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. 植物被昆虫植食者攻击的程度，大概率取决于其表观度（apparency）以及产生防御性状的能力，而此类性状可被邻株与非生物环境所调控。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修饰，目前仍不明确。 2. 本研究依托法国西南部的树木多样性实验样地，对纯林以及与栎树、松树或二者混交的林分中的白桦（Betula pendula）叶片开展三项测定：昆虫植食作用（包括咀嚼式与潜叶式昆虫危害）、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（总多酚与缩合单宁）含量。我们对一半样地内的树木进行灌溉以缓解水分胁迫，另一半样地则不浇水，使树木处于干旱胁迫状态。 3. 整体而言，异株邻伴下的白桦叶片受昆虫植食程度更高，这符合关联易感效应（associational susceptibility）。与该结果一致，异株邻伴下的白桦叶片抗植食防御物质含量更低。进一步而言，干旱胁迫环境下昆虫造成的叶片损伤更严重，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效应依赖于干旱条件：仅在干旱胁迫的白桦个体中才会出现关联易感效应。在控制白桦表观度与叶片化学组成的影响后，邻株多样性与灌溉对叶片植食作用的独立与交互效应仍显著，这表明未被测定的植物性状，或是与植物性状变异及表观度无关的其他机制，可能参与了观测到的植食作用模式。 4. 研究总结：本研究证实关联效应依赖于非生物限制因子，为理解气候梯度下物种多样性与抗虫性之间关系的驱动机制提供了全新视角。