Positive Interactions Between An Exotic Invader And Moss Biocrusts Vary Across Life Stage And Correspond With The Effect Of Water Pulses On Soil Nitrogen

1. The size arid frequency of resource pulses can affect plant interactions and increase the abundance of invasive species relative to native species. We examined resource pulses generated during the desiccation and rehydration of communities of native biological soil crust (biocrust)-forming mosses, in the context of positive associations between biocrusts and the invasive forb, Centaurea stoebe.2. We surveyed Centaurea and biocrust cover and evaluated how interactions among Centaurea, biocrusts and water pulses influenced plant biomass and soil nitrogen in a field experiment. Centaurea seedling and biocrust interactions were also compared in a greenhouse experiment to evaluate differences related to life stage.3. In field surveys, Centaurea and biocrusts were positively associated. Across water pulse treatments, biocrust biomass decreased when Centaurea was removed, indicating that Centaurea facilitated biocrusts. Biocrusts did not affect adult Centaurea in the field, but Centaurea seedling biomass was greater when grown with biocrusts in the greenhouse. Water pulses did not affect plant biomass, but interactions between Centaurea and biocrusts corresponded with variation in the effect of water pulses on soil nitrogen which were not evident when Centaurea or biocrusts were grown alone. Twenty-four hours after large water pulses were added, soil NO3- was nine times higher in plots where biocrusts and Centaurea co-occurred compared with small water pulse plots. In these same plots, soil NH4+ tended to be lower at the end of the experiment.4. These results highlight positive interactions between an invasive exotic forb and native moss biocrust. Water pulses influenced soil nitrogen availability when both plants co-occurred, but did not affect plant biomass, suggesting that resource pulses and species interactions can interact to affect ecosystem processes.