Data from: Anti-herbivore defences and insect herbivory: interactive effects of drought and tree neighbours

1. How much a plant is attacked by insect herbivores likely depends on its apparency and ability to produce defensive traits, which may be modified by neighbouring plants and abiotic conditions. Yet, how much the direct and trait-mediated effects of neighbours on herbivory is modified by abiotic factors is still unknown. 2. By using a tree diversity experiment in SW France, we measured leaf insect herbivory (chewers and miners), nutritional quality (water content, C/N ratio, sugar and starch content) and chemical defences (total polyphenolics and condensed tannins) on birch (Betula pendula) trees growing in monocultures and mixtures with oak, pine or both species. We alleviated water stress by irrigating trees in half of the plots while trees remained unwatered (i.e., drought-stressed) in the other half. 3. Overall, insect herbivory was higher among heterospecific neighbours, which corresponds to associational susceptibility. Consistent with this finding, leaves had lower amount of anti-herbivore defences among heterospecific neighbours. In turn, insects caused more damage in drought-stressed conditions, but such effect was independent of leaf chemistry. We also found that the effect of tree species diversity on herbivory was contingent of drought conditions as associational susceptibility only occurred in drought-stressed trees. The independent and interactive effects of neighbour diversity and irrigation on leaf herbivory remained significant after accounting for birch apparency and leaf chemistry, suggesting that unmeasured plant traits or some other mechanisms not associated with plant trait variation and apparency might be involved in the observed herbivory patterns. 4. Synthesis. By demonstrating that associational effects are contingent upon abiotic constraints, we bring new insights into our understanding of the mechanisms driving diversity - resistance relationships across climatic gradients1. 植物遭受植食性昆虫危害的程度，主要取决于其可察觉性（apparency）与合成防御性状的能力，而邻株与非生物环境可对这些特性产生调控作用。然而，邻株对植食作用的直接效应与性状介导效应，在多大程度上会被非生物因子调控，目前仍无定论。 2. 本研究依托法国西南部的树木多样性实验平台，对纯林以及与栎树、松树或二者混交林分中的白桦（Betula pendula）开展了多项测定：包括叶片植食性昆虫取食危害（咀嚼类与潜叶类昆虫）、叶片营养品质（含水量、碳氮比、糖与淀粉含量）以及化学防御物质水平（总多酚与缩合单宁）。我们对半数样地内的白桦进行灌溉以缓解水分胁迫，另一半样地则不进行补水，维持植株处于干旱胁迫状态。 3. 总体而言，异种邻株生境中的白桦受植食性昆虫危害的程度更高，这与邻株易感效应（associational susceptibility）的规律相符。与之一致的是，异种邻株生境下的白桦叶片抗植食防御物质含量更低。进一步分析显示，干旱胁迫条件下昆虫造成的危害更为严重，但该效应与叶片化学组成无关。本研究还发现，树木物种多样性对植食作用的影响取决于干旱环境：邻株易感效应仅在干旱胁迫的植株中出现。在纳入白桦可察觉性与叶片化学组成的影响后，邻株物种多样性与灌溉处理对叶片植食危害的独立效应与交互效应依然显著，这表明未被测定的植物性状，或是与植物性状变异及可察觉性无关的其他机制，可能参与了本次观测到的植食危害格局的形成。 4. 研究总结：本研究证实邻株效应取决于非生物约束条件，为解析气候梯度下植物多样性-抗虫性关系的驱动机制提供了全新视角。