物种共存机制及多样性维持机理一直是群落生态学家最为关注的核心生态学问题之一。物种间对环境资源的竞争和分割是群落构建的重要力量。与土壤资源相比，光资源对物种共存和群落构建的作用的研究相对较少。但在森林群落中，光是最为稀缺的资源之一且具有高度异质性和变异性。因而，光资源对物种共存及群落的构建必然有着至关重要的影响。本研究通过收集黑石顶常绿阔叶林50公顷样地中154个物种的光合特性并结合光环境、土壤环境数据，探究林下物种的光生态位分化格局，并从以下几方面探讨光在物种共存和群落构建中起到的重要作用： 耐荫性是衡量物种光资源竞争能力的重要指标，准确地量化物种的耐荫能力有助于更深入全面地探讨物种光资源竞争过程中的共存机理。但是目前耐荫性的衡量存在各种操作上或者准确率方面的问题，人们需要一个合适的方法或指数客观地量化不同物种的耐荫能力。林下小树占该物种总多度的比例常被用于简单地比较两物种耐荫性的高低。通过数据模拟，本研究表明使用该法比较物种的耐荫能力存在误判的风险。本研究提出了一个适用于野外调查的耐荫性指数用于量化不同树种的耐荫能力。该指数同时结合树种的光合性状和低光环境小树的多度信息，能准确、客观地量化物种的耐荫能力，同时能克服传统方法数据获取困难和结果模糊等缺点。 竞争排斥作用与环境过滤作用是影响群落构建的两个重要的生态过程。为探讨物种对光及其他资源的竞争与环境过滤作用对黑石顶常绿阔叶林群落构建的重要性，本研究通过对物种性状的差异、谱系距离与物种共存指数(Schoener指数)的分位数回归分析，发现与耐荫能力密切相关的性状(木材密度、树木最大分支数等)与物种的共存指数呈正相关关系。该结果表明物种的光资源竞争对物种的共存起着重要的作用。而共存物种的其他性状较相似及亲缘关系较近，说明了其他环境因子的过滤作用对群落构建同样起着作用。 对光资源需求不同的类群，可能在分布和共存过程中受到竞争或环境过滤作用的影响程度不同。深入探讨其中的差异将有助于更全面地诠释群落物种共存机理和群落构建机制。通过对物种性状的差异与共存指数的分位数回归分析，发现决定不同耐荫性类群的物种共存的作用力不同。耐荫种的光生态位分化更明显，非耐荫物种的共存更多地受到光环境的过滤作用影响。而对土壤资源，耐荫种更多地受到土壤因子的环境过滤的限制，非耐荫种则受到竞争排斥作用影响更明显。 光资源在亚热带森林林下是限制性资源之一，物种间对稀缺资源的竞争必然导致生态位的分化。本文比较了物种光合性状值与多度沿光资源梯度的分布的关系，结果显示物种光合性状取得最优值的光环境与多度取得最大值的光环境的差值显著比随机群落的小，证明林下树种间存在明显的光生态位分化现象。 植物的光合作用和荧光反应参数常被用于描述植物对光环境的响应与适应。但大多情况下，人们仅用两者相互佐证，很少能通过两者的差异研究物种对各种资源的响应或需求的差异。本研究将荧光性状和光合性状结合起来，利用叶片光饱和点与荧光饱和点的差值和多度与各环境资源的多元线性回归中光资源对应的偏回归系数的相对重要值作比较，发现它们呈现显著的负相关性。证明光饱和点与荧光饱和点的差异能够很好地描述物种多度与光资源和其他资源的关联程度的差异。此结果为结合光合与荧光性状研究物种对光及其他环境因子响应的差异提供有意义的参考。 在邻体空间共存的物种对光资源偏好的差异也是光生态位分化的表现。本研究通过计算物种多度与各环境资源的多元线性回归中光资源对应的偏回归系数的相对重要值，并对其种间差值与物种共存指数作分位数回归分析，发现共存物种之间多度受光资源影响的程度差异更大。这说明不同物种受光资源影响程度的分异是维持群落物种共存的重要机制之一。同时，不同耐荫性类群的上述回归分析结果不同。共存耐荫种、中度耐荫种受光资源影响程度趋于分异，而共存非耐荫种之间受光资源影响程度趋于相同。该结果进一步证明了不同耐荫性类群间物种共存机制存在差异的结论。 本研究提出了一个新的耐荫性指数。该指数能更好地量化树种的耐荫能力，有助于深入研究物种间光利用策略的差异对物种共存的意义。结合物种的光合性状与物种多度沿光资源梯度的分布，阐明了林下物种的光分割格局。通过分析各种功能性状及系统发育的结构，证明了物种间光资源的竞争作用对群落构建的重要影响及其他环境因子过滤作用的影响。同时，本研究发现不同耐荫性类群的共存机制存在差异，这种差异影响着群落的构建。在邻体空间共存的物种间多度受光资源影响的程度趋于分异，进一步证明了物种对光资源的偏好差异对共存具有促进作用。本研究将荧光和光合性状结合起来，发现两者的差异能衡量物种多度受光资源影响程度的差异，该结果为研究物种对光资源及其他资源利用策略的差异提供了有用的参考。本研究结合物种的光合特性，从各个角度全面揭示了物种对光资源的分割格局，证明了光生态位分化在黑石顶常绿阔叶林的物种共存和群落构建中的关键作用。 关键词：生态位，光分割，耐荫性，共存，光合性状，亚热带森林

 Understanding the mechanisms of species coexistence and community assembly rules is fundamental in community ecology. Many studies have emphasized the importance of soil resources in determining niche differentiation, with much less attention paid to light partitioning although it has long been recognized that light is a key limiting resource in forest ecosystems. Light distribution is found highly variable and heterogeneous within forests, resulting in light niche differentiation. Basing on photosynthetic traits of 154 tree species and environment data (including light and soil properties), this thesis studies the pattern of light niche differentiation and discusses the important role light plays in determining species coexistence and community assembly in Heishiding evergreen broad-leaved forest. The degree of shade tolerance is a widely used surrogate of light preference of plant but is poorly defined for tree species in forest. More problematic is that there is lack of a means to measure shade tolerance. There is a need to develop a practical method to measure shade tolerance. Sapling ratio is often employed to compare the difference of shade tolerance between species. But this method is found to be unreliable and incorrectly ranks species shade tolerance in most cases in a simulation test conducted in this study. This thesis proposes a new index to measure shade tolerance of tree species in forest. This index considers information of photosynthetic performance and species abundance and it can quantify the difference of shade tolerance between species more accurately and objectively. Furthermore, the index can overcome the drawbacks of traditional methods. Competitive exclusion and environmental filtering are two major ecological processes in determining the local co-occurrence of species. In order to evaluate the relative importance of these two forces in community assembly, this thesis studies the relationship between trait difference of species and co-occurring index (Schoener index) by a quantile regression analysis. The results show that traits-defined shade tolerance, including wood density, maximum branch number, tends to be dissimilar among co-occurring species. These results demonstrate that light niche difference of species plays an important role in promoting species coexistence. For non-light traits, co-occurring species are found to share similar traits, indicating that