Temperature controls ecosystem CO2 exchange of an alpine meadow on the northeastern Tibetan Plateau

Alpine ecosystems are extremely vulnerable to climate change. To address the potential variability of the responses of alpine ecosystems to climate change, we examined daily CO2 exchange in relation to major environmental variables. A dataset was obtained from an alpine meadow on the Qinghai-Tibetan Plateau from eddy covariance measurements taken over 3 years (2002-2004). Path analysis showed that soil temperature at 5 cm depth (T-s5) had the greatest effect on daily variation in ecosystem CO2 exchange all year around, whereas photosynthetic photon flux density (PPFD) had a high direct effect on daily variation in CO2 flux during the growing season. The combined effects of temperature and light regimes on net ecosystem CO2 exchange (NEE) could be clearly categorized into three areas depending on the change in T-s5: (1) almost no NEE change irrespective of variations in light and temperature when T-s5 was below 0 degrees C; (2) an NEE increase (i.e. CO2 released from the ecosystem) with increasing T-s5, but little response to variation in light regime when 0 degrees C <= T-s5 <= 8 degrees C; and (3) an NEE decrease with increase in T-s5 and PPFD when T-s5 was approximately > 8 degrees C. The highest daily net ecosystem CO2 uptake was observed under the conditions of daily mean T-s5 of about 15 degrees C and daily mean PPFD of about 50 mol m(-2) day(-1). The results suggested that temperature is the most critical determinant of CO2 exchange in this alpine meadow ecosystem and may play an important role in the ecosystem carbon budget under future global warming conditions.