Estimating leaf area index and photosynthetic production in canopies of the mangrove Rhizophora apiculata

Comparative estimates of the leaf area index (LAI) of a mangrove forest canopy in Peninsular Malaysia were obtained by 3 independent methods: (1) from direct measurement of canopy leaf area above a prescribed ground area; (2) from simple measurements of light flux density above and beneath the canopy; (3) from measurements of direct beam transmittance through the canopy. Measurements were also made of the rate of net photosynthesis at different levels in the canopy. LAIs obtained by direct measurement ranged from 2.2 to 7.4 over 4 replicate 0.25 m(2) quadrats, with a mean of 4.9. This high degree of variability was attributed partly to the small quadrat size and partly to the spatial heterogeneity of the canopy. In contrast, estimates of LAI obtained from log averaged measurements of light transmission over a larger area of forest varied by less than 5% between replicates, with an average of 5.1. Measurements of direct beam transmittance yielded an average canopy LAI of 4.4. Average rates of net photosynthesis ranged from 4 mu mol CO2 m(-2) s(-1) at the bottom of the canopy to about 10 mu mol CO2 m(-2) s(-1) at the top of the canopy. The LAI weighted average rate of net photosynthesis integrated over the entire canopy was 9 mu mol CO2 m(-2) leaf s(-1). Assuming an effective daylength of 8 h, total daily net photosynthetic carbon fixation was estimated to be 155 kg C ha(-1) d(-1), giving an annual net photosynthetic production of 56 t C ha(-1) yr(-1). This is substantially higher than previously reported for mangrove forests.