Data from: Latitudinal variation in plant chemical defences drives latitudinal patterns of leaf herbivory

A long-standing paradigm in ecology holds that herbivore pressure and thus plant defences increase towards lower latitudes. However, recent work has challenged this prediction where studies have found no relationship or opposite trends where herbivory or plant defences increase at higher latitudes. Here we tested for latitudinal variation in herbivory, chemical defences (phenolic compounds), and nutritional traits (phosphorus and nitrogen) in leaves of a long-lived tree species, the English oak Quercus robur. We further investigated the underlying climatic and soil factors associated with such variation. Across 38 populations of Q. robur distributed along an 18° latitudinal gradient, covering almost the entire latitudinal and climatic range of this species, we observed strong but divergent latitudinal gradients in leaf herbivory and leaf chemical defences and nutrients. As expected, there was a negative relationship between latitude and leaf herbivory where oak populations from lower latitudes exhibited higher levels of leaf herbivory. However, counter to predictions there was a positive relationship between leaf chemical defences and latitude where populations at higher latitudes were better defended. Similarly, leaf phosphorus and nitrogen increased with latitude. Path analysis indicated a significant (negative) effect of plant chemical defences (condensed tannins) on leaf herbivory, suggesting that the latitudinal gradient in leaf herbivory was driven by an inverse gradient in defensive investment. Leaf nutrients had no independent influence on herbivory. Further, we found significant indirect effects of precipitation and soil porosity on leaf herbivory, which were mediated by plant chemical defences. These findings suggest that abiotic factors shape latitudinal variation in plant defences and that these defences in turn underlie latitudinal variation in leaf herbivory. Overall, this study contributes to a better understanding of latitudinal variation in plant-herbivore interactions by determining the identity and modus operandi of abiotic factors concurrently shaping plant defences and herbivory.生态学领域长期存在一项经典范式：植食动物取食压力及由此关联的植物防御能力，均随纬度降低而增强。然而近期研究对该预测提出了挑战——部分研究未观测到二者与纬度的显著关联，或呈现相反趋势：即植食作用或植物防御能力在高纬度地区反而更高。 本研究以长寿乔木物种欧洲夏栎（English oak，Quercus robur）为研究材料，对其叶片的植食程度、化学防御物质（酚类化合物，phenolic compounds）及营养性状（磷、氮）的纬度变异展开测试，并探究了与该变异相关的潜在气候与土壤因子。 我们选取了分布于18°纬度梯度上的38个欧洲夏栎种群，覆盖该物种几乎完整的纬度与气候分布范围。结果显示，叶片植食程度、化学防御物质与营养成分均存在显著但分化迥异的纬度梯度格局：正如预期，纬度与叶片植食程度呈负相关关系，低纬度地区的栎树种群叶片受植食程度更高。但与预测相悖的是，叶片化学防御能力与纬度呈正相关关系，高纬度种群的防御能力更强。类似地，叶片磷、氮含量亦随纬度升高而增加。 路径分析（path analysis）结果表明，植物化学防御物质（缩合单宁，condensed tannins）对叶片植食程度存在显著负向影响，表明叶片植食程度的纬度梯度是由防御投入的反向纬度梯度所驱动的。叶片营养成分对植食程度无独立影响。进一步研究发现，降水量与土壤孔隙度对叶片植食程度存在显著间接效应，且该效应通过植物化学防御物质介导。 上述研究结果表明，非生物因子塑造了植物防御能力的纬度变异，而这些防御能力反过来又决定了叶片植食程度的纬度差异。总体而言，本研究明确了同时调控植物防御与植食程度的非生物因子的种类与作用模式，从而增进了学界对植物-植食动物相互作用纬度变异的理解。