Global patterns of leaf defenses in oak species

Plant defensive traits drive patterns of herbivory and herbivore diversity among plant species. Over the past 30 years, several prominent hypotheses have predicted the association of plant defenses with particular abiotic environments or geographic regions. We used a strongly supported phylogeny of oaks to test whether defensive traits of 56 oak species are associated with particular components of their climatic niche. Climate predicted both the chemical leaf defenses and the physical leaf defenses of oaks, whether analyzed separately or in combination. Oak leaf defenses were higher at lower latitudes, and this latitudinal gradient could be explained entirely by climate. Using phylogenetic regression methods, we found that plant defenses tended to be greater in oak species that occur in regions with low temperature seasonality, mild winters, and low minimum precipitation, and that plant defenses may track the abiotic environment slowly over macroevolutionary time. The pattern of association we observed between oak leaf traits and abiotic environments was consistent with a combination of a seasonality gradient, which may relate to different herbivore pressures, and the resource availability hypothesis, which posits that herbivores exert greater selection on plants in resource-limited abiotic environments.植物防御性状主导不同植物物种间的植食作用模式与植食者多样性分布格局。近三十年来，多项经典假说均提出植物防御机制与特定非生物环境或地理区域存在关联。本研究依托得到高支持率的栎属系统发育树，检验56个栎树物种的防御性状是否与其气候生态位（climatic niche）的特定组分存在关联。无论单独还是联合分析，气候因素均可有效预测栎树的叶片化学防御与物理防御性状。栎树叶片防御性状在低纬度地区更为显著，且该纬度梯度完全可由气候因素解释。通过系统发育回归（phylogenetic regression）分析方法，本研究发现：分布于温度季节波动较低、冬季温和且最低降水量偏少区域的栎树物种，其防御性状通常更为突出；同时植物防御性状可能在宏观进化时间尺度上缓慢追踪非生物环境的变化。本研究观测到的栎树叶片性状与非生物环境的关联模式，与两类假说的综合结论相符：一是与不同植食者压力相关的季节梯度假说，二是资源可用性假说（resource availability hypothesis）——该假说指出，在资源受限的非生物环境中，植食者会对植物施加更强的选择压力。