Erodium cicutarium, an invasive annual forb in the Colorado Desert, experiences increased benefits of water and nitrogen over native annual forbs

Background/Question/Methods

Anthropogenic nitrogen deposition acts as an artificial fertilizer enhancing soil nitrogen levels in the otherwise nutrient-poor deserts of Southern California. Exotic invasive species are often more nitrophilous than native desert species, which are adapted to low nutrient soils. This can create a nitrogen deposition-invasive species feedback loop, where invasives benefit from increases in soil nitrogen more than native species. Here, a greenhouse experiment evaluated the effects of six nitrogen levels (0, 2.5, 5, 10, 15, 20 mg-N kg-soil^-1) and two watering treatments (20% and 60% free drainage) on the growth of one invasive and three native annual forbs common to the Colorado Desert.

Results/Conclusions

The high water treatment (60%) typically led to significantly larger increases in plant growth than the low (20%) watering treatment, highlighting the fact that this system is primarily water-limited. Although both native and invasive annuals experienced benefits of nitrogen additions, the invasive forb Erodium cicutarium demonstrated more consistent increases in growth and reproductive effort with each successive increase in nitrogen. For example, E. cicutarium basal rosette width doubled from the 0 mg-N kg^-1 soil, 20% water treatment to the 20 mg-N kg^-1 soil, 60% water treatment, suggesting that aboveground competition could intensify under high water and high nitrogen inputs. Annual forbs tended to experience maximum growth benefits at intermediate levels of nitrogen additions, such as 10 or 15 mg-N kg^-1 soil. Percent tissue nitrogen concentration measurements showed the most similarity between the invasive and native species, with all species increasing in tissue nitrogen concentrations up to the highest soil nitrogen level. The ability of E. cicutarium to utilize soil nitrogen additions in a stepwise fashion suggests that it has the ability to be a dominant competitor in the desert, especially under high nitrogen deposition. Habitat conservation and restoration efforts should focus on areas of high nitrogen deposition, where E. cicutarium is most likely to have severe competitive effects on natives.