Studies on carbohydrate dynamics on trees in dry evergreen forest, Thailand

In order to manage a forest ecosystem properly, it is important to clarify the ecophysiological characteristic among the trees composing the forest. Light environment within a forest is complex. Under various light condition, it is important to know how the trees in different environment (e.g., canopy and understory) utilize the photosynthates efficiently. In this study, diel carbohydrate dynamics on trees was clarified by analyzing the carbohydrate content and making a compartment model in Dry Evergreen Forest (DEF) in Thailand, where the forest has been highly disturbed by human activities. Diel carbohydrate dynamics were studied on a canopy-1ayer species of Hydnocarpus ilicifolius. The measurement was conducted in the beginning of dry season in 1997. Leaves, twigs, stems and roots were sampled every 3 hours during a 24hours period, and contents of sucrose, hexose and starch in ail tissues were quantified. The fluctuations of total non-structural carbohydrate (TNC) content in every organ seemed to have a baseline with a few peaks. The minimum values of TNC contents in all organs were around 100 mg g(-1) of DW and they suggested a "baseline" of the fluctuation. The baseline would define a long-term carbohydrate storage in the organ and the peak showed the temporary increment. Three to nine hours after the TNC content reached the peak in the upper organ, the TNC content showed the highest value in the lower organ. The different peak occurrences seemed to reflect the mass carbohydrate movement because the time difference in the peaks reasonably fit with the reported value of translocation rate. The ranges of diel fluctuations were in the range of the seasonal change reported in the past. It was suggested that studies of carbohydrate contents in plants should take this large variation into consideration. To analyze carbon dynamics between leaves and the other plant parts we developed a compartment model and compared the differences between canopy-layer species and underlayer species. Hydnocarpus ilicifolius was selected as a canopy species and Glycosmis parva as an underlayer species. Using the model, the carbon export rate was estimated by subtracting the changes in carbohydrate content from carbon gain by photosynthesis during a continuous 24hour period. The results for H. ilicifolius suggested that assimilated carbon was stored as starch in the morning, but released as monosaccharides for the rest of the daytime. Carbon export occurred not only in the night, but also during the day when the light intensity was low for H, ilicifolius. In case of G. parva, the translocation occurred throughout 24hour period, with the highest carbon export during periods of sunflecks. Thus, translocation of carbon appeared to occur during night and under low light intensity in both species. Furthermore, accumulated carbohydrate might change its form corresponding to the environmental changes. Our study showed that carbon flow in tree species could be more flexible than has generally been thought and that the dynamics differed largely depended upon the species as has been revealed in H. ilicifolius and G. parva.