Abstract:

Aims Water availability influences community composition and function in semi-arid areas. Habitat water availability may differentiate functional traits on water and carbon assimilation use efficiencies of dominant species, which may induce different response patterns of communities to climatic change. Our objectives are to 1) quantitatively analyze long-term net primary productivity (NPP) dynamics of typical grassland communities along a water gradient based on field measurements, long-time monitoring results and a process-based model; 2) analyze trends of temperature and precipitation based on local long-term meteorological data and predict future climate change scenarios and 3) predict NPP responses of the communities along the water gradient to climate change and interpret the results by means of water stress index (WSI) and carbon stress index (CSI).

Methods We selected four grassland communities along a water gradient in the Xilingol River Basin of China as our study sites: Stipa baicalensis, Leymus chinensis, Stipa grandis, and Stipa krylovii. We conducted field surveys and measurements of physiological parameters and soil parameters in summer 2005 and used local meteorological data (1953 to 2005) to analyze trends of precipitation and temperature and prescribe future scenarios. A process-based model, BIOME-BGC, was parameterized and validated using field data and long-term monitoring data and run on daily steps to simulate NPP dynamics of the four communities under the current climate and future scenarios. We calculated water and carbon stress indices for each community under each scenario to interpret possible mechanisms.

Important findings Minimum and maximum temperatures increased from 1953 to 2005, while the precipitation varied. Average NPP of the four communities decreased 14.2% under the scenario of increasing temperature, increased 13.2% under the scenario of increasing precipitation, and declined 2.7% under the scenario of increasing both. Precipitation is the predominant factor on NPP dynamics in this semi-arid area, while temperature mainly influences plant respiration and evapotranspiration and also affects NPP. Due to the differences in water availability and functional traits of dominant species, the four communities presented different responses and sensitivities to the climate changes in precipitation and temperature, which closely related to variations in WSI and CSI. Stipa krylovii community maintains a high WSI and temperature, reflected by variations in WSI and CSI, shows great dependence on precipitation mainly because of the poor habitat water availability. Stipa baicalensis community has higher NPP under both current and future climate change scenarios largely because it has 1) lower WSI because of better habitat water availability and 2) lower CSI because its higher carbon to nitrogen ratio contributes to less photosynthetic products consumed by maintenance respiration.

Key words: typical steppe, BIOME-BGC, NPP (Net primary productivity), climate change, stress index