Carbon and Water Vapor Exchange Processes in the Semiarid Typical Steppe Ecosystem in Inner Mongolia

Carbon and water vapor exchange processes in the semiarid typical steppe ecosystems in Inner Mongolia

Liu HuiZhi Wang Lei Du Qun

State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry，Institute of Atmospheric Physics, Chinese Academy of Science，Beijing 100029

Typical steppe ecosystems are mainly distributed in semiarid areas and degradation has been accelerated due to climate change and increasing grazing intensity. The effects of grazing intensity on water vapor and carbon exchange process were investigated using eddy covariance measurement data during the growing season (May to September) from 2005 to 2008 over typical steppes in Inner Mongolia, China. Four grazing intensities were examined (UG79, ungrazed since 1979; WG, winter grazed; CG, continuously grazed; HG, heavily grazed). The main results are: (1) Grazing decreased evapotranspiration (ET) on a seasonal scale. The most important climatic factor controlling ET on daily scale shifted from SWC to Net radiation (Rn) when grazing intensity increased. SWC, Rn and air temperature (or vapor pressure deficit) can explain 59%-71% of the variation in daily ET. The effect of grazing reducing leaf area index (LAI) on ET is not significant in this ecosystem. Soil evaporation compensates for most of the loss in transpiration due to reduced LAI. (2) Both the amounts and distribution of precipitation during the growing season affected the net ecosystem exchange (NEE). The effects of grazing on the CO₂ flux increased with the grazing intensity. During the peak growth stage, heavy grazing and winter grazing decreased the saturated NEE (NEEsat) and gross primary production (GPP) due to leaf area removal. Both ecosystem respiration (RE) and Q₁₀ were clearly reduced by heavy grazing. Heavy grazing changed the ecosystem from a CO₂ sink into a CO₂ source. The effects of grazing on the CO₂ flux also varied with the vegetation growth stages and SWC. (3) Grazing effects on ET partitioning could vary with soil water conditions in this ecosystem. SWC significantly influenced daily T/ET by affecting T and E in different ways. On a monthly scale, T/ET at UG79 increased with vegetation growth. In the normal precipitation year (2006), WG and HG reduced T/ET because they significantly reduced T through the removal of leaf area. In the dry year (2005), this negative effect on T was depressed by summer droughts, while WG and HG decreased E primarily due to the decreased interception of rainfall by the canopy.

Key words: semiarid, eddy covariance, grazing intensity, carbon exchange, evapotranspiration, evapotranspiration partitioning