Production of two exotic grasses and one desert native annual under altered precipitation and nitrogen regimes: Implications for fire in the desert

Although fire was once rare in the deserts of the U.S., exotic grass invasion coupled with nitrogen deposition and years of above average precipitation have increased fire frequency and severity.  Because both nitrogen and water are considered limiting in arid environments, these two factors were examined through a greenhouse experiment to determine the levels of each necessary to produce sufficient biomass of exotic grasses and native annuals to sustain fire.  Three water treatments were crossed with four nitrogen fertilization treatments for two exotic grasses, Bromus madritensis and Schismus barbatus, and one native annual, Amsinckia tessellata, implicated in fires in Southern CA.  Water treatments corresponded to water potentials of 0.950, 0.068, and 0.031 MPa for the drought, intermediate, and high precipitation treatments respectively.  Nitrogen concentrations in the soil ranged from 8 mg-N kg-soil^-1 to 28 mg-N kg-soil^-1, bracketing observed field concentrations.  Shoot weight, root:shoot ratio, and C:N ratio were measured on all plants and analyzed using ANOVA. Exotic grass production was significantly affected by nitrogen addition in all water treatments (P<0.0001), and there was a significant water-nitrogen interaction (P<0.0001).  The native annual also showed a significant response to water and nitrogen, although the maximum growth leveled off at a lower nitrogen fertilization level than the exotic grasses. When extrapolated to the field, these results can indicate to land managers which years may be most vulnerable to fire given the precipitation and nitrogen deposition that occurred during the growing season and will inform regulations for critical loads of N from air pollution.