Weed Risk Assessment in California

California is a biodiversity hotspot for native plants and has become something of a hotspot for invasive plants as well, with >200 non-native, invasive plants causing impacts in California wildlands. More than half of the invasive plants in California were introduced as ornamental species. At the same time, horticultural plants are one of the largest agricultural crops in the state. Weed Risk Assessment can provide information on where invasive plants may spread and which new species may become the next invaders. Natural resource managers need to know where these plants may spread, especially as climate change progresses, in order to direct their early detection and control efforts. Meanwhile, it would be valuable for the horticultural industry to know which plants are or have the potential to become invasive so the industry can take proactive measures.

This presentation will describe several risk assessment projects by the nonprofit California Invasive Plant Council (www.cal-ipc.org) and our collaborators. First, we are working to develop projections showing where invasive plants have the most potential to spread. One of the strongest determinants of a plant becoming invasive is its invasiveness in another region with a similar climate. We started with a group of 35 invasive plants and used Climex software to develop habitat suitability models that compare the global native and invasive range of each plant to the climate of California. We are now studying an additional 30 species, focusing on plants that are of concern in the Sierra Nevada, an area expected to be heavily impacted by climate change. We also surveyed lists of invasive plants in other mediterranean-type regions and compared them to the plants already naturalized (but not considered invasive) in California and to plants sold in the horticultural trade in California, in order to identify possible future invasive plants.

In California, the California Horticultural Invasives Prevention (Cal-HIP) partnership and its PlantRight campaign (www.plantright.org) are working to develop a Weed Risk Assessment based on the Pheloung model to screen new introductions. Since 2004, Cal-HIP has brought together environmental groups, horticultural industry groups, and government agencies to pursue voluntary changes to reduce the sale of invasive horticultural plants. Cal-HIP works to remove invasive plants from the trade, identify research questions, and provide information to nursery owners, growers, and the gardening public.