Heterogeneity of light availability and its effects on simulated carbon gain of tree leaves in a small gap and the understory in a tropical rain forest

To clarify the small-scale heterogeneity of light regimes in a rain forest, photosynthetic photon Awe density (PFD) was measured at 1-min intervals du ring six days at 12 microsites in each of two plots, a small gap and an und erstory in Pasoh Forest Reserve, Peninsular Malaysia. Frequency distributio n of microsite PFD was unimodal with the peak value between 16 and 32 mu mo l/m(2)/sec in the small gap, bur between 8 and 16 mu mol/m(2)/sec in the un derstory. In the small gap, PFD was more variable among microsites; total d aily PFD and daily sunfleck PFD exceeding 10 mu mol/ m(2)/sec tended to be higher (P < 0.05; t-rest) compared to those in the understory. Sunfleck PFD exceeding 50 mu mol/ m(2)/sec, however, showed no difference between the t wo plots. Diffuse PFD transmittance, defined as the ratio of PFD in the For est to that measured at 43 m above ground during the periods 0800-0810 and 1750-1800 h, was significantly higher in the small gap than in the understo ry plot. Diffuse PFD transmittance was also positively correlated with micr osite total daily PFD. To examine the effects of the subtle heterogeneity o f light regimes on leaf carbon gain, we simulated carbon gain by sun and sh ade leaves in a typical shade-tolerant species, Brosimum alicastrum Sw. (Mo raceae). Despite the similarity in total daily PFD, total daily carbon gain was considerably higher in the gap than in the understory for both sun and shade leaves. This study suggests that frequency distribution of PFD is cr itical in describing microsite PFD regimes and determining leaf carbon gain in the tropical forest Boor.