Retention of green leaves not brown leaves increases spring cynipid diversity on large valley oaks

Abstract

Plants can retain either physiologically active green leaves or inactive brown leaves over winter. Research has suggested that leaf retention incurs a cost due to higher herbivore load in the following year; however, no distinction has, thus, far been made between retention of green and brown leaves. We surveyed the over-winter retention of physiologically active green and inactive brown leaves of valley oaks (Quercus lobata) and examined their relationship with the diversity and density of 15 gall-making cynipid wasp species. Cynipid diversity in the spring was 8.2-fold greater on larger trees with more green leaf retention. Brown leaf retention was not associated with spring cynipid diversity, but was related to a substantial 20-fold reduction in spring cynipid densities on large trees. Retention of either leaf type was generally a poor predictor of summer cynipid diversity and density. Overall, green leaf retention better explained cynipid diversity, but brown leaf retention better explained cynipid densities. These differing effects may be explained by the fact that green leaves provide a common cue used by herbivores to find a suitable host. Retained brown leaves, however, may be an ecological trap for over-winter gall wasps that normally drop to the ground.