Non-linear shift from grassland to shrubland in temperate barrier islands

The encroachment of woody plants into grasslands is global phenomenon that has been observed in arctic, alpine, desert, and coastal ecosystems. This abrupt replacement of grasses with woody plants has significant impacts on ecosystem structure, functioning, and the provision of ecosystem services. In the last century, the woody plant species Morella cerifera L. has encroached into grass covered swales on Virginia barrier islands, which are known for being vulnerable to climate change. These islands lie at the northern limit of the latitudinal range of M. cerifera. M. cerifera expansion has not been affected by direct human disturbances since 1930s, therefore the ongoing encroachment of M. cerifera and the warming effect induced by the impact of M. cerifera on the surface energy balance indicate that a positive feedback between vegetation cover and microclimate could exist due to the cold intolerance of M. cerifera, however, the mechanisms underlying it are still unclear. Our experimental data revealed a significant difference ( 2^oC) in the mean of the 5% and 10% lowest minimum temperatures between shrubland and grassland. The modelling results showed that the warmer microclimate conditions in shrublands interacting with climate warming can induce a non-linear shift in ecosystem state from grassland to an alternative stable state dominated by M. cerifera. This work provides a general theoretical mechanism for the emergence of bi-stable vegetation dynamics in shrubland-grassland ecotones and advances our understanding and prediction of how the stability of these ecosystems may nonlinearly change under future climate change scenarios.