Gas Exchange and Turgor Maintenance of Tropical Tree Species in Pasoh Forest Reserve

Abstract

Net photosynthetic rate (Pn), stomatal conductance (Gw), water use efficiency (WUE), and osmotic potential (ψ s0) were studied for 46 tropical tree species, including 24 tree species found in Pasoh Forest Reserve, in order to clarify their ecophysiological traits. The maximum value of Pn (Pnmax) varied from 2.5 to 24.2µmolCO2 m-2s-1 with an average of 9.1µmolCO2 m-2 s-1. The maximum value of Gw (Gwmax) varied from 30 to 1,300 molH2O m-2 s-1 with an average of 340 mmolH2O m-2 s-1. These values were relatively low compared to those of temperate tree species. Intrinsic water-use efficiency (IWUE, Pnmax/Gwmax) of the tropical species was also relatively low compared to the temperate tree species. ψ s0 of the tropical species was relatively high (less negative) compared to the temperate tree species, suggesting that the capacity of leaves to maintain positive turgor is relatively low in the tropical species. The lower photosynthetic rate in the tropical species was due not only to lower stomatal conductance, but also to lower photosynthetic efficiency, compared to the temperate species. The rapid growth of tropical tree species may be due to favorable environmental factors such as relatively constant temperature and moisture levels, which permit photosynthesis all the year-round.