scales.On the other hand,the phylogenetic diversity showed an increasing trend while the NRI index decreased with increasing scales.Variabilities of the biodiversity indices among different spatial scales indicated the strong scale-dependent effects of the underlying ecological processes surrogated by different facets of biodiversity.In summary,we found that the biodiversity varied significantly along spatial gradients and among habitat types.All the diversity indexes had an increasing trend from flat ground through gentle slope and steep slope to ridge.Interactions among individuals might dominate at smaller scales,while the environmental filtering might play more roles at larger scales in community assembly process.5.By performing ANOVA on community-level plant functional traits among habitat types within the FDP,we found that all the measured functional traits varied significantly among habitat types.Species in ridge habitat tended to adopt conservative strategies with lower specific leaf area?SLA?,potential maximum height(H_max),leaf nutrient concentration and higher leaf dry matter content?LDMC?and wood density?WD?,while those in flat ground habitat tended to adopt acquisitive strategies with higher SLA,H_max,leaf nutrient concentration and lower LDMC and WD.By comparing with simulation of null models with observed value,we found that environmental filtering played a more important role in nutrient-poor sites,while limiting similarity acted more under fertile soil conditions.In addition,we found that limiting similarity probably acted at scales<20 m×20 m and environmental filtering showed the strongest influence at scales between 20 m×20 m and 40m×40 m.In summary,plant functional traits and the representing ecological process varied along spatial gradient and among habitat types.The life history strategy of speices surrogated by functional traits changed from conservative to acquisitive from flat ground to ridge habitat.In addition,environmental filtering had a more important role under poor soil conditions and at larger scales,while limiting similarity was dominated under fertile soil conditions and at smaller scales in community assembly process.6.We conducted point pattern analysis to examine the conspecific negative density dependence?CNDD?of 123 species with abundance?60 in the FDP,and found that 89.1%of the examined species in the FDP showed CNDD.Besides,most species?94.1%pf the species showing CNDD?exhibited CNDD at smaller scales less than 5 m.Results of linear regression showed that the strength of CNDD was negatively associated with species abundance.Further analysis indicated that the strength of CNDD depended on the relative species abundance at local scales not on its relative abundance at the whole FDP scale.The species would suffer weaker CNDD if it was less abundant at local scales even if it was abundant at the community scale.The strength of CNDD was significantly correlated with species'habitat preference.Species often exhibited weaker CNDD in preferred habitats.Results of correlations of the strength of CNDD with plant functional traits(including SLA,LDMC,LNC,LPC,WD,H_max)showed that species with acquisitive ecological strategies?higher SLA,Hmax,leaf nutrient content,lower LDMC,WD?suffered stronger CNDD.In summary,the strength of CNDD depended on the relative density of species at local scales.Typically,species tended to suffered more at fertile conditions.However,they would exhibit weaker CNDD when in their preferred habitats.The strength of CNDD was significantly associated with plant functional traits.Species with acquisitive strategies showed stronger CNDD.7.Results of performing the individual species-area relationship framework?ISAR?showed that no key species was found which strongly drove the diversity pattern in the FDP.Results of null models showed that interspecific facilitation contributed greatly to diversity maintenance at local scales?<15 m?.Species functional traits had a significant influence on the neighbor diversity.Species with conservative ecological strategies had higher neighbor species richness.In addition,species with conservative strategies was proved to have higher neighbor functional divergence,which also implied the importance of limiting similarity in dominating community assembly process at local scales.In summary,the interspecific facilitation played an important role in community assembly at small scales and the species with conservative strategies had a significant role in maintaining higher species richness in neighboring area.By systematically performing spatial heterogeneity on abiotic environment and community structures within the world's largest FDP,we found significant spatial heterogeneity in soil variables,community structure,plant functional traits and diversity patterns.The plant functional traits,interactions among individuals,biodiversity patterns and the underlying community assembly rules varied significantly along spatial gradients and among habitat types.By conducting quantitative analysis on multiple indicators within the FDP,we found that the FDP was actually a plant-environment complex with strong spatial heterogeneity.We must consider the effect of spatial heterogeneity characteristics within it when exploring the ecological patterns,processes and mechanisms within a FDP.The results of this study are significant in further elucidating biodiversity maintenance mechanisms and community assembly rules in species rich forest ecosystems.In addition,they also provided scientific guidelines for the rational conservation and sustainable management of tropical montane rainforest ecosystems.