This study evaluated the effectiveness of two bioswales eight years after construction in Davis, California. The treatment bioswale measured 9 m × 1 m × 1 m (L × W × D). Engineered soil mix (75% native lava rock and 25% loam soil) replaced the native loam soil. Four Red Tip Photinia (Photinia × fraseri Dress) trees and two Blueberry Muffin Hawthorn (Rhaphiolepis umbellata (Thunb.) Makino) shrubs were planted in the bioswale. Runoff flowed into the bioswale from an adjacent 171 m2 panel of turf grass. An identically sized control bioswale consisting of non-disturbed native soil was located adjacent to the treatment bioswale. Surface runoff quantity and quality were measured during three experiments with different pollutant loads. When compared to the control, the treatment bioswale reduced surface runoff by 99.4%, and reduced nitrogen, phosphate, and total organic carbon loading by 99.1%, 99.5%, and 99.4%, respectively. After eight years, tree growth characteristics were similar across both sites.

This study used tree data from field plots in urban areas to describe forest structure in urban areas throughout California. The plot data were used with numerical models to calculate several ecosystem services produced by trees. A series of transfer functions were calculated to scale-up results from the plots to the landscape using urban tree canopy (UTC) mapped at 1-m resolution for each combination of 6 land use classes and climate zones. California's UTC covered 15% of the urban area and contained 173.2 million trees, five per city resident. UTC per capita was lowest among U.S. states (90.8 m2), indicating ample opportunity for tree planting. Oaks were the most abundant taxon (22%) and overall plantings were youthful. The annual value of ecosystem services was estimated at $8.3 billion and the urban forests asset value was $181 billion. Assuming an average annual per tree management cost of $19 and benefit of $47.83, $2.52 in benefit was returned for every dollar spent. The threat posed by Invasive Shot Hole Borer (Euwallacea sp.) illustrates that urban forests are a relatively fragile resource whose contributions to human health and well-being can be suddenly jeopardized. One scenario projected that should Southern California cities lose 50% (11.6 million) of all susceptible trees, the value of ecoservices foregone over 10 years was $616.6 million. The approximate cost of removing and replacing the trees was $15.9 billion. Strategies to reduce the risk of catastrophic loss by increasing the resilience of California's urban forests are discussed.

This report represents a collaborative effort between the Information Center for the Environment, Department of Environmental Science and Policy, University of California, Davis (UCD-ICE); the Urban Ecosystems and Social Dynamics Program, Pacific Southwest Research Station, USDA Forest Service (USFS-PSWRS); and the California Department of Forestry and Fire Protection, Fire and Resource Assessment Program (CAL FIRE-FRAP), to assess the current status of the tree canopy and associated benefits within the urban areas of California. In addition, the current status of environmental threats to the urban area population were also examined, in order to highlight the communities most vulnerable and potentially the most likely to benefit from tree plantings and maintenance.

The authors developed a simple screening-level model of exposure of aquatic species to pyrethroid insecticides for the lower American River watershed (California, USA). The model incorporated both empirically derived washoff functions based on existing, small-scale precipitation simulations and empirical data on pyrethroid insecticide use and watershed properties for Sacramento County, California, USA. The authors calibrated the model to in-stream monitoring data and used it to predict daily river pyrethroid concentration from 1995 through 2010. The model predicted a marked increase in pyrethroid toxic units starting in 2000, coincident with an observed watershed-wide increase in pyrethroid use. After 2000, approximately 70% of the predicted total toxic unit exposure in the watershed was associated with the pyrethroids bifenthrin and cyfluthrin. Pyrethroid applications for aboveground structural pest control on the basis of suspension concentrate categorized product formulations accounted for greater than 97% of the predicted total toxic unit exposure. Projected application of mitigation strategies, such as curtailment of structural perimeter band and barrier treatments as recently adopted by the California Department of Pesticide Regulation, reduced predicted total toxic unit exposure by 84%. The model also predicted that similar reductions in surface-water concentrations of pyrethroids could be achieved through a switch from suspension concentrate-categorized products to emulsifiable concentrate-categorized products without restrictions on current-use practice. Even with these mitigation actions, the predicted concentration of some pyrethroids would continue to exceed chronic aquatic life criteria.