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

AbstractHuman activities alter disturbance regimes and spatio-temporal distribution of soil resources that can affect plant invasion outcomes. The objective of this study was to test whether fire history and cattle activity alter soil resource gradients, thereby affecting patterns of annual brome invasion. 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. At 100-meter transect increment points, we measured the distance to the two nearest cowpats and then established two random points and measured the density, height, biomass, and seed production of red brome, soil moisture and inorganic N. 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), likely due to increased soil moisture and inorganic N. Post-fire conditions and cowpats increased soil inorganic N, which was strongly correlated with red brome density, height, biomass and seed production (R2= 0.60-0.85). Transgenerational legacy effects were evident as seeds from red brome next to cowpats had 27% higher germination than seeds collected from random points. The results suggest that positive responses of red brome to increases in inorganic N related to fire and cattle activity could significantly reinforce fine fuel infill that drives invasive grass-fire cycles in deserts of North America.