Global change effects on the spread of cool-season exotic plants in the Colorado Front Range

Background/Question/Methods

Over the last several decades, the Front Range of Colorado has been experiencing increased invasion of exotic cool-season grasses and forbs. Over the same time period, winter precipitation and nitrogen (N) availability have increased due to climate change and anthropogenic nitrogen deposition.  To determine whether these agents of global change are facilitating increased invasion by exotic species, we set up an in-situ full factorial experiment consisting of two nitrogen treatments (low-N and ambient N) and two precipitation treatments (high and ambient winter precipitation)  in a mixed-grass meadow just north of Boulder, CO (ele ~1500m).  We are using sugar additions (500g C m^-2 year^-1) to immobilize N and irrigating plots (+20 mm H₂O m^-2 year^-1, representing a 12% increase over ambient conditions) to increase winter precipitation.  Before treatments began (in spring 2012), we measured species composition with a 1 m² point-intercept frame in the center of 96 9-m² plots.  During the first year post-treatment, in April 2013, we measured response of the cool-season exotic annual grass, Bromus tectorum, in a subsample of 20 plots.  We counted the number of seedlings within 1-m² quadrats and calculated and compared the percent change in B. tectorum density from pre- to post-treatment.

Results/Conclusions

Sugar additions had a marginally significant negative effect on change in B. tectorum density (Kruskal-Wallis chi-squared = 3.29, df = 1, p= 0.069), but winter precipitation had no effect (Kruskal-Wallis chi-squared = 0.21, df = 1, p = 0.65).  B. tectorum density was highly influenced by pre-treatment conditions: plots that had high B. tectorum cover before the experiment began also had high B. tectorum seedling density in April regardless of treatment (r²= 0.70, p=0.0006).  Additionally, plots that initially had high species richness experienced lower B. tectorum density in April (r²=-0.63, p=0.003).  These preliminary results suggest that increased nitrogen deposition in the Front Range may be facilitating the expansion of invasive species, but small scale variation in invasive dominance across the landscape is probably driven by differences in propagule pressure and/or site conditions (e.g., disturbance history, species composition, soils).  Further monitoring and experimentation (on both B. tectorum and other exotic cool-season species) will help us identify mechanisms of invasion spread and determine the degree to which global change has facilitated changes in species composition.