Growth strategies of a shade-tolerant tropical tree: the interactive effects of canopy gaps and simulated herbivory

1 We hypothesized that the survival of shade-tolerant juvenile trees in the understorey depends on allocation strategies that slow their growth but enhance recovery from herbivore damage. In contrast, allocation patterns should maximize height growth in gaps where competitors grow rapidly. We tested the interactive effects of canopy gaps and simulated herbivory (by removing the apical meristem, or 10%, 50% or 90% of tissue from all leaves) on juvenile (<1 cm diameter at breast height) Shorea quadrinervis Sloot (Dipterocarpaceae), a dominant canopy tree in Borneo. 2 Damage did not diminish survival over 8 months, except for 90% foliage removal from understorey plants. Height growth in the understorey was negligible in both control and damaged plants. In gaps, height growth was stimulated by removal of either the apical meristem or 10% of leaf tissue; growth was reduced only after 90% foliage removal. 3 Higher damage levels increased subsequent net leaf loss. Leaf production was much greater, but leaf retention much lower, in gaps than in the understorey. Ninety per cent foliage removal drastically reduced production in gaps and retention in the understorey. After 10% defoliation, height growth was enhanced in gaps despite approximately 30% net leaf loss. 4 In a separate experiment. juveniles were moved from a shade house to gaps. After 8 months, gap plants had more leaves and more total biomass, but only half the root : shoot ratio of plants left in the shade. 5 Allocation patterns, together with the survival of understorey plants after all but the highest levels of damage, indicate that root : shoot allocation there tends to reduce the risk of mortality following herbivore or mechanical damage at the expense of growth. In contrast, in gaps, where interference competition for light is high, resource allocation apparently maximizes height growth. The interaction between resource availability and herbivore damage provides the basis for a more synthetic theory of resource allocation and growth strategies than one based on resource availability alone.