西双版纳热带季节雨林土壤养分空间异质性对乔木树种多样性的影响

生态位理论认为,养分空间异质性分布会减少种间竞争排斥而有助于物种共存;而中性理论则认为群落树种呈独立于环境特征的随机分布。为研究土壤养分异质性与乔木树种多样性的联系,在西双版纳热带季节雨林随机设置了16个1 hm~2样方,调查了各样方乔木树种多样性,计算了各样方土壤有效氮(AN)、有效磷(EP)、有效钾(EK)、有机质(OM)、p H、总氮(TN)、总磷(TP)、总钾(TK)的变异系数以代表各样方土壤养分空间异质性分布的相对水平。Pearson相关分析表明,TK变异系数与乔木树种丰富度、Shannon-wiener指数呈显著正相关(P0.05),表明该区域不同植物可能在钾资源的利用上存在明显的生态位分化,钾的异质性有助于树种共存;OM、AN变异系数与Pielou均匀度指数呈明显正相关(P0.1),在一定程度上说明了这些养分的空间异质性缓解了种间竞争压力,树种多度分布相对均匀,有助于树种共存。除TK外,其它土壤指标的变异系数与乔木树种多样性的正相关性均不显著(P0.05),表明这些养分的空间异质性分布对乔木树种多样性的影响相对较小,中性或其它生态学过程可能掩盖了这些养分的空间异质性分布对乔木树种多样性的影响。这说明,土壤养分空间异质性可能在一定程度上促进了树种共存,但同时应当重视中性过程等在西双版纳热带雨林群落构建中的作用。

Relationship between soil resource heterogeneity and tree diversity in Xishuangbanna Tropical Seasonal Rainforest, Southwest China

The niche theory predicts that soil resource heterogeneity promotes species coexistence because of reduced competitive exclusion among species, while the neutral theory predicts that species coexistence is more strongly determined by stochastic processes. To study the relationship between soil resource heterogeneity and tree diversity, 16 plots of 1 hm² area each were randomly set up in Xishuangbanna tropical seasonal rainforest, southwest China. This forest is characterized by abundant tree species and complex community structure. The laterite soil developed from siliceous rocks, and the average rainfall is 1493 mm. We investigated the tree diversity of each plot and analyzed the coefficient of variance (CV) of AN (Ammonium Nitrogen), EP (Extractable Phosphorus), EK (Exchangeable Potassium), OM (Organic matter), pH, TN (Total Nitrogen), TP (Total Phosphorus), and TK(Total Potassium) to show the relative heterogeneity of soil nutrients. We divided the 1hm² plot into 25 quadrats, each with an area of 400 square meters (20 m ×20 m). All four corners of each quadrat were set as sampling points. After the litterfall was cleared from the soil surface, we collected 500 g soil at 1-10 cm depth under the soil surface for soil nutrient analysis. The soil nutrient content in each 20 m ×20 m was calculated as the mean of the samples taken from the four corners. We calculated the CV of the soil nutrient in 25 quadrats to show the relative heterogeneity within a 1hm² plot. We randomly selected five 20m ×20m small quadrats in a 1hm² plot for tree evaluation and all the trees with a DBH (diameter at breast height) above 1cm have been investigated. The trees within five quadrats were summarized to show the relative levels of tree diversity within a 1hm² plot;besides, voucher specimens were made for all trees which surveyed in all of the 16 plots with each area of one hectare and conserved in Laboratory of Plant Phylogenetics & Conservation Group, Xishuangbanna Tropical Botanical Garden, Chinese Academy of Sciences. We used Pearson correlation analysis to explore the correlations between soil nutrient heterogeneity (indicated by the CV of each soil factor within a 1hm² plot) and tree diversity (indicated by tree richness, the Shannon-Wiener index, and the Pielou evenness index within a 1hm² plot) in the Xishuangbanna topical seasonal rainforest. Before the analysis, the K-S (Kolmogorov-Smirnov) test was implemented to inspect the normal distribution of all variables. The results showed that all variables (CV of soil factors and tree diversity) followed a normal distribution (P > 0.05), which was suitable for Pearson correlation analysis. A significant positive correlation was found between the CV of TK and tree diversity (tree richness, Shannon-Wiener index) (P < 0.05), indicating that potassium heterogeneity promotes tree coexistence and niche differentiation among trees based on potassium utilization. OM and AN were positively correlated with the Pielou evenness index (P < 0.1), indicating that the heterogeneity of these nutrients moderates competitive exclusion among trees and promotes tree coexistence. Except for TK, no significant positive correlation was found between nutrient heterogeneity and tree diversity. The heterogeneity of these nutrients may have relatively small effects, and the neutral or other ecological process may weaken the effects of the nutrient spatial heterogeneity on tree diversity. Our study showed that soil nutrient heterogeneity may promote the coexistence of trees to a certain extent, and that the neutral process should be considered when studying community assembly in Xishuangbanna tropical seasonal rainforest.