Evidence for Recent Invasion of Historically Resistant Chaparral Shrublands to Grasslands

Although historically resistant to invasion and type-conversion, there are strong indications that native shrublands in southern California are often increasingly degraded, and in many cases have begun transitioning into herbaceous grasslands. Chaparral shrublands in particular, which are characterized by a closed evergreen canopy composed of multiple species, represent a critical habitat for many native fauna and also play a significant role in soil stabilization and water partitioning throughout much of Southern California. However, in response to interactive global changes, these ecosystems may be transitioning into invasive-dominated deciduous grasslands. Through the use of a novel, phenology-driven vegetation classification, we examine the extent of such type-conversions through analysis of increases in seasonal changes (i.e. deciduousness). Estimates of phenological variation in greenness (NDVI) developed through analysis of Landsat 4-8 imagery were calibrated to observed seasonal NDVI variation as developed through high-resolution ground-based imagery platforms to develop estimates in the change of percent cover by evergreen shrubs and drought-deciduous herbs and grasses. This study evaluates long-term changes in invasive cover from 1985 through 2011. These analyses indicate substantial type-conversion of native chaparral over this period, as with differences in local elevation representing the dominant factor in the degree of long-term type-conversion at broad landscape-scales, with high elevation sites being the least susceptible to type conversion from chaparral to invaded grassland.