Thursday, August 9, 2007

PS 59-71: Ecological management of serpentine grasslands in the face of nitrogen deposition and invasive grasses

Christal Niederer and Stuart B. Weiss. Creekside Center for Earth Observation

Coyote Ridge contains approximately 7000 acres of nutrient-poor serpentine soil in South San Jose, CA and supports unique native wildflower displays and core populations of the threatened bay checkerspot butterfly (Euphydryas editha ssp. bayensis). Atmospheric nitrogen deposition, estimated at 10-20 kg-N ha-1 year-1, drives dense annual grass invasions that can overrun bay checkerspot host plants and nectar sources. Local development projects have been required to mitigate for indirect N-deposition impacts through habitat acquisition and long-term management funding.

Moderate cattle grazing is effective management, because cattle prefer nitrogen-rich annual grasses and create open sites favored by native annuals. In 2006, annual grass cover averaged 24.6% at grazed sites, compared to 47.1% on nearby ungrazed sites. Annual forbs, including host and nectar plants for the butterfly, averaged 29.5% at the grazed site, and 8.1% at the ungrazed site.

Burning is effective for short-term reduction of annual grasses. The year following an accidental spring burn, annual grass cover was 37.4% on burned and 72.1% on adjacent unburned sites. Annual forb cover was 35% on burned and 7.5% on unburned plots.

A new threat to serpentine grassland is barb goatgrass (Aegilops triuncialis). This silica-rich grass is unpalatable to cattle and can invade even in the absence of nitrogen deposition. Treatment strategies include burning, herbicides, and mechanical control.