Data from: Historical and ecological controls on phylogenetic diversity in Californian plant communities

We addressed the classic question of whether community diversity is determined from the bottom up by the breadth and partitioning of niche space or from the top down by historical and evolutionary forces. Specifically, we contrasted local and regional explanations for the diversity of Californian plant communities using phylogenetic and functional analyses. Our communities were sets of four field plots that sampled alpha (within-plot) and beta (among-plot) sources of variation in diversity. We sampled 93 such communities nested within 78 larger regions for which regional species pools could be independently estimated, spanning the California Floristic Province. We measured phylogenetic and functional diversity within plots and between plots on neighboring soils and slopes. We also measured the phylogenetic diversity of regional species pools and analyzed them in terms of biogeographic groups. We found no evidence linking the phylogenetic diversity of communities to within-plot functional diversity or among-plot beta diversity. Instead, we found that the phylogenetic diversity of communities depends on that of regional species pools. In turn, phylogenetically diverse pools were those with high proportions of species of northern biogeographic affinity, which have relatively mesic distributions and traits. This supports what we call the climatic refuge hypothesis rather than the biogeographic crossroads hypothesis.

本研究旨在解答经典科学议题：群落多样性究竟由生态位空间的广度与分化自下而上塑造，还是由历史与进化力量自上而下决定？具体而言，本研究采用系统发育（phylogenetic）与功能多样性（functional diversity）分析方法，对比局域与区域尺度的两类解释框架，以解析加利福尼亚植物群落多样性的驱动机制。
本研究的群落以每组4个野外样地为基本单元，用于取样多样性的alpha（alpha diversity，样地内部）与beta（beta diversity，样地间）变异组分。我们共采样93个此类群落，这些群落嵌套分布于78个可独立估算区域物种库（regional species pools）的更大区域内，研究区域覆盖整个加利福尼亚植物区省（California Floristic Province）。
我们在相邻土壤与坡地生境中，测定了样地内部及样地间的系统发育多样性与功能多样性。此外，我们还测定了区域物种库的系统发育多样性，并按生物地理类群（biogeographic groups）开展分析。
研究未发现群落系统发育多样性与样地内功能多样性或样地间beta多样性存在关联的证据。与之相反，本研究发现群落系统发育多样性实则依赖于区域物种库的系统发育多样性。进一步分析显示，系统发育多样性较高的区域物种库，往往包含较高比例的具有北部生物地理亲缘性的物种——这类物种的分布区相对湿润，且携带相应的湿润适应性状。该结果支持本研究提出的气候避难所假说（climatic refuge hypothesis），而非生物地理交叉路口假说（biogeographic crossroads hypothesis）。