Physiological Response of Tropical Tree Species to Elevated CO_2 Levels at Seedling Stage

摘要

The Increasing CO_2 concentration in the atmosphere levels often stimulates the photosynthesis and biomass. However, the duration and magnitude of this stimulation on physiological processes of tropical tree species is unknown. The objective of this experiment was to examine the physiological response of four commercially important tropical tree species to elevated CO_2levels. Tectona grandis (teak). Azadirachta indica (neem), Ailanthus excelsa (maharukh) and Bambusa bambos (bamboo) seedlings were exposed to ambience (380 ppm) or elevated CO_2(600 and 900 ppm) levels using an automated open top chamber (AOTC). The seedlings were exposed to elevated CO_2levels for 180 days. Leaf gas exchange characters were measured in the second or third fully expanded leaves of the seedlings with a portable infra-red gas analyser (Li-Cor 6400XT) at the end of the study period. The elevated CO_2 levels significantly affected the physiological processes and did not show identical response in the studied tree species. Maharukh and bamboo plants were recorded optimistic response in terms of photosynthesis under elevated CO_2 even up to 900 ppm. Teak also registered a high photosynthesis up to 600 ppm level of CO_2, but it showed a decreased photosynthetic rate under900 ppm of CO_2. Contrary to these three species, neem Showed a negative response to the elevated CO_2 at both 600 and 900 ppm levels. The response of the species on transpiration (E) mmol m~(-2)S~(-1) was similar to that of the photosynthetic rate (Pn).This study recommend that we should not generalise the response of tropical tree species to elevated CO_2 However, the commercially important tropical tree species should be assessed individually for the physiological functions to elevated CO_2