The climate models all project increased intensity and duration of drought in the future due partly to climatic warming. Such climate changes may significantly affect terrestrial ecosystem’s carbon fluxes, resulting in positive or negative feedback to the atmospheric CO2 concentrations. Manipulative experiments have been extensively conducted to explore the responses of ecosystem to warming and/or altered precipitation, but the most previous studies did not report how warming interacts with climate extremes such as drought in affecting ecosystem carbon balance. The goal of this research is to assess the grassland ecosystem’s responses to experimental warming and clipping under extreme precipitation, including drought or extreme wet conditions. The experiment was conducted in the Great Plains in central Oklahoma, USA to examine the effects of warming (+2°C) and clipping (to mimic hay harvesting) on prairie ecosystems since 1999. Treatment combinations are unclipped control (UC), unclipped warming (UW), clipped control (CC), and clipped warming (CW). Monthly measurements of carbon fluxes, i.e., net ecosystem CO2 exchange (NEE), gross primary production (GPP), and ecosystem respiration (ER), and yearly measurements of above-ground net primary production (ANPP) were carried out from 2009 to 2015, during which 2011 was extreme dry year (549 mm), 2015 was extreme wet year (1605 mm), and the rest of the years were considered as normal years with an average of 841 mm.
Results/Conclusions
A significant carbon source in the dry period was occurred, resulting from enhanced ER under warming condition from July to September in 2011 (p < 0.05). The warming effects on ANPP across the whole period were significant (p < 0.01) while there were no clipping effects. The magnitude of warming effects on ANPP varied among the three periods (+42.73 ± 9.12, +96.35 ± 9.92, and +185.42 ± 27.16 gm-2y-1 in dry, normal and wet periods, respectively). In addition, ANPP under experimental warming (UW and CW) was significantly decreased by drought (p < 0.05). ER under warming in drought year was significantly higher than normal years due partly to higher plant biomass under warming than ambient temperature, demanding more energy for plants to survive under drought. Due to significantly enhanced ER under warming in drought, stimulation of ANPP by warming in the dry period was much lower than the other two periods. Our results indicated that extreme drought would decrease ANPP and increase ER under warmer climate, triggering a positive feedback to climate.