A Comparison of Carbon Cycling and the Surface Energy Balance between Native Perennial and Exotic Annual Grass Communities in Northern Coastal California

Summary Grassland ecosystems of California's Coastal and Central Valley regions have undergone dramatic changes, with the almost complete replacement of native perennial grasses by exotic annuals. Our research compares the ecosystem properties and processes that govern the exchange of carbon, water, and energy in California grasslands before and after the invasion of exotic annual grasses at two sites in northern coastal California. To perform these comparisons, we make use of coastal research sites in which native vegetation is found growing alongside locations that have undergone exotic invasion. The broader goal is to understand how shifts in plant community composition can affect global climate change through 1. shifting the balance of carbon storage between terrestrial stocks and the atmosphere, and 2. altering the water and energy regimes that heat or cool the earth's surface. Our preliminary results indicate that soil carbon storage, the primary carbon stock in these grassland ecosystems is greater in regions dominated by native perennial grass communities. At a site near Bolinas, CA, we found that exotic grass invasion has resulted in a transfer of 50 metric tons of carbon per hectare from the soil to the atmosphere. Over the years 2004‐2006, we found energy partitioning into latent and sensible heat fluxes similar among annual and perennial grasses during the portions of the year when water is not limited. When water becomes scarce in the late spring, however, annual grasses die, but the more deeply rooted perennial grasses persist by exploiting deep soil water reserves. Because native perennials remain active, they devote more energy towards latent heat flux and less towards sensible heat flux relative to exotic annuals. During the dry summer months, this difference in energy partitioning and surface drying can lead to soil temperatures of up to 4 o C higher in exotic annual communities, but may not appreciably affect air temperatures. Lower albedo, and thus higher energy capture in exotic annuals during the summer months, can raise surface temperatures an additional 6 o C relative to native perennials. In sum, this study reveals that the invasion of exotic annual grasses has contributed to both global and local warming in California grasslands.