Effects of Severe Defoliation on the Long-Term Resistance to Insect Attack and on Leaf Chemistry in Six Woody Species of the Southern African Savanna

Abstract

Severe defoliation as occurs in insect outbreaks can alter the chemistry and food value of woody-plant leaves for insects for several years after defoliation ceases. Two hypotheses, the active-defense hypothesis (AD) and the hypothesis of a carbon/nutrient balance (CNB), attempt to explain the mechanism of such responses to defoliation. We tested these hypotheses by studying the responses to severe defoliation by six southern African savanna woody species. Manual defoliation of the three fast-growing deciduous species we studied (Grewia flavescens, Acacia tortilis, Dichrostachys cinerea) resulted in increased resistance to insect attack in the next year. Concomitantly, nitrogen (N) and phosphorus (P) concentrations in leaves decreased and leaf total phenol and condensed tannin concentrations increased. The three slowly growing species that we studied (Burkea africana, Ochna pulchra, Euclea natalensis) responded differently to manual defoliation. In the year following defoliation, leaf resistance to insect attack declined, concentrations of total phenols and condensed tannin in leaves decreased, and concentrations of N and P increased. These opposite responses are consistent with the expectations of CNB rather than AD. We also found that defoliation of B. africana by caterpillars of one of its major defoliators, the saturniid moth Cirina forda, resulted in a greater loss of resistance than did manual defoliation of B. africana. This result further suggests that the AD does not apply to B. africana. We conclude that inherently slow growth constrains the long-term responses of woody plants to severe defoliation. Most of our results indicate that the CNB is a useful working model of woody-plant responses to defoliation. However, CNB's current emphasis on evergreenness as a control over the way slowly growing woody species respond to severe defoliation cannot explain our results. Thus, we suggest that a low photosynthetic capacity is also an important control over these responses.