In arid and semi-arid ecosystems, resources are spatially and temporally heterogeneous, leading to differential seedling recruitment over time. Interactions among seedlings and existing vegetation also influence seedling establishment, particularly if existing vegetation improves conditions experienced by seedlings. The degree of benefit received and the facilitative mechanisms involved, if any, depend upon species life histories and ecophysiological characteristic, and may differ significantly for native versus invasive species.  We asked if a native legume, Lupinus argenteus, facilitates seedling establishment of native versus invasive species, and the potential mechanisms involved, in unburned and burned sagebrush steppe. We chose six treatments to identify specific mechanisms by which L. argenteus likely influences establishment:  1) live L. argenteus; 2) dead L. argenteus; 3) no L. argenteus; 4) no L. argenteus with L. argenteus litter; 5) no L. argenteus with inert litter; and 6) mock L. argenteus. We examined burn and treatment effects on environmental variables (soil nutrient availability, soil moisture, soil temperature, and light) and seedling establishment of the native perennial grass Elymus multisetus, native perennial forb, Eriogonum umbellatum, and non-native invasive annual grass, Bromus tectorum.
Results/Conclusions

In both communities, temperature and light levels were highest in no L. argenteus plots and least in mock and live L. argenteus plots. Further, in both communities L. argenteus litter plots had the highest extractable N. In unburned communities, seedling emergence was unaffected by treatment. Increased N availability from L. argenteus litter increased B. tectorum survival, biomass and seed number. Survival of native species was unaffected by L. argenteus treatment, but height of E. multisetus was higher in both L. argenteus litter and inert litter treatments. In burned communities litter reduced emergence of E. umbellatum and increased emergence of E. multisetus, but treatment had no effect on emergence of B. tectorum. Reduced light and temperature fluctuation in litter treatments increased E. multisetus survival and height. All species were larger in burned communities, but higher N levels likely masked treatment differences. B. tectorum had higher emergence and survival than either native species regardless of treatment or community type. Modification of the resource environment by native L. argenteus can increase plant establishment and growth of the native and non-native study species, but higher establishment and growth rates give the non-native, B. tectorum, a greater advantage.