Plant community change in response to directional shifts in nitrogen deposition and precipitation seasonality in the Colorado Front Range
Global change drivers are altering climatic and edaphic conditions of ecosystems across the globe, and we therefore expect novel plant communities to become more common though documented cases are still rare. Over the last several decades, changes in the composition of the mixed-grass prairie plant community of the Colorado Front Range have occurred in conjunction with shifts in the climatic and edaphic conditions. To test whether increases in winter precipitation and N availability have been responsible for the observed shift in species composition, we conducted an in-situ manipulative experiment in a mixed-grass meadow near Boulder, CO. We simulated historical conditions by reducing N availability (via addition of 500g C m-2 year-1) and winter precipitation (with rain-out shelters placed from October to March) for two years (2013-2014) and compared vegetation response to these treatments to that of ambient conditions. The site experienced an extreme precipitation event in fall 2013 which saturated soils into the next growing season and allowed us to compare an exceptionally wet year (2014) to an average year (2013). We measured pre-treatment species composition in the summer of 2012, and treatment responses of cool-season and warm-season species 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 dry-low N plots, particularly the invasive annual grass Bromus tectorum which was reduced by 55% in 2013 (p < 0.0001) and 62% in 2014 (p < 0.0001) in the low N treatment. These same species responded strongly to the extreme event with large increases (B. tectorum increased by 300% from 2013 to 2014), while native grasses and forbs showed little change in productivity or composition under varying climatic or edaphic conditions. This work demonstrates that extreme events that increase soil resource abundance combine with climate and atmospheric chemistry drivers to enhance directional change in plant communities.