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

Authors: EDWARDS, BRANDON - New Mexico State University; ALLEN, SCOTT - Eth Zurich; BRAUD, DEWITT - Louisiana State University; KEIM, RICHARD - Louisiana State University Agcenter

Submitted to: Forest Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/21/2019
Publication Date: 3/25/2019
Citation: Edwards, B.L., Allen, S.T., Braud, D.H., Keim, R.F. 2019. Stand density and carbon storage in cypress-tupelo wetland forests of the Mississippi River delta. Forest Ecology and Management. 441:106-114. https://doi.org/10.1016/j.foreco.2019.03.046.
DOI: https://doi.org/10.1016/j.foreco.2019.03.046

Interpretive Summary: Baldcypress-water tupelo swamps are the dominant forest type on the Mississippi River delta. These unique forests are could potentially convert to marsh or open water because of relative sea level rise and increasing salinity. We classify cypress-tupelo forest condition on the delta and investigate how stand density and carbon storage potential relate to distance from the coast and broad-scale coastal change processes. Overall, approximately 29, 50, 21 % of forest was classified as full canopy, intermediate, and open canopy, respectively; if all these forests were to degrade to an open canopy condition, an estimated 7 megatonnes of standing carbon stock would be lost. There are well defined gradients of forest density in the coastward direction. Full canopy forest was concentrated in upper basins and along major distributary ridges, intermediate forest makes up the largest proportion of forest in mid-basin reaches, and the proportion of open forest increases in the coastward direction. In turn, aboveground carbon stocks and forest production decrease toward the coast. The similarities between these patterns and well-documented marsh loss patterns in the region raises the possibility that the processes driving marsh loss at the marsh-marine boundary may also manifest in coastal forest losses at the marsh-forest boundary. This work provides an initial assessment for dtecting change in forest canopy density moving forward.

Technical Abstract: Forested wetlands play a vital role in the coastal zone, but their vulnerability to drivers of 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, 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 forest on the Mississippi River delta.