Woody plant encroachment in coastal ecosystems as a phase transition induced by microclimate feedbacks

Abstract:

The spatial pattern of vegetation patchiness may follow universal characteristic rules when the system is close to critical transitions between alternative states. This improves the anticipation of ecosystem-level state changes which are currently difficult to detect in real systems. Climate warming has been suggested to facilitate woody plant encroachment in ecotones worldwide where woody plant growth is primarily constrained by extreme cold events including the Virginia barrier islands. This phenomenon can be enhanced by vegetation–microclimate feedbacks, whereby the establishment of woody plants creates a warmer microclimate thereby reducing the mortality of cold-intolerant woody seedlings. It is unknown whether the woody vegetation patterning exhibits some characteristic properties in these woodland-grassland ecotones and whether the spatial patterns arise from local positive feedbacks. In this study, we integrate high-resolution imagery data over four decades with a novel stochastic cellular automata model to investigate the spatial patterning of woody patches on Hog Island (Virginia). We found that the spatial patterns of vegetation exhibited signs of critical phenomena as evidenced by the emergence of power law distribution of woody patch size preceding the abrupt woody plant expansion (1986-1990). The modelling results confirmed the observational results that power law distribution only occurred when the background minimum temperature approximated a critical threshold, suggesting that the power law patterning arises from the local positive feedback between woody plants and microclimate. Our findings highlight the critical role of local positive feedbacks in driving these critical phenomena and a phase transition in plant dominance. Therefore, this study improves our understanding of vegetation spatial patterning and ecosystem stability under global climate change in coastal ecosystems. Our work also provides both empirical and theoretical evidence of whether the observed patterning can be considered as a general early warning signal of critical transitions from grassland to woodland in coastal and potentially other ecosystems.