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 CO2 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 CO2 fixation capacity.