Photosynthetically active radiation heterogeneity within a monodominant Congolese rain forest canopy

Photosynthetic photon flux density (PPFD) regimes within vegetation canopies are intrinsically heterogeneous in time and space. Photosynthesis responds to changes in the intensity and temporal variability of PPFD in a non-linear way. Therefore, it is important to quantify PPFD heterogeneity at short time scales in order to better understand and assess plant-atmosphere carbon exchange from the scale of the individual leaf to the entire canopy. Here, we report the intensity and temporal heterogeneity of sunflecks at three heights within a 42 m tall, monodominant Gilbertiodendron dewevrei tropical rain forest canopy in central Africa over a 12-day measurement period. Overall, sunflecks occurred during an average of 0.7% of the total sample time and contained 9.5% of the total recorded energy. These values ranged in magnitude with canopy height, as did the average intensity of PPFD contained in sunflecks. Probability distributions of sunfleck intensities were positively skewed at all three measurement heights. The temporal separation of sunflecks was bimodal at all three measurement heights; for example, at 3 m above ground level (AGL), 85% of all sunflecks were separated by either less than 30 s (43%) or more than 32 min (42%). Sunflecks were clustered in time such that 68, 84, and 89% of all sunflecks at respective heights of 3, 24, and 34 m AGL fell within clusters where no more than 2 min of non-sunfleck PPFD separated temporally adjacent sunflecks. This, plus the fact that a large percentage of the time within the clusters is comprised of sunfleck-level PPFD, indicates that sunfleck clusters are of great importance for photosynthesis within the canopy. We discuss the data from this site in the context of recent measurements and models of dynamic photosynthesis and present some possible implications for photosynthetic processes within a dense tropical forest canopy. (C) 2000 Elsevier Science B.V. All rights reserved.