BlOTROPlCA 31(2): 268-278 1999 zyxwvut 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' Yanhong Tang' Global Environmental Research Division, National Institute for Environmental Studies, Tsukuba, lbaraki 305, Japan Naoki Kachi Department of Biology, Faculty of Science, Tokyo Metropolitan University, Hachioji, Tokyo 192-03, Japan Akio Furukawa Department of Biology, National Institute for Environmental Studies, Tsukuba, lbaraki 305, Japan and Muhamad B. Awang Centre for Environmental Technology and Natural Resources, Universiti Putra Malaysia, 43400 UPM, Serdang, Selangor, Malaysia ABSTRACT zyxwvuts To clarify the small-scale heterogeneity of light regimes in a rain forest, photosynthetic photon flux density (PFD) was measured at 1-min intervals during six days at 12 microsites in each of two plots, a small gap and an understory ill Pasoh Forest Reserve, Peninsular Malaysia. Frequency distribution of microsite PFD was unimodal with the peak value between 16 and 32 p,mol/m2/sec in the small gap, but between 8 and 16 p,mol/m2/sec in the understory. In the small gap, PFD was more variable among microsites; total daily PFD and daily sunfleck PFD exceeding 10 p,mol/ m2/sec tended to zyxwvutsrq be higher zyxwvuts (P< 0.05; t-test) compared to those in the understory. Sunfleck PFD exceeding 50 Fmoli m2/sec, however, showed no difference between the two plots. Diffuse PFD transmittance, defined as the ratio of PFD in the forest to that measured at zyxwvutsr 43 m above ground during the periods 0800-0810 and 1750-1800 h, was significantly higher in the small gap than in the understory plot. Diffuse PFD transmittance was also positively correlated with microsite total daily PFD. To examine the effects of the subtle heterogeneity of light regimes on leaf carbon gain, we simulated carbon gain by sun and shade leaves in a typical shade-tolerant species, Brosimum alicasmm Sw. (Moraceae). 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 critical in describing microsite PFD regimes and determining leaf carbon gain in the tropical forest floor. Kty words: tropical trees. carbon gain; gaps: light environments; Malaysia; microsites; Pasoh Forest Reserve; sunjecks; tropical rain forest; MANY 5TUDIFS HAVE REVFALED THAT LlGI I1 REGIMES in tropical forests are spatially and temporally het- erogeneous (Whitmore & Wong 1959, Yoda 1974, Pearcy 1983, Chazdon & Fetcher 1984, Smith et al. 1992). Most of these studies focused on the variation of light regimes on large spatial scales be- tween the gaps and the understory, or among gaps of various sizes (e.g., Chazdon & Fetcher 1984). A multilayered forest canopy, however, may result in variations of light regimes on much smaller spatial ' Received 14 November 1996; revision accepted 10 Au- gust 1997. Corresponding author: Tang Yanhong, Global Environ- mental Research Division, National Institute for Environ- mental Studies, Tsukuba, Ibaraki 305, Japan; E-mail: tangyh@nies.go. jp scales, such as that of individual leaves or seedlings within a gap or within the understory. Very little is known about the heterogeneity of light regimes on these small scales in tropical forests (Oberbauer et al. 1988, Chazdon 1992). Photosynthetic photon flux density (PFD) af- fects the growth, survival, and regeneration of trop- ical rain forest plants (Chazdon 1988). Light re- gimes also influence the diversity of canopy trees (Nufiez-Farfan & Dirzo 1988, Poulson 1989). The spatial heterogeneity of light regimes on the scale of individual plants may play an important role in the regeneration of seedlings under canopies (Pear- cy et al. 1994), since it is the light regime around each individual plant that affects its performance and fate (Grubb 1977, Harper 1977, Tang et al. 268