Gas exchange analysis for estimating net CO₂ fixation capacity of mangrove (Rhizophora stylosa) forest in the mouth of River Fukido, Ishigaki Island, Japan

Mangrove trees have been considered to possess a higher carbon fixation capacity than terrestrial trees although a reliable method to estimate their CO₂ fixation capacity has not been established. In this study, net CO₂ fixation in above-ground of Rhizophora stylosa was estimated as the difference between photosynthetic absorption and respiratory emission of CO₂. In order to estimate these parameters, photosynthetic rates of single-leaves in response to light and temperature and the respiratory rates of leaves and branches in response to temperature were measured. Furthermore, we established a model of diurnal change in temperature. Monthly averages of the diurnal temperature change were used for correcting the CO₂ absorption and emission. The effect of temperature modification on the estimation of net CO₂ fixation was examined, and the net CO ₂ fixation capacity estimated with and without temperature modification was compared. Biomass accumulation estimated without temperature modification (i.e. corrected only for the light intensity) was 6.1 tons ha ^-1 yr^-1, while that estimated with temperature modification (i.e. corrected for both light intensity and temperature) was 13.0 tons ha^-1 yr^-1. A doubling of the estimated values of net CO₂ fixation as observed in this study was caused by the decrease in respiratory CO₂ emission by half, which results from temperature modification. These findings suggest that temperature modification in gas exchange analysis could improve the accuracy of estimation of the net CO ₂ fixation capacity.