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 en hance recovery from herbivore damage. In contrast, allocation patterns shou ld maximize height growth in gaps where competitors grow rapidly. We tested the interactive effects of canopy gaps and simulated herbivory (by removin g the apical meristem, or 10%, 50% or 90% of tissue from all leaves) on juv enile (<1 cm diameter at breast height) Shorea quadrinervis Sloot (Dipteroc arpaceae), a dominant canopy tree in Borneo. 2 Damage did not diminish survival over 8 months, except for 90% foliage re moval from understorey plants. Height growth in the understorey was negligi ble in both control and damaged plants. In gaps, height growth was stimulat ed by removal of either the apical meristem or 10% of leaf tissue; growth w as 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 unders torey. Ninety per cent foliage removal drastically reduced production in ga ps and retention in the understorey. After 10% defoliation, height growth w as 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 on ly half the root : shoot ratio of plants left in the shade. 5 Allocation patterns, together with the survival of understorey plants aft er all but the highest levels of damage, indicate that root : shoot allocat ion there tends to reduce the risk of mortality following herbivore or mech anical damage at the expense of growth. In contrast, in gaps, where interfe rence competition for light is high, resource allocation apparently maximiz es height growth. The interaction between resource availability and herbivore damage provides the basis for a more synthetic theory of resource allocation and growth st rategies than one based on resource availability alone.