State transition of US Great Plains prairie under experimental warming

Background/Question/Methods It has been well documented in the historical and paleoecological records that vegetation states changed on multidecadal timescales during the last century, in the mid-Holocene, and other geological ages. Dynamic global vegetation models (DGVM) have often predicted changes in ecosystem states that are induced by and, in turn, feedback to climate change.  Studies in tropical regions often suggest multiple equilibria in climate and vegetation that are maintained by positive feedbacks between them.  However, there is, to the best of our knowledge, no experimental study in the field to examine fundamental mechanisms and processes of ecosystem state changes. We conducted an experiment with both warming and clipping treatments in a prairie of Oklahoma, USA since November 1999.  The experimental site is located in an ecotone between the mixed and tall grass prairies. We have measured changes in ANPP, plant nitrogen dynamics, and species composition in addition to precipitation, temperature, and soil moisture.

Results/Conclusions Long-term experimental warming induced state transition of a US Great Plains prairie with its dominant species changing from mixed to tall grass prairie types.  Our study showed two-stage stimulations of aboveground net primary production (ANPP) with small increases in the first seven years followed by distinctly large increases under experimental warming in comparison with those under control. The two-stage ANPP stimulations were accompanied with shifting dominant species within the plant community over time but not related to changes in physical environment. Our results suggested that the chronic experimental warming exerted differential impacts on individual species, and climate extremes in 2005-2007 aggravated the warming impacts on dominant species to the extent that plant community structure and ecosystem functions were on a trajectory of changing to a different state.