COS 60-6 - Assessment of spatio-temporal patterns of aboveground biomass using in-situ data and remote sensing time series-based indicators: An application in the Atchafalaya and Terrebonne Basins, Louisiana, US

Background/Question/Methods

Coastal basins in the Mississippi River Deltaic Plain (MRDP) are increasingly threatened by global climate change and anthropogenic activities. Future rates of sea level rise and altered sediment supply due to land use change may have an impact on future coastal wetland ability to keep pace with rising seas. Healthy vegetation is a dominant factor influencing the stability and elevation of tidal marshes as sea level rises. We used a combination of in-situ biomass measurements and remote sensing techniques to estimate how vegetation dynamics changes in response to seasonality and salinity gradients across different hydrogeomorphic zones (supratidal, intertidal) in two coastal basins – Atchafalaya and Terrebonne – with contrasting sediment delivery and hydrