Black mangrove forest resistance and resilience to winter climate extremes: Implications for range expansion under climate change
In tidal wetlands of the northern Gulf of Mexico, climate change is expected to result in the poleward expansion of black mangrove (Avicennia germinans) forests at the expense of some salt marshes. Since woody plant encroachment into tidal grasslands would alter ecosystem properties and the supply of some ecosystem goods and services, there is a need to better understand how changing winter air temperature extremes would affect mangrove-marsh interactions in northern Gulf of Mexico tidal wetlands. In this study, we investigated the resistance and resilience of black mangrove forests to winter climate extremes. We refer to resistance as the capacity to withstand change and resilience as the capacity to recover after perturbation. Our analyses focus primarily upon a transition zone in Louisiana where disturbance-induced mangrove-marsh competition has been intense and where salt marshes are particularly vulnerable to future mangrove expansion under climate change. We quantified temporal and spatial variation in winter air temperature extremes using historical records dating back to the 1820s as well as spatially-explicit data spanning the land-to-sea transition zone in coastal Louisiana. Black mangrove forest resistance and resilience were measured using aerial imagery interpretations (1978, 1993, 2000, and 2011) along with field-based measurements following recent freeze events (2014).
We used diverse data sources to refine climate-coastal wetland linkages and to better understand the vertical and horizontal microclimatic gradients that affect black mangrove resistance and resilience to winter climate extremes. We quantified mangrove forest damage and recovery from extreme freeze events that occurred in the 1980s. Our analyses identify positions in the landscape where black mangrove forests are likely to be most resistant, resilient, and dominant. Resistance and resilience were highest in areas close to the ocean and far from land, where winter temperature extremes were buffered by large expanses of water and saturated soil. Under climate change, such locations are likely to serve as local “hubs” or “hotspots” for mangrove dispersal, growth, expansion, and displacement of salt marsh. Collectively, our results improve our understanding of the current and potential future distribution of black mangrove forests and salt marshes in Louisiana as well as along other parts of the northern Gulf of Mexico coast.