European exotics drive a North American invasion: the role of fire and cattle on soil resource gradients and patterns of annual brome invasion

Background/Question/Methods

Human activities alter disturbance regimes and spatio-temporal distribution of soil resources raising questions about how they influence plant invasion outcomes. The objective of this study was to test whether fire history and cattle activity increase soil resources, thereby affecting patterns of annual brome invasion, a widespread problem in deserts of North America. Six paired burned and unburned transect lines (1-kilometer in length) were established in the northeast Mojave Desert along the boundaries of four independent wildfires that occurred in 2005. At each 100-meter transect increment point, we measured the distance to the two nearest cowpats and two random points and measured the density, height, biomass, and seed production of red brome, and soil moisture and mineral N.

Results/Conclusions

Cattle activity was 29% greater along burned transects compared to unburned transects. Red brome height, density, and seed production were 11% to 34% greater along burned transects than unburned transects (P < 0.05). Red brome height, biomass, density, and seed production were 2- to10-fold greater next to cowpats compared to random points (P < 0.05). Post-fire conditions and cowpats significantly increased soil mineral N availability, which was strongly correlated with red brome density (R2 = 0.60), height (R2 = 0.72), biomass (R2 = 0.83) and seed production (R2 = 0.85). Maternal effects were also evident as seeds from red brome growing next to cowpats had 27% higher germination. The positive responses of red brome to mineral N gradients created by fire and cattle activity could significantly reinforce fine fuel infill that drives invasive grass-fire cycles in deserts of North America.