Data from: Processes of community assembly in an environmentally heterogeneous, high biodiversity region

Despite decades of study, the relative importance of niche-based versus neutral processes in community assembly remains largely ambiguous. Recent work suggests niche-based processes are more easily detectable at coarser spatial scales, while neutrality dominates at finer scales. Analyses of functional traits with multi-year multi-site biodiversity inventories may provide deeper insights into assembly processes and the effects of spatial scale. We examined associations between community composition, species functional traits, and environmental conditions for plant communities in the Kouga-Baviaanskloof region, an area within South Africa's Cape Floristic Region (CFR) containing high α and β diversity. This region contains strong climatic gradients and topographic heterogeneity, and is comprised of distinct vegetation classes with varying fire histories, making it an ideal location to assess the role of niche-based environmental filtering on community composition by examining how traits vary with environment. We combined functional trait measurements for over 300 species with observations from vegetation surveys carried out 1991/2 and repeated in 2011/2. We applied redundancy analysis, quantile regression, and null model tests to examine trends in species turnover and functional traits along environmental gradients in space and through time. Functional trait values were weakly associated with most spatial environmental gradients and only showed trends with respect to vegetation class and time since fire. However, survey plots showed greater compositional and functional stability through time than expected based on null models. Taken together, we found clear evidence for functional distinctions between vegetation classes, suggesting strong environmental filtering at this scale, most likely driven by fire dynamics. In contrast, there was little evidence of filtering effects along environmental gradients within vegetation classes, suggesting that assembly processes are largely neutral at this scale, likely the result of very high functional redundancy among species in the regional species pool.尽管已有数十年的研究，但群落构建中基于生态位（niche）的过程与中性过程的相对重要性，迄今仍大体不明晰。近期研究表明，基于生态位的过程在较粗的空间尺度下更易被检测，而中性过程则主导更精细的空间尺度。结合多年多站点生物多样性清查与功能性状（functional traits）分析，或可加深我们对群落构建过程以及空间尺度效应的理解。我们针对南非开普植物区（Cape Floristic Region, CFR）内的库加-巴维亚恩斯克鲁夫（Kouga-Baviaanskloof）区域开展研究，该区域拥有极高的α和β多样性，我们分析了其中植物群落的群落组成、物种功能性状与环境条件之间的关联。该区域存在显著的气候梯度与地形异质性，且包含具有不同火历史的多种植被类型，因此是通过探究性状随环境的变化规律，评估基于生态位的环境过滤对群落构建作用的理想场地。我们将针对300余种物种的功能性状测定结果，与1991/1992年度开展、并于2011/2012年度重复进行的植被调查观测数据进行整合。我们采用冗余分析（redundancy analysis）、分位数回归（quantile regression）以及零模型检验（null model tests），探究物种周转与功能性状沿空间和时间环境梯度的变化趋势。功能性状数值与多数空间环境梯度仅存在微弱关联，仅在植被类型以及火后时间尺度上呈现出变化趋势。然而，相较于零模型的预期结果，调查样地的群落组成与功能性状随时间的稳定性更高。综合来看，我们发现不同植被类型间存在显著的功能性状差异，表明该尺度下存在强烈的环境过滤作用，其驱动因素极有可能为火动态过程。与之相反，在植被类型内部的环境梯度上，几乎未发现环境过滤作用的相关证据，这表明该尺度下群落构建过程以中性过程为主，这一结果大概率源于区域物种种库内物种间极高的功能冗余性。