Data from: Ontogenetic changes in insect herbivory in birch (Betula pubesecens): the importance of plant apparency

1. Several theories aim at predicting changes in the interactions between plants and herbivores over the lifetime of a plant. Hypotheses based on ontogenetic changes in resource allocation to plant defence and in plant apparency lead to partly opposing predictions regarding the differences in levels of herbivory between juvenile and mature plant individuals. 2. We tested these predictions by measuring background foliar losses to insects in saplings and mature trees of downy birch (Betula pubescens) in ten sites along a latitudinal gradient from 60 to 69ºN in boreal forests of Northern Europe. 3. The percentage of consumed leaf area increased, and the variation in the levels of herbivory among plant individuals decreased, for tree sizes ranging from small saplings (2−12 cm tall) to large saplings (13−80 cm tall) and then to mature trees (3−18 m tall). 4. Small saplings had higher foliar quality for insects, as indicated by better performance of leafmining larvae of Eriocrania semipurpurella and by greater specific leaf area, compared with large saplings and mature trees. The average percentage of leaf area consumed from a damaged leaf, which reflects the inducibility of local defence responses to insect damage, did not vary among the birch size classes. 5. The foliar losses to insects decreased nearly fivefold with an increase in latitude from 60 to 69ºN, and the relative differences in these losses among the birch size classes were independent of latitude. 6. Our findings fit well with the predictions based on a plant apparency hypothesis, but do not support predictions based on ontogenetic changes in resource allocation to plant antiherbivore defences. We conclude that the generally overlooked effects of apparency on plant damage by insects can explain, at least in some cases, the frequently observed lack of correspondence between the levels of plant defences and herbivory.1. 已有多项理论旨在预测植物整个生命周期中与植食动物之间互作关系的变化。基于资源分配至植物防御的个体发育变化，以及植物显见性提出的假说，针对幼龄与成熟植株的植食危害水平差异，得出了部分相悖的预测结果。 2. 我们通过在北欧寒温带森林中沿60°N至69°N的纬度梯度设置的10个样地，对柔毛桦（Betula pubescens）的幼树与成熟林木的昆虫背景性叶片取食损失量进行测定，以此检验上述预测。 3. 随着林木株型从小型幼树（株高2−12 cm）到大型幼树（13−80 cm）再到成熟林木（株高3−18 m），被取食的叶面积百分比逐渐升高，而不同植株个体间的植食危害水平差异则逐步降低。 4. 与大型幼树及成熟林木相比，小型幼树的叶片昆虫适口性更佳：具体表现为紫翅狭蛾（Eriocrania semipurpurella）的潜叶幼虫生长表现更好，且比叶面积（specific leaf area）更高。而反映昆虫取食诱导本地防御反应程度的单片受害叶平均被取食叶面积百分比，在不同株型的桦树间并无显著差异。 5. 随着纬度从60°N升高至69°N，昆虫造成的叶片损失量下降了近五倍，且不同株型桦树间的植食危害相对差异不受纬度影响。 6. 我们的研究结果与基于植物显见性假说（plant apparency hypothesis）的预测高度吻合，但并不支持基于资源分配至植物抗植食防御的个体发育变化所提出的预测。综上，此前常被忽视的显见性对昆虫造成植物损伤的影响，至少在部分案例中，可以解释为何频繁观测到植物防御水平与植食危害程度之间并不匹配这一现象。