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Invasive Weeds Deserve Study

National Science Foundation Awards $2 Million Grant to UC Riverside

Nitrogen in Air Pollution is Acting as Weed Fertilizer

(September 30, 2004)

Edith Allen

Edith Allen

RIVERSIDE, Calif. (Sept. 30, 2004) -- The invasion of the over-fertilized weeds continues, but a $2 million National Science Foundation grant to UC Riverside may eventually help alleviate the state’s ongoing weed problem.

The five-year grant, which begins Oct. 1, is the largest study of its kind ever attempted, said principal investigator Edith Allen, a professor in the UCR Department of Botany and Plant Sciences. She will lead a team of UC Riverside and UC Berkeley researchers who will monitor weed growth in the state, and make appropriate recommendations. Weeds are important because they out-compete native species and they can contribute to the spread of wildfires.

“Nitrogen deposition occurs at high levels in southern California, and is fertilizing our wildlands,” Allen said. “While growers and gardeners may appreciate this free fertilizer, it promotes the growth of weedy species in our forests, shrublands, deserts and grasslands. The invasion of weeds is a huge problem for maintenance of our fragile biodiversity, which is already impacted by development.”

Allen says that most of the nitrogen is the result of automobile emissions. Some of this is converted to nitrate or nitric acid, the form of nitrogen (known as N) that is used by plants as fertilizer.

“California has been subject to weed invasions since the arrival of the first European settlers, but the southern California coastal sage scrub and deserts have been experiencing weed invasions primarily in the last 40 years, since the level of nitrogen deposition has increased,” Allen said. “Some of these weeds have been present for the past 100 years, but they are just now becoming dense, and they are replacing native vegetation.”

The weeds are primarily from the Mediterranean region, and include annual grasses and flowering plants such as stork's bill and mustard. The grasses are problematic, Allen said, because they form a fine fuel that can help carry fire during the dry season, more so than any of the state’s native species. The grass invasion also kills native plants and animals. Once the grass and fire cycles have been established, native plants can no longer establish themselves, and the conservation value of the land for biodiversity is reduced.

The biocomplexity grant will model the emissions and rate of deposition across the landscape, mainly in Riverside and San Bernardino Counties.

“We will use remote sensing and ground vegetation surveys to determine the current and historical rate of change of vegetation, examining when invasive grasses became large scale problems in different areas,” Allen said.

“The subject has never been studied on this scale,” Allen said. “People have long been concerned about the impact nitrogen deposition has, and other concerns about weed invasion, but there has never been a landscape-scale study that links nitrogen with invasive species.”

The study will determine the amount of N required to build up different levels of invasive grass fuel, impacting fire frequency. The threshold at which N affects different vegetation types from forest to shrubland to desert is different, and the study will determine the different levels of sensitivity for each vegetation type. Future vegetation of the area will be modeled based on different levels of N emissions and different urban development scenarios.
This is the second large biocomplexity grant awarded to UC Riverside recently. The first, for $1.5 million, was to help monitor the unintended spread of engineered crop genes.


The University of California, Riverside ( is a doctoral research university, a living laboratory for groundbreaking exploration of issues critical to Inland Southern California, the state and communities around the world. Reflecting California's diverse culture, UCR's enrollment has exceeded 21,000 students. The campus opened a medical school in 2013 and has reached the heart of the Coachella Valley by way of the UCR Palm Desert Center. The campus has an annual statewide economic impact of more than $1 billion.

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