Partial compensation in Psychotria marginata (Rubiaceae) after simulated defoliation increases photon capture and photosynthesis.

摘要

We used Y-plant, a computer-based model of plant crown architecture analysis, to simulate effects of defoliation on daily canopy carbon gain in Psychotria marginata (Rubiaceae) plants under two contrasting irradiances. The plants are growing in a forest understorey that has been largely undisturbed for at least 50 years in the Smithsonian Tropical Research Institute field station, Barro Colorado Island, Panama. Five levels of defoliation were simulated using two different types of leaf blade damage (bite and edge type damage). Compensatory increases in photon-saturated photosynthetic capacity (Pmax) of 25, 50, and 100 defoliation were also simulated. In all simulations, the daily photon capture and CO2 assimilation increased with defoliation. However, without a compensatory response, daily canopy carbon gain also decreased with defoliation. Under high irradiance, reduction in daily canopy carbon gain was less than what would be expected if the response was proportional to leaf area reduction. Thus, 25 and 50 defoliation resulted in only 20 and 41 of daily canopy carbon gain reduction, respectively. In the scenario where 25 of the leaf area was removed, if the Pmax value was increased by 25, the remaining leaves compensated for 94 of the daily canopy carbon relative to an undamaged non-compensated plant. At the same defoliation level, increasing Pmax values by 50 and 100 resulted in overcompensation. It is concluded that the increase in photon capture and CO2 assimilation resulting from herbivory could only partially contribute to post-defoliation compensation of daily canopy carbon gain. Because the increase of these two factors is a passive consequence of the reduction in leaf area instead of an active response, under the conditions presented here, photosynthetic compensation could be only possible through an active mechanism such as the increase of Pmax values..