Linking N deposition to invasive plant biomass, fires, and diversity loss in the California deserts

Background/Question/Methods

Anthropogenic nitrogen (N) deposition has increased in the last several decades in California deserts downwind of urban areas. Invasive annual grasses have also increased their range, as have fires in the desert. We tested the hypotheses that N deposition increases the productivity of invasive species to a greater degree than native species, that increased annual productivity is related to increased fires, and that both N deposition and fire result in reduced native herbaceous species diversity. The study area was in and around Joshua Tree National Park. A N deposition gradient occurs across the region, ranging from 16 kg ha^-1 yr^-1 west of the Park to 3 kg ha^-1 yr^-1 in the eastern Park. We used three approaches to test the impacts of N deposition and fires on native herb diversity. We 1) measured annual vegetation response along the N deposition gradient 2) and in fertilized plots at four sites (two high elevation and two low elevation, each in low and high N deposition areas) in the Park at levels of 0, 5 and 30 kg N ha^-1 yr^-1, and 3) observed successional changes in annual plant composition after fires.

Results/Conclusions

The dominant invasive grasses were Schismus barbatus at the lower elevations in creosote bush scrub and Bromus madritensis at the higher elevations in pinyon-juniper woodland. Some 90 species of native herbaceous species were recorded over 5 years. Exotic grass biomass increased significantly with 30 kg N/ha at three of the four sites during a year with moderate precipitation, and with 5 kg N/ha at two sites during a year of high precipitation. The richness of native forbs declined with fertilization at sites with high initial exotic grass cover, but native richness and cover increased with fertilization at a site with low grass cover. Richness measured at the scale of 36 m² fertilized plots was not affected by elevated N, but at the scale of 0.5 m² sample quadrats there was decreased native richness in two of the four sites in the Park. Outside the Park where N deposition is higher the herbaceous vegetation has been converted to exotic annual vegetation, indicating that long-term high N input has a negative effect. Burned areas had a further decline in native diversity. The study shows that both the direct effect of N on invasive species and the indirect effect via fire reduce diversity of native vegetation.