Effects of precipitation change on population dynamics of Bromus tectorum and abundance of the pathogen Ustilago bullata

Background/Question/Methods

Shifts in precipitation patterns resulting from global climate change have the potential to influence success of invasive plant species.  In the Front Range of Colorado, Bromus tectorum (cheatgrass) and other non-native winter annuals are invading grassland communities.  As the climate warms, yearly precipitation may shift to a more winter-wet pattern, and this shift could benefit early-growing winter annuals, such as B. tectorum, to the detriment of native species.  This study measures responses of B. tectorum to simulated changes in precipitation patterns in a grassland ecosystem near Boulder, Colorado, USA.  In 2010, three precipitation treatments were created based on climate model predictions for Colorado: winter-wet, winter-wet/summer-wet, and winter-wet/summer dry.  In addition, a winter-dry/summer-wet treatment was added to simulate historical conditions.  B. tectorum was hypothesized to have greater survival, growth, and seed production in treatments receiving supplemental winter precipitation, and lower survival and seed production in the control and winter-dry treatment.  In 2011 and 2012, cover, survival, fecundity, and biomass of B. tectorum were measured in the different treatments.  In 2012, demographic data were used to estimate population growth rates of B. tectorum under different precipitation scenarios.

Results/Conclusions

In spring 2011, B. tectorum was more abundant in winter-wet treatments than in the control or winter-dry treatment.  However, biomass and seed production of B. tectorum did not differ between treatments.  In 2012, cover, biomass, and seed production were greater in winter-wet treatments than in the control or winter-dry treatment.  Population growth rates were highest for B. tectorum growing in the winter-wet treatment, and negative (indicating population decline) in the winter-dry/summer-wet treatment.  Contrasting responses of B. tectorum in 2011 and 2012 can be partially explained by different patterns of ambient precipitation in winter and spring of those years, and these results demonstrate the sensitivity of B. tectorum to precipitation timing.  An unexpected finding was that B. tectorum individuals in the winter-wet treatment were more often infected with the head smut pathogen (Ustilago bullata) than individuals in other treatments.  Overall, this study suggests that B. tectorum may become more invasive in grasslands along the Front Range of Colorado as the climate changes.  However, pathogens may also benefit from precipitation change and decrease success of B. tectorum populations.