Correlation Of Stem Density And Species Richness Of The Tropical Montane Rain Forest In Jianfengling, Hainan Island

Biodiversity is the basis of human life, which has attracted many research interests duringthe past several decades. Many fundamental questions about biodiversity had been proposed byecologist, including the relationship between stem density (N) and species richness (S).However, the causes of the relationship between N and S have not been fully understood. Tosolve this problem, based on the60hm~2dynamic plot dataset of tropical montane rain forest inJianfengling, Hainan Island, we analyzed the effects of spatial scales, topography (elevation,convexity and slope) and DBH on the relationship between stem density and species richness.It is aim to learn the plant spatial distribution patterns and coexistence mechanisms and providescientific basis for biodiversity protection and sustainable utilization in tropical forest areas inthe future.Jianfengling has abundant plant species. There are304species found in the60hm~2dynamic plot, except of58individuals unidentified. They are belonging to154genera and65families. Lauraceae and Rubiaceae are the most dominant families. Livistona saribus,Cryptocarya chinensis, Gironniera subaequalis, Alseodaphne hainanensis and Prismatomeristetrandra are the top five dominant species.The effects of spatial scales, topography and DBH on the relationship between stemdensity and species richness are illustrated as below.(1) Effects of spatial scales on the relationship between stem density and species richnessSeven spatial scales (5m×5m,10m×10m,20m×20m,40m×40m,60m×60m,80m×80m,100m×100m) are used to analyze the relationship between stem density and speciesrichness. Results showed that the relationship between stem density and species richness isrelated to changes of spatial scales. There are significant positive correlation between stemdensity and species richness on the spatial scales of5m×5m,10m×10m,20m×20m and40m×40m. There are not significant correlation between stem density and species richness on thespatial scales of60m×60m,80m×80m and100m×100m. On the spatial scale of20m×20m to40m×40m, there are obvious sliding intervals whenspecies richness changed with stem density. On the scale of20m×20m, increase rate ofspecies richness became slowly when stem density larger than300. On the spatial scale of40m×40m, there are two different shapes when species richness changed with stem density.Species richness increased with stem density when stem density smaller than1200, while S didnot related to stem density when stem density larger than1200.In conclusion, the correlation between stem density and species richness decreased withthe increase of spatial scales.20m×20m is the key spatial scale and an ideal unit used toanalyze the relationship between species richness and stem density in the tropical montaneforest.On the scale of20m×20m, we also found that there are obvious differences on speciescomposition before and after the turnpoint (stem density equals to300) between speciesrichness and stem density. There are more light-demanding species in the quadrats with stemdensity larger than300than those smaller than300. It is estimated that light may be one of thekey factors caused the differences.(2) Effects of topography on the relationship between stem density and species richnessAll20m×20m quadrats were divided into eighteen types by combining elevation,convexity and slope, to analyze their effects on the relationship between species richness andstem density.First, in the quadrats with convexity smaller than0, the significance of the relationshipbetween stem density and species richness show a unimodal form with the increase of elevation.In the quadrats with convexity larger than0, the significance of the relationship between stemdensity and species richness decreased with the increase of elevation.Second, there is much more significant relationship between stem density and speciesrichness for those types with larger convexity. While for the types at the middle elevation andwith slope between15°and30°, the relationship between stem density and species richness arestrongest when convexity smaller than0. Finally, we also find that with the increase of slope, the significance of the relationshipbetween stem density and species richness increased.(3) Effects of DBH on the relationship between stem density and species richnessAll stems in the60hm~2plot were divided into three DBH types: DBH≥1.0cm, DBH≥2.5cm and DBH≥7.5cm. Then the effects of DBH types on the relationship between stem densityand species richness were analyzed for every seven spatial scales. Results showed that there aresimilar trends between stem density and species richness after separating all stems into threeDBH types. The significance of the relationship between stem density and species richness forDBH≥2.5cm and DBH≥7.5cm types are more similar than those for DBH≥1.0cm type. Thereis much more stronger correlation between stem density and species richness for those stemswith larger DBH. It is inferred that negative dependence and self thinning are two possiblemechanisms caused the phenomenon.