Many ecosystems have faced species composition changes related to invasive species; vast areas are characterized by loss of species or functional diversity. When the process of community disassembly is advanced, changes in water resource pool dynamics can provide a resource for establishment of other species populations, especially in water-controlled ecosystems. Bromus tectorum (cheatgrass) is an invasive annual grass that has become dominant in extensive areas previously occupied by big sagebrush-steppe communities. In years when near monocultures of this annual Bromus tectorum experience fall mortality post germination, soil moisture pools in the top 50 cm are not completely used and the system is more likely to be invaded, particularly by other annuals. The objective of this study was to assess whether more complete utilization of upper soil moisture by B. tectorum acts to reduce the potential for new invaders. Soil moisture was measured in a near-monoculture cheatgrass stand during two periods, 2000-2002 and 2007-2009 when fall mortality of B. tectorum was followed by rapid community change to annual forbs. Measurements and soil water modeling were used to generate a continuous set of volumetric water contents for both periods. Cheatgrass biomass, densities, growth rates, percent cover, and species composition were measured.

Results/Conclusions

Between the period 2000-2002 and 2007-2009, the cheatgrass dominated system rapidly changed to an annual forb dominated community following years when fall mortality of seedling B. tectorum occurred. When cheatgrass did not experience early mortality (2000, 2002) and dominated ground cover, the water content remaining in the upper soil pool at mid-summer as a percentage of the water content at the start of the spring growing season was 57.9% in 2000 and 42.6% in 2002 for the 30 cm layer and 70.5% (2000) and 70.1% (2002) for the 45 cm layer. However, when B. tectorum experiences fall mortality (late 2000 and 2007), these ratios the following summer are 84.5% (2001) and 101.7% (2008) for the 30 cm layer and 88.6% (2001) to 114.5% (2008) for the 45 cm layer. While the annual forbs dominated for only one year, our results show the importance of the interplay between available soil moisture, and rapid community change and invasion potential. Arid or semi-arid areas in advanced stages of community disassembly may be especially vulnerable to community changes and new species invasions linked to changes in water pool dynamics.