Sunfleck Contribution to Leaf Carbon Gain in Gap and Understory Tree Seedlings of Shorea macrophylla

Abstract

Dynamic gas exchange in response to sunflecks has been studied extensively through laboratory experiments and by modeling. However, model estimates of photosynthetic changes in response to sunflecks vary widely depending on the local light environment and on species-specific differences. Moreover, it remains unclear how much sunflecks contribute to leaf carbon gain in natural forests because of a lack of field measurements of dynamic photosynthetic responses. Since sunflecks may increase photosynthetic carbon gain by providing photosynthetic energy and by decreasing limitations on the induction of photosynthesis, we hypothesized that a large proportion of photosynthetic photon flux density (PPFD) received from sunflecks would increase the proportion of carbon gain contributed by sunfleck PPFD. We measured dynamic changes in photosynthesis of Shorea macrophylla tree seedlings experimentaly planted under a canopy gap microsite and under the forest canopy in the Pasoh Forest Reserve (Pasoh FR) of Malaysia. The total proportion of daily leaf carbon gain that resulted from sunfleck utilization varied from 26% to 83%, depending on the microsite, during 7 days of measurements. Sunfleck utilization efficiency decreased with increasing threshold values for sunfleck PPFD. The results indicated that total daily carbon gain was closely related to total daily PPFD and total sunfleck PPFD, but was weakly related to diffuse background PPFD.