高寒草甸初级生产力时间稳定性机制及其相互关系研究

Recent theoretical and empirical work suggests that species diversity (via asynchrony, portfolio, overyielding, dominance effect, facilitation, and/or weak interactions) can enhance the temporal stability of community. However, it remains unclear how the mutuality of their affection occurred among potential mechanisms driving temporal stability, which are more important for performing the stabilizing function, and how did the transformation from complementary effect to selection effect happen. Through a long-term field manipulation experiment, which was conducted in 2007 in alpine meadow, this project aims to answer these questions by identifying temporal stability mechanisms of net aboveground primary productivity (NAPP) and mutuality of their affection in the meadow, comparing the relative contributions of different stabilized mechanisms to NAPP temporal stability of treatment community, and by showing the law of transformation from complementary effect to selection effect. We employed a split-plot design with clipping treatment in the whole plot using three clipping levels. Subplots were treated with fertilizer and without fertilizer. Based on species-mediated response to clipping and fertilizing, we will firstly test the mechanisms of temporal stability in NAPP and the patterns of stability effect in different communities, performes numerical simulations of generalised diversity-interactions models (GDIM) with changing values of δij, Pi and θ, and fitting GDIM with observed values of both community NAPP and species relative abundances in order to ascertain the relationships between these patterns and treatment conditions. Then, using structural equation modeling (SEM), we will quantity the relative contributions of different stability mechanisms to temporal stability in NAPP within and among treatment communities, and estimate the main stability mechanisms and a matchup of the main mechanism with θ value. Based on stability analysis of dominant species and community that was controlled respectively by complementary effect and selection effect, comparing the relative contributions of community evenness, leaf dry matter content (LDMC) of dominant species and θ value to stability of dominant species and community, the efficiency index representing transformation from complementary effect to selection effect will be identified. Then we will analyze the correlation between the efficiency index and temporal stability of dominant species and community in order to obtain the threshold value of the efficiency index, and clarify the crucial effect of changes in community evenness, LDMC of dominant species, and/or θ values on switch in temporal stability mechanism. Finally, we will draw critical hypothesis which can predicate correlations between θ values and mechanisms interactions of temporal stability. Our objectives are to provide a more generalized explanation for understanding changes in mechanisms of temporal stability under changing environment.

针对群落时间稳定性机制的相互关系、不同机制的相对重要性、以及互补和选择效应两大机制的转变等问题，依托野外长期刈割与施肥控制实验，通过对高寒草甸生产力的时间稳定性比较与稳定性机制的验证、广义多样性—相互作用模型(GDIM)的数值模拟、群落实测数据的GDIM拟合研究，解释时间稳定性机制相互关系变化的原因。采用结构方程模型(SEM)分析恒定环境(群落内)和变化环境(群落间)中不同时间稳定性机制的相对贡献，确定各群落主要的时间稳定性机制，进一步明确成对物种相互作用效应(θ值)与主要稳定性机制的对应关系。通过对群落均匀度、优势种叶片干物质含量(LDMC)、以及θ值的分析，确定能够预测互补和选择两大机制转变的有效指标及其临界值，揭示优势度效应产生的原因。实现上述目标可以深化该领域的理论和实验研究，阐明全球变化背景下主要扰动因素对高寒草甸生产力稳定性的影响及其维持机制变化的原因。

本项目依托野外长期刈割与施肥控制实验，揭示了高寒草甸群落时间稳定性机制间的相互关系、不同机制对时间稳定性的相对贡献以及互补效应和选择效应两大机制的转变，主要结论如下：.1. 不同的时间稳定性维持机制可以同时存在于特定群落中，但它们对群落稳定性的贡献不同，而且是扰动依赖的。平均种群的时间稳定性、投资组合效应、选择效应和异步性效应对群落时间稳定性的相对贡献分别为0.80、0.58、0.53和0.22。种群时间稳定性和群落时间稳定性正相关。.2. 种间相互作用及其多样性效应可以解释群落时间稳定性机制的变化。通过确认广义多样性—相互作用模型(GDIM)中的种间相互作用关系及其多样性效应，能够预测群落时间稳定性的主要维持机制。.3. 刈割强化了投资组合效应，而施肥则加强了选择效应，刈割和施肥是引起这两种机制发生转变的重要驱动因素，但施肥介导的选择效应或优势度效应对群落时间稳定性的影响更大。.4. 叶片干物质含量（LDMC）并非始终是预测种群时间稳定性的最佳指标，在多变的群落环境下，有效的预测指标可以是叶片的P含量、N含量或是比叶面积（SLA）。此外，由于LDMC、叶片P、N含量或SLA都与种群时间稳定性线性相关，而种群稳定性与群落稳定性正相关，因此，也不存在能够表征选择效应和互补效应相互转变的临界性状值。.5. 施肥主要通过增加物种的个体大小非对称光竞争以及诱导植物向“快速-回报”策略转变而降低了物种丰富度，这符合光竞争假说关于养分富集引起物种丧失机制的预测。施肥引起的“快速-回报”策略还降低了种群的时间稳定性，而刈割并不会改变此关系。.6. 物种对刈割和施肥扰动引起的补偿响应动态是决定群落时间稳定机制的关键生物因素。我们的结果表明传统的补偿生长指数能准确地表征这种补偿响应，而总耐牧性指数并不是描述该响应的理想指标。.上述研究深化了该领域的研究，阐明了刈割和施肥对高寒草甸生产力稳定性及其维持机制的影响。.