Abstract:

Tropical forests play an important role in altering the carbon budgets of terrestrial ecosystems. We examined patterns of diurnal and seasonal net ecosystem CO₂ exchange (NEE) in a tropical seasonal rainforest of Xishuangbanna on clear days between November 2003 and October 2004. We found that the diurnal dynamics of NEE showed a single_peaked curve. During daytime throughout the year, NEE increased with solar radiation after sunrise, but fluctuated after sunset. Values of NEE (absolute values) in the foggy_cool and wet seasons were greater than those in the dry_hot season during daytime. During the night, NEE values were dominated by soil temperature and soil water content and were highest in the wet season, followed by those in the dry_hot season and lowest in the foggy_cool season. NEE, water vapor deficit (VPD) and air temperature (T_a) curves all showed pronounced seasonal variation, but photosynthetically available radiation (PAR) did not vary significantly. Maximum photosynthesis rates (P_max) and dark respiration rates (R_e) were greater when VPD≥16 hPa than whenVPD<16 hPa, whereas photon density (α) was the opposite. R_e was also greater when T_a≥25 ℃ than whenT_a<25 ℃ in the three seasons.α was reduced when T_a≥25 ℃ in the dry_hot and wet seasons. TheP_max increased in the dry_hot season and decreased in the wet season when T_a≥25 ℃. Our data suggested thatPAR is the main factor influencing NEE diurnal dynamics, whereas both VPD and T_a play a major role in regulating NEE seasonal dynamics.

Key words: Xishuangbanna tropical seasonal rainforest, Net ecosystem CO₂ exchange, Eddy covariance, Diurnal and seasonal variation