Canopy‐level photosynthetic compensation after defoliation in a tropical understorey palm

Abstract 1. Increases in photosynthesis of leaves remaining after defoliation may result from shifts in leaf photosynthetic characteristics or from an improved light penetration in the canopy. However, few studies have tried to estimate the relative contribution of these factors to mitigating the negative effects of defoliation. We present a quantitative framework for such an analysis. 2. In a field and greenhouse experiment, plants of the rain forest understorey palm Chamaedorea elegans were subjected to three levels of defoliation (0, 50 or 66% of leaves removed) and, in the greenhouse, grown at two irradiances (5 or 16% of daylight, ‘low light’ and ‘high light’ hereafter). For each plant, leaf photosynthesis and light penetration in the canopy were measured and these data were used to calculate whole‐canopy carbon gain. 3. Defoliation significantly increased the light available to the remaining leaves. In the field and in the ‘high‐light’ greenhouse plants, defoliation also resulted in an increase in the light‐saturated photosynthesis per unit leaf area (Pmax), although in the ‘low‐light’ plants this was not the case. 4. A sensitivity analysis revealed that in the ‘low‐light’ plants, a proportional increase in leaf Pmax did not result in increased canopy‐level carbon gain, while in the ‘high‐light’ and field plants, it did. This suggests that the lack of plasticity in Pmax in response to defoliation, at low light, may be the appropriate pattern to maximize carbon gain. 5. Defoliated plants had 10–18% more average photosynthesis per unit leaf area (Parea) than the control plants, and this increase was mostly the result of an improved light penetration in the canopy. 6. A relatively small fraction (5–30%) of the total estimated loss in canopy carbon gain caused by the removal of leaves was compensated for by an increase in Parea. This suggests that in rain forest understorey plants, an increase in diurnal photosynthesis of remaining leaves has limited potential for mitigating the negative effects of defoliation.