environmental filtering and competitive exclusion both contribute to community assembly. It is possible that species with different light utilization strategies show different trait dispersion patterns, which might suggest that more complicated and detailed mechanisms that may regulate the whole communities. The study finds different trait dispersion patterns among different shade-tolerant groups of species, which are classified according to their shade tolerant abilities. This thesis shows shade tolerant species to have obvious light niche differentiation pattern, while the group of shade-intolerant species is more influenced by environment filtering effect. However, for soil resources utilization, shade tolerant species are more affected by environment filtering while intolerant species are more affected by competitive exclusion. Light is limited in the understory of subtropical forest, resulting in light niche differentiation. In order to reveal the light partitioning pattern, this thesis compares the value of photosynthetic traits and species distribution along the light gradient and finds light level in which trait values is optimal is consistent with light level in which species abundance is highest, compared to a null model. The result confirms the existence of light niche differentiation among species. Although photosynthetic traits and chlorophyll fluorescence traits are often employed together to describe plant's performance, these two traits are seldomly combined to quantify species different needs or responses to diverse resources. This thesis combines photosynthetic traits and chlorophyll fluorescence traits from their respective saturated light intensity, and finds their difference is negatively related to the ratio of light resource preference to other resources of species. The result shows that the difference between photosynthetic light saturate point and chlorophyll fluorescence light saturate point can well describe the difference between light resource preference and other resource preference, and hence can be a useful physiological surrogate to compare plant performance of resources utilization strategies. The fact that the nearest neighbors of species show different light preferences, indicates that light partitioning of species greatly affects species coexistence. This thesis finds the ratio of light resource preference to other resources of co-occurring species are more dissimilar, suggesting that dissimilarity of resource preference might promote species coexistence. Moreover, patterns of the ratio of light resource preference to other resources of co-occurring species between different shade tolerance groups are different. The ratio of light resource preference to other resources of co-occurring shade tolerant or moderate shade tolerant species are more dissimilar, while intolerant species are more similar. The results verify the conclusion that the mechanisms of coexistence between different shade tolerant species are different. In summary, this thesis proposes a new index to quantify shade tolerance of tree species and it contributes to understanding the mechanisms of species coexistence by more precisely distinguishing species light utilization strategies. By analyzing the relationship between photosynthetic traits values and species distribution along the light gradients, this thesis finds evidence of light niche differentiation between species in forest understory. The results of trait dispersion patterns confirm that light niche partitioning and environmental filtering both play important roles in shaping the community. This thesis also discovers that different shade-tolerant species undergo different mechanisms of coexistence. Exploring this difference would help us to infer community assembly mechanisms. The result that neighboring species showing dissimilar ratio of light resource preference to other resources also confirms light partitioning greatly affects species coexistence. This thesis also shows that the difference between photosynthetic and chlorophyll fluorescence traits is a useful surrogate to compare plant performances of resources utilization strategies. Therefore, this thesis reveals light partitioning pattern of species from several aspects, and demonstrates the important role of light niche differentiation in shaping the subtropical forest community. Key words: niche, light partitioning, shade tolerance, coexistence, photosynthetic traits, subtropical forest