Resource availability and growth responses to defoliation in seedlings of three early-successional, tropical, woody species

Abstract

A growth experiment was conducted using seedlings of three early- successional, tropical, woody species: Dillenia suffruticosa (Dilleniaceae), Macaranga heynei (Euphorbiaceae) and Trema tomentosa (Ulmaceae). These species are characteristic of different positions along a soil fertility gradient in Singapore, with D. suffruticosa being the least and T. tomentosa the most demanding of high nutrient availability. The seedlings were grown in vermiculite at either low or high nutrient availabilities supplied by watering with different concentrations of a commercial plant food. Half the seedlings were subjected to a 50% defoliation at the start of the experiment by cutting off the distal half of each leaf. After 9 weeks the plants were harvested. Macaranga heynei and T. tomentosa seedlings showed no significant difference in parameters of growth such as total dry weight and total leaf area between the defoliated and control seedlings. The seedlings compensated completely for the loss of leaf area. Dillenia suffruticosa did show significant reductions in growth in some parameters due to defoliation, and these were more pronounced under the high nutrient treatment. These findings support the hypothesis that plants characteristic of resource-rich sites can readily recover from herbivory through fast growth, probably associated with a rapid turnover of leaves, whereas species of resource-poor habitats cannot easily replace losses due to herbivory and are adversely effected by defoliation.