Woody plant encroachment and ecological bistability through microclimate feedbacks in California

Woody plant encroachment is a major shift in plant distribution and abundance around the globe that has important impacts on ecosystem functioning and services. Climate warming has been suggested to facilitate the expansion of many cold-sensitive woody species into grassland. Woody plant expansion can also be enhanced by vegetation-microclimate feedbacks, whereby the establishment of woody plants reduces the frequency of extreme cold events and the mortality of woody seedlings. At the same time woody plant establishment may be counteracted by biotic factors such as herbivory. Currently, field evidence documenting the effects of these abiotic and biotic factors are still missing. In this study, we conducted field experiments at coastal field sites in northern and southern California to investigate the impact of shrub thickets on the microclimate. We also developed a process-based model to quantitatively evaluate to what extent the vegetation is expected to undergo a relatively abrupt transition to woody plant dominance in response to climate warming. We found that during the winter season the mean minimum nighttime temperatures increased by nearly 2°C within shrub thickets with respect to the adjacent open grassland areas. Shrub seedlings transplanted under shrub canopies experienced less freeze-induced damage but significantly greater herbivore damage than those transplanted in adjacent open grasslands. The xylem specific hydraulic conductivity of the two shrub species included in this study decreased significantly when temperatures dropped to -2°C. The modelling results show that a small increase in minimum temperature can induce an abrupt and highly irreversible shift in ecosystem state from a stable state without woody cover to another stable state with woody plant dominance. These bistable dynamics are induced by vegetation-microclimate feedbacks, while their resilience is also affected by herbivory. Our findings highlight the complex joint effects of global climate warming, vegetation-microclimate feedbacks and herbivory on woody vegetation dynamics and stability in coastal ecosystems.