Structural heterogeneity in above vs. belowground biomass pools differ for Spartina alterniflora monocultures, with consequences for forecasting ecosystem resiliency

Background/Question/Methods: We estimated structural heterogeneity in above vs belowground biomass of Spartina alterniflora, a common salt marsh macrophyte. S. alterniflora typically occurs in monoculture. Despite this, structural heterogeneity in plant biomass is important in these ecosystems. For example S. alterniflora vegetation heights and aboveground biomass typically decline precipitously along a marsh edge to interior gradient. These aboveground changes are caused by well-known elevation-related differences in salinity and tidal flooding that occur in these ecosystems. However, the influence of these environmental gradients on the structure of belowground biomass is less well known. Understanding salt marsh belowground structural heterogeneity is important because belowground productivity contributes to coastal marsh resiliency both through sub-surface expansion, allowing marsh surface elevation to keep pace with sea level rise, and through soil stabilization, preventing lateral marsh erosion from wave action. Thus, sustained declines in belowground biomass may forecast future conversion of marsh areas to mudflats or open-water. We developed an open-source remote sensing method to estimate change in S. alterniflora above and belowground biomass, where we modeled belowground biomass via aboveground biophysical proxies. We then scaled these plant biophysical models through the use of freely available Landsat 8 satellite data and examined spatio-temporal patterns.

Results/Conclusions: Our models predicted aboveground biomass from validation data with a root mean squared error (RMSE) of < 90 g m^-2, and belowground biomass with a RMSE < 350 g m^-2 (field data range: 20–1000 g m^-2 and 200–3500 g m^-2, respectively). Over the 4-year span of available data, aboveground biomass change was positive across the study area when averaged by marsh elevation. However, mean belowground biomass decreased steadily over time at the lowest marsh elevations. Thus, patterns in belowground biomass change differed substantially from aboveground change. These belowground declines were also negatively correlated with tidal flood heights. This suggests that sea level rise may be overwhelming the marsh capacity to accrete vertically, likely by depriving plant roots of oxygen. Patterns of declines were spatially explicit and concentrated in low-lying marsh interior areas, where declines of as much as 400 g m^-2 were observed. Given that rates of sea level rise are accelerating, it will be increasingly important to use these kinds of observations to decide when and whether to initiate early interventions to prevent marsh loss. As these methods develop, our goal is to map spatio-temporal changes in belowground productivity in coastal marshes more broadly.