COS 189-6 - Climate-driven community reassembly in retreating forest along the mid-Atlantic coast, USA

Abstract: Sea-level rise is a primary driver of ecosystem turnover across coastal landscapes. One of the most striking examples is occurring along the mid-Atlantic coast of the US, a sea-level rise hotspot where retreating upland forest is rapidly being replaced by low-lying, non-regenerating forest (transition forest) and salt marsh. Yet data tracking vegetation changes occurring during ecosystem turnover are scarce. To better understand the dynamics of community reassembly from forest to marsh accompanying sea-level rise in this region, we evaluated vegetation characteristics across five zones spanning gradients of salinity and elevation (high elevation forest, mid elevation forest, low elevation forest, forest-to-marsh transition, and high marsh) at five sites: Milford Neck, DE; Monie Bay, MD; Moneystump, MD; Phillips Creek, VA; and Goodwin Island, VA. Across sites we found that, 1) species richness of mature trees decreased from high elevation forest to the forest-to-marsh transition zone, generally along a gradient of increasing soil salinity; 2) although tree seedlings were generally present in forest understory, relatively few saplings were present, indicating a decline in forest regeneration in otherwise seemingly healthy forest stands; and 3) tree canopy cover declined and salt marsh cover increased abruptly with increased soil salinity, suggesting there is a salinity threshold beyond which upland forest is not resilient, which leads to a state shift from forest to salt marsh. These trends were weakest at Milford Neck, where the rate of sea-level rise is the lowest among the sites (3.60 mm y^-1). Tree canopy cover decreased below 50% and salt marsh cover exceeded 50% at soil salinities above 5 ppt at Monie Bay, and above 3 ppt at Moneystump. At Phillips Creek, canopy cover decreased below 50% and salt marsh cover reached 100% at salinities above 6 ppt. At Goodwin Island, where the rate of sea-level rise is highest among the sites (4.93 mm y^-1), canopy cover fell below 50% at salinities above 2 ppt and marsh cover reached 100% across the range of soil salinities. Our findings highlight the structural and compositional changes occurring in coastal forest in a region where high rates of sea-level rise are driving wholesale reorganization of coastal ecosystems.