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 semipurpure...