EFFECTS OF DEFOLIATION ON COMPETITIVE INTERACTIONS IN EUROPEAN WHITE BIRCH

Abstract

Defoliation can affect plant competition in at least two ways. Both negative effects on the performance of defoliated plants and positive effects due to neighbor defoliation can potentially alter competitive relations between plants. Furthermore, if there is a trade-off between defense investment and growth or competitive ability, then selective herbivory may contribute to coexistence of defended and undefended plants. We tested these ideas in an experiment using two types of European white birch (Betula pendula). We compared growth (height) in two types that are known to differ with respect to palatability to small mammalian herbivores. The plants were grown from seeds in a greenhouse for three years and were potted either singly or in pairs. The pairs consisted either of two plants of the same type (pure pairs) or of one plant of each type (mixed pairs). In the second year, we defoliated randomly preselected plants twice. The experiment was concluded at mid-season in the third year. Both competition and defoliation (of the target plant) negatively affected growth. Competition reduced growth by 19–36%, the palatable type consistently being more affected than the unpalatable type. The negative effects of defoliation were about the same (9–13%) for both types. When grown singly, the palatable type grew taller (13%) than did the unpalatable type. The unpalatable type was the stronger competitor of the two types. As compared with plants grown singly, palatable plants grown in pure pairs were on average 24% shorter, while those grown in mixed pairs were 36% shorter. The corresponding data for the unpalatable type were 19% and 20%. The effects of neighbor defoliation differed between the types. The only significant effect of neighbor defoliation was that in palatable plants grown in mixed pairs neighbor defoliation compensated fully for the competitive effects, i.e., they were not significantly different from undefoliated plants grown singly. There were no significant effects of neighbor defoliation—neither on palatable plants grown in pure pairs, nor on unpalatable plants. Consequently, the effects of defoliation may not only depend on the presence of neighbors, but also on the composition of the neighborhood.