Examination on Processes and Mechanisms Involved in Carbon Dynamics in an Alpine Meadow on the Qinghai-Tibetan Plateau

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The Qinghai-Tibetan Plateau is the highest plateau in the world. Covering mainly by alpine grasslands, the plateau plays an important role in the region's carbon budget. The plateau is characterized by high irradiance, low atmospheric pressure, and low temperature. To examine the carbon dynamics of grassland ecosystems on the plateau, we developed a new simulation model, the carbon dynamics model for alpine grasslands (CDMag). CDMag is composed of a gross photosynthesis (Ac) submodel including pbotoinhibition simulation, an ecosystem respiration (ER) submodel that considers both autotrophic and heterotrophic respiration, and an intercellular CO_2 partial pressure and canopy conductance submodel. A photoinhibition simulation is included to describe the gross production of alpine ecosystems in response to the high-irradiance environment A multilayered irradiance transfer scheme is adopted to estimate the irradiance on sunlit and shaded leaves within grassy canopies, and a coupled canopy … More conductance - net photosynthesis function is integrated into the Ac submodel. CDMag was used to simulate the carbon dynamics of the most extensively distributed alpine grassland, a Kobresia meadow ecosystem on the Qinghai-Tibetan Plateau. CDMag described well the diurnal and seasonal variation of net ecosystem exchange (NEE) and nighttime ER data obtained from 2001 to 2003 using the eddy-covariance method. The mean measured NEE was about 1.62 μmol m^<-2>s^<-1> for the measured periods of the 3 years ; the mean of the deviations between measured and simulated NEE was -0.16 to 0.33 μmol m^<-2> s^<-1>. Owing to photoinhibition, daily gross photosynthesis lost about 5.3%-5.6%. The losses were mainly contributed by the 7.5%-7.8% reduction in Ac of sunlit leaves of the upper canopy layer. In addition, the light-use efficiency of the alpine meadow had seasonal dynamics, with the highest values in the middle of the growing season. We have generalized CDMag for other alpine grasslands on the entire plateau.In addition, high UV irradiation on the Qinghai-Tibetan plateau could also affect photosynthetic CO_2 uptake. We found that neither the photosynthetic CO_2 uptake rate nor the photosynthetic O_2 evolution rate changed significantly after a long period of artificially enhanced UV-B radiation. However, UV-B-absorbing compounds, detected by absorbance at 300 mm, seemed to increase after enhanced UV-B irradiation. Both biochemical components including chlorophyll and other pigments, and leaf thickness increased significantly under high UV-B radiation, suggesting that these alpine plants changed their leaf morphology and biochemistry to avoid a significant decrease in CO_2 uptake under UV-B elevation. Less