Can three native Mojave Desert perennials adapt to increasing atmospheric nitrogen deposition and annual grass invasion in Joshua Tree National Park?

Abstract

Anthropogenic emissions have increased atmospheric nitrogen deposition nearly four-fold in the industrial era. Although increased N availability can be beneficial for individual plants, it can increase community susceptibility to invasion and have detrimental effects on native plant communities. We grew three native Mojave Desert perennials, Ambrosia salsola, Hilaria rigida, and Stipa speciosa, collected across an N deposition/invasion gradient in Joshua Tree National Park, in greenhouse and field common gardens to determine if populations possessed the heritable variation required to adapt to increasing N deposition and annual grass invasion, and asked if there was any evidence for adaptation to these factors. We collected seed from individual plants (families) at six sites representing three levels of N deposition and invasion within the park, and grew plants in a greenhouse for 90 days with and without competition from the invasive Bromus madritensis ssp. rubens at varying N levels. We analyzed differences in leaf number, total biomass, allocation to above and below ground biomass, and allocation to fine and coarse roots using a nested ANOVA. As expected, B. madritensis competition decreased biomass of all native plants, and N addition increased biomass, except for H. rigida. Families of all species differed in many measured traits, and there were site differences in some traits for A. salsola and H. rigida. Populations from different N deposition and invasion levels showed some differences in growth strategies, especially in A. salsola, for which plants from high N/invaded sites saw a smaller relative decrease in biomass and leaf number when grown with B. madritensis, consistent with adaption to annual grass competition. We also conducted a reciprocal transplant experiment using seed collected from the same six sites. Plants were transplanted reciprocally after one year in the greenhouse. Transplant survival, vigor, and size were affected by transplant site and N status, whereas origin site and N status did not significantly impact these traits. Although there was no evidence of evolutionary response to N deposition level for any species, families exhibited differences in responses to N, suggesting that there may be a small amount of heritable variation that could allow for future adaptation to conditions that are likely to be prevalent in the future. Use of populations or individuals that show increased performance under likely future conditions could allow for more successful restoration projects.