Carbondioxide exchange characteristics and biomass of tropical tree species under various environmental conditions

Study on carbon dioxide exchange characteristics of tropical tree species in the present study was carried out at the Mae Klong Watershed Research Station, Ampur Thong Pha Phum, Changwat Kanchanaburi. The study was emphasized at the mixed deciduous forest type only. The gas exchange processes were the photosynthesis and respiration processes. The measurements were carried out with tree seedlings under nursery condition, small trees and mature trees under natural condition as well as the changes of canopy structure which is the most important part where gas exchanges occur. The present report concentrated on the analysis of all the basic results in order to simulate the exchanges of carbon dioxide and the biomass production of deciduous forest in the tropic. The result showed that leaf photosynthetic rates of all species varied diurnally and seasonally. The variation caused by changes of environmental factors such as light intensity, temperature, vapor pressure and carbon dioxide concentration. The leaf photosynthetic rates can be simulated with mathematical model using photosynthetic photon flux density (PFD), vapor pressure difference (VPD) and the carbon dioxide concentration as the input parameters. The estimation on canopy photosynthesis production can be estimated by using direct light model and diffuse light model with different reliability depending on season. However, the diffuse light model is much easier than the direct light model. Canopy structure was evaluated by using a hemispherical photography technique. The canopy area index (CAI) as well as relative light intensity (RLI) can then be calculated from the image. The results showed a remarkable seasonal variation in canopy area index with the value between 2.52-4.15. The RLI was negatively correlated with CAI. The highest CAI was in July (4.15) while the highest RLI was in March (19.50 percent). The estimation of annual net photosynthesis production of deciduous forest and 17-year old teak plantation showed that deciduous forest produce higher net production or has higher potential in uptaking carbon dioxide from the atmosphere. The carbon dioxide uptake by deciduous forest and teak plantation were 86.55 and 59.83 ton/h/yr. The loss of production by respiration from leaves and stems were 61.10 and 42.19 ton/ha/yr. The average dry matter of litter production of the forest was 4.58 ton/ha/yr. The average primary biomass production of the deciduous forest and teak plantation were estimated to be 9.35 and 5.04 to DM/ha/yr.