UC Delivers

Research Proves Nitrogen Deposition Harms Native Plants

The Issue

Research Proves Nitrogen Deposition Harms Native Plants
Joshua Tree National Park. Photo courtesy of National Park Service.
Research by scientists from the University of California, Riverside has documented the harmful effects of air pollution, specifically nitrogen deposition, on coastal sage scrub and desert native vegetation. The nitrogen emissions originate from automobiles as NOx and from agriculture as ammonium. Nitrogen deposited on the soil promotes the growth of non-native invasive grasses that can quickly replace and out-compete native plants.
The loss of native plant communities can displace native wildlife that relies on the native species for nutrition, shelter, or nesting. The spread of invasive grasses has also been linked to increases in the frequency and severity of wildfires. The understory vegetation in deserts and coastal sage scrub consists of native wildflowers that, unlike invasive grasses, provide sparse fuels for fires. Invasive grasses grow quickly when soil nitrogen increases under nitrogen deposition, and provide increased amounts of biomass to fuel fires, which allows fires to spread over large areas.

What Has ANR Done?

Natural Resource Specialist Edith Allen of the University of California, Riverside, has been working with a multi-agency (EPA, National Park Service, Forest Service) working group to determine critical loads of nitrogen deposition that will cause harmful ecosystem impacts to southern California shrublands. Relatively low amounts of N deposition (5-10 kg N/ha/yr, compared to natural background levels of 2 kg N/ha/yr) will cause increases in invasive grass biomass that fuel fires, and will cause losses in diversity of native wildflower species. The research has been funded by grants to Dr. Allen through the National Park Service and the National Science Foundation. Dr. Allen has authored or co-authored several chapters of a monograph and publications on critical loads of nitrogen deposition that will be presented to regulators.

The Payoff

Critical Loads of Nitrogen Deposition are Set to Reduce Invasive Grass Fuel for Fires

The data collected by Dr. Allen and her research group will enable regulators to set air quality standards to avoid major ecosystem impacts. Air quality standards are set based on human health standards. However, in some cases, even lower amounts of atmospheric NOx pollution than affect human health will have negative impacts on native California ecosystems. Fires have been occurring in the desert where fires are historically unknown, and are more frequent in coastal sage scrub, that are fueled by invasive grasses responding to nitrogen deposition. These negative impacts of nitrogen deposition on native ecosystems have previously been unknown, and now for the first time critical load values are available to regulators to consider ecosystem as well as human health.


Dr. Edith Allen
Natural Resource Specialist and Professor of Plant Ecology
Department of Botany & Plant Sciences
2129 Batchelor Hall
(951) 827-2856