Causes and feedbacks to widespread grass invasion into chaparral shrub dominated landscapes

Context: This study provides a unified, holistic framework for predicting the dynamics of shrub-grass conversion throughout Mediterranean-climate shrublands. This work focuses specifically on the California chaparral, which until recently has been considered resistant to invasion by exotic grasses, but in recent years appears to have undergone substantial type conversion. Objectives: To synthesize current understanding of the feedbacks and anthropogenic impacts that both enhance and reduce the susceptibility of southern California chaparral to invasion and its corresponding ability to recolonize invaded areas. Methods: We review the existing literature pertaining to the factors that enhance or reduce the susceptibility of chaparral to invasion, and organize these factors and their interactions into a single unified framework of environmental drivers, ecological interactions, and historical legacies associated with the distribution and rate of such invasion. Results: A myriad of processes interact to mediate the invasion of exotic grasses into intact chaparral. In addition, we demonstrate that feedbacks exist within both chaparral shrublands and exotic grasslands that modify the landscape in ways that can enhance their own survival, act as barriers to conversion into alternate cover types, and in some cases weaken the resistance of adjacent vegetation to invasion. We posit a methodological framework from which the many climatic, anthropogenic, edaphic, and biotic feedbacks that determine the mosaic of invasion can be modeled. Conclusions: This study demonstrates that substantial conversion of chaparral into deciduous grasslands has recently occurred in southern California and presents a unified framework for forecasting the dynamics of shrub-grass conversion throughout Mediterranean-climate shrublands.