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Stand density and carbon storage in cypress-tupelo wetland forests of the Mississippi River delta

Edwards, Brandon L., Allen, Scott T., Braud, DeWitt H., Keim, Richard F.
Forest ecology and management 2019 v.441 pp. 106-114
Landsat, Nyssa aquatica, Taxodium distichum, canopy, carbon, carbon sequestration, carbon sinks, coastal forests, coasts, ecosystem services, ecosystems, forest types, lowland forests, reflectance, river deltas, salt marshes, sea level, stand density, thematic maps, Mississippi River
Forested wetlands play a vital role in the coastal zone, but their vulnerability to coastal change—and the impact to high-value ecosystem services—is not as well established as that of more seaward systems such as saltmarsh and mangroves. To address this need, we develop field-based stand density classes, then classify baldcypress-water tupelo (Taxodium distichum (L.) Rich var. distichum; Nyssa aquatica L.) stand density on the Mississippi River delta using a multitemporal ordination of reflectance from Landsat Thematic Mapper imagery. Approximately 29, 50, and 21% of forest was classified as full canopy, intermediate, and open canopy, respectively. We estimate stand-level live-stem carbon stocks and annual accumulation rates of 96, 67, and 39 t-C ha−1 and 1.9, 1.4 and 0.8 t-C ha−1 yr−1 for full-canopy, intermediate, and open-canopy forest, respectively. Regional live-stem carbon stocks are ∼ 69 t-C ha-1, and total carbon and annual increment for all forest analyzed are ∼17 Mt-C and ∼0.34 Mt-C yr-1. Much of the cypress-tupelo forest on the delta stores carbon at rates significantly below the potential of fully stocked stands, yet overall rates of accumulation are comparable to other forest types and coastal systems. Delta-wide, there is a well-defined coastward gradient of stand density, which has important implications for the future of deltaic forests and their ecosystem services. The similarities between forest density gradients and well-documented marsh loss patterns in the region suggest that the same processes driving coastal marsh loss—relative sea level rise exacerbated by human activity—are likely responsible for a second, more gradual interface of land loss at the marsh-forest boundary. Results highlight the potential response of coastal forests to continued environmental change—and associated impacts to the carbon cycle—and provide a baseline for detecting future change to forests on the Mississippi River delta.