Tidal freshwater forest accretion does not keep pace with sea level rise

Abstract

AbstractSoil properties, accretion, and accumulation were measured in tidal freshwater forests (tidal forests) of the Ogeechee, Altamaha, and Satilla rivers of the South Atlantic (Georgia USA) coast to characterize carbon (C) sequestration and nutrient (nitrogen‐N, phosphorus‐P) accumulation in these understudied, uncommon, and ecologically sensitive wetlands. Carbon sequestration and N and P accumulation also were measured in a tidal forest (South Newport River) that experiences saltwater intrusion to evaluate the effects of sea level rise (SLR) and saltwater intrusion on C, N and P accumulation. Finally, soil accretion and accumulation of tidal forests were compared with tidal fresh, brackish and salt marsh vegetation downstream to gauge how tidal forests may respond to SLR. Soil accretion determined using 137C and 210Pb averaged 1.3 and 2.2 mm yr−1, respectively, and was substantially lower than the recent rate of SLR along the Georgia coast (3.0 mm yr−1). Healthy tidal forest soils sequestered C (49–82 g m−2 yr−1), accumulated N (3.2–5.3 g m−2 yr−1) and P (0.29–0.56 g m−2 yr−1) and trapped mineral sediment (340–650 g m−2 yr−1). There was no difference in long‐term accretion, C sequestration, and nutrient accumulation between healthy tidal forests and tidal forests of the South Newport River that experience saltwater intrusion. Accelerated SLR is likely to lead to decline of tidal forests and expansion of oligohaline and brackish marshes where soil accretion exceeds the current rate of SLR. Conversion of tidal forest to marshes will lead to an increase in the delivery of some ecosystem services such as C sequestration and sediment trapping, but at the expense of other services (e.g. denitrification, migratory songbird habitat). As sea level rises in response to global warming, tidal forests and their delivery of ecosystem services face a tenuous future unless they can migrate upriver, and that is unlikely in most areas because of topographic constraints and increasing urbanization of the coastal zone.