Delayed Budbreak: A Defensive Response of Mountain Birch to Early-Season Defoliation?

Abstract

Delayed budbreak has been suggested to be a potentially defensive characteristic of deciduous trees against early-feeding lepidopteran larvae. We therefore tested the effects of early (in June) and late (in August) manual defoliations on leaf development in mountain birch. Early defoliation induced a significant delay in budbreak, whereas late defoliation influenced only slightly leaf flush during the next growing season. This supports the defense hypothesis. However, the responding units were separate branches rather than entire birch trees, since leaf growth was delayed on defoliated branches relative to untreated branches on the same trees. Furthermore, as artificial defoliation induced the delay in leaf flush, the presence of larvae is not necessary for triggering these localized responses in mountain birch. The fact that both early and late defoliation reduced final leaf size may suggest that the observed defoliation effects were associated with localized resource deficiencies suppressing leaf growth. Still, a delay in budbreak on previously defoliated branches, due to resource shortage or other causes, could be expected to adversely affect immobile early-feeding larvae. Mobile larvae, on the other hand, should be generally capable of overcoming within-tree variation in the predictability of their food resources.