Effects of defoliation on the saplings of a gap-colonizing neotropical tree

Abstract. This paper describes a defoliation experiment on saplings of the gap-colonizing pioneer tree Heliocarpus appendiculatus in the tropical rain forest of Los Tuxtlas (Southeast Mexico). Four levels of defoliation (0, 25, 50, and 75% of the leaf area removed) were applied to naturally established plants in a medium-sized forest gap. Records were made of growth (height, diameter, and leaf production) and of whole-plant and leaf survival. Statistical comparisons for all variables showed that plants that had not been defoliated grew better than defoliated ones, but there were no statistically significant differences between the defoliation levels. The risk of death was significantly lower for control plants than for defoliated plants at any level of damage. Leaf survivorship was highly irregular. The most consistent pattern was that the leaves of intact plants always showed higher survivorship, while the most heavily defoliated ones always had the poorest survival. The survivorship pattern of leaves at intermediate defoliation levels was irregular. The results illustrate the lack of a monotonic response to a wide range of defoliation levels, and suggest the potential effect of herbivores as reducers of vegetative growth and survival in pioneer tropical species, and as limiting agents of plant establishment in regenerating forest gaps.