NSF Grants $2 Million to Study California Weed Growth

The invasion of the overfertilized weeds continues, but a $2 million National Science Foundation grant to University of California (UC) Riverside may eventually help alleviate the state's ongoing weed problem.

The five-year grant, which began Oct. 1, is the largest study of its kind ever attempted, says 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 says. "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 most of the nitrogen is the result of automobile emissions. Some of this is converted to nitrate or nitric acid, the form of nitrogen (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 says. "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 says, 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 says.

"The subject has never been studied on this scale," Allen says. "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.