Woody plant encroachment into grasslands is a global phenomenon that has been observed in arctic, alpine and desert ecosystems. The abrupt replacement of grasses with woody plants has significant impacts on ecosystem structure, functioning, and the provision of ecosystem services. This change in plant dominance is also affecting coastal ecosystems, including barrier islands, which are known for being vulnerable to the effects of climate change. In the last century, the woody plant species Morella cerifera L. (Myricaceae), has encroached into grass covered swales in many of the barrier islands of Virginia along the Atlantic seaboard. These islands lie at the northern limit of the latitudinal range of M. cerifera and specifically, M. cerifera expansion has not been affected by direct human disturbances since 1930s, therefore the ongoing encroachment of M. cerifera and the possible warming effect through the surface energy balance may indicate that a positive feedback between vegetation cover and microclimate could exist due to the cold intolerance of M. cerifera, though the underlying mechanisms remain poorly understood. In this study, we use a combination of experimental and modeling approaches to identify the cold tolerance thresholds of M. cerifera and quantitatively assess the coupling effects of regional climate change and local microclimate modification on ecosystem stability and the potential occurrence of bi-stable states in grassland-shrubland ecotones.
Results/Conclusions
Nighttime air temperatures were significantly higher in myrtle shrublands than grasslands, particularly in the winter season. The difference in the mean of the 5% and 10% lowest minimum temperatures between shrubland and grassland calculated from two independent datasets ranged from 1.3 to 2.4°C. The stem hydraulic conductance significantly decreased at -15°C (P < 0.0001) and further reduced to ~0 at -20°C due to the freezing-induced cavitation. The model results clearly show that a small increase in near-surface temperature can induce a non-linear shift in ecosystem state from a stable state with no shrubs 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.