Imposing antecedent global change conditions rapidly alters plant community composition in a mixed-grass prairie.

Global change drivers are altering climatic and edaphic conditions of ecosystems across the globe, and we expect novel plant communities to become more common as a result. In the Colorado Front Range, compositional changes have occurred in the mixed-grass prairie plant community in conjunction with shifts in winter precipitation and atmospheric nitrogen (N) deposition. To test whether these environmental changes have been responsible for the observed plant community change, we conducted an in situ manipulative experiment in a mixed-grass meadow near Boulder, CO. We simulated historical conditions by reducing N availability (+500 g C m(-2) year(-1)) and winter precipitation (with rainout shelters) for 2 years (2013-2014) and compared vegetation response to these treatments with that of ambient conditions. The site experienced an extreme precipitation event in autumn 2013 which allowed comparison of an exceptionally wet year with an average year. We measured pre-treatment species composition in 2012, and treatment responses in the spring and summer of 2013 and 2014. As predicted, simulating historical low N-winter dry conditions resulted in a plant community dominated by historically abundant species. Cool-season introduced species were significantly reduced in low N-winter dry plots, particularly the annual plants Bromus tectorum and Alyssum parviflorum. These same species responded strongly to the extreme precipitation event with large increases, while native grasses and forbs showed little change in productivity or composition under varying climatic or edaphic conditions. This work provides clear evidence linking on-going global change drivers to altered plant community composition in an otherwise relatively undisturbed grassland ecosystem.