Growing after attack: what predicts tree responses to herbivores?

Trees can face significant threats from herbivores, particularly ones that defoliate (i.e. eat leaves and reduce leaf area). After being attacked by defoliators, trees can respond by changing their growth patterns and producing defence chemicals. In extreme cases, trees are unable to mount an effective response to defoliation, killing the tree. On a forest-wide scale, these responses can affect the entire ecosystem, through changes in water runoff and forest productivity. A crucial question in tree-herbivore interaction research is whether we can predict trees’ responses to defoliation.

In a recent article from Tree Physiology, Dr. Jane Foster set out to answer this question by analyzing the results from over 30 studies examining how trees respond to defoliation from herbivores (called a meta-analysis). In general, evergreen coniferous trees that had long-lived leaf tissues were more likely to die than deciduous trees with shorter-lived leaves when they were attacked by herbivores. Why might this be? Dr. Foster suggests that it comes down to where the trees maintain energy reserves. In deciduous trees, the carbohydrates produced through photosynthesis are stored in stem and root tissues by necessity, since there are no leaves to store carbohydrates in during the winter or dry season, while evergreen trees can store carbohydrates in their leaves. Conifers also tend to rely on leaf energy stores for producing new leaves. After a defoliation event, deciduous trees still have substantial energy stores in their stems and roots, while conifers will have lost a large proportion of their energy stores in old leaves to rebuild new leaves, which increases their risk of dying.

These results can help us assess the mortality risk of trees during a defoliation event using traits such as leaf longevity and how carbohydrates are stored in the species. Such information could then be used in models of tree growth and survival to predict which trees and forests may need protective measures (e.g. biocontrol of pests, pesticide application) in advance of a defoliation event.