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 18º 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的经典范式认为：植食动物压力及其驱动的植物防御，随纬度降低而增强。然而，近期研究对该预测提出了质疑——部分研究未观测到二者存在关联，另有研究得到相反趋势：植食作用或植物防御在高纬度地区反而更强。本研究以长寿树种英国栎（Quercus robur）为对象，检测其叶片的植食作用、化学防御（酚类化合物（phenolic compounds））与营养性状（磷、氮）的纬度变异规律，并探究了驱动该变异的潜在气候与土壤因子。研究团队在沿18°纬度梯度分布的38个英国栎种群中开展采样分析，该梯度覆盖了该物种几乎全部的纬度与气候分布范围，结果观测到叶片植食作用、化学防御与营养成分的纬度梯度呈现显著但分化的模式。正如预期，纬度与叶片植食作用呈负相关：低纬度地区的栎树种群叶片植食程度更高。然而，与预测相悖的是，叶片化学防御与纬度呈正相关，高纬度种群的防御能力更强。同理，叶片磷与氮含量随纬度升高而增加。路径分析显示，植物化学防御物质（缩合单宁（condensed tannins））对叶片植食作用具有显著负向影响，表明叶片植食作用的纬度梯度是由防御投入的反向梯度驱动的。叶片营养成分对植食作用无独立影响。此外，研究发现降水与土壤孔隙度对叶片植食作用存在显著间接效应，该效应通过植物化学防御介导。上述结果表明，非生物因子塑造了植物防御的纬度变异，而这些防御反过来又决定了叶片植食作用的纬度差异。总体而言，本研究通过明确同时调控植物防御与植食作用的非生物因子的种类与作用方式，增进了学界对植物-植食动物相互作用纬度变异的理解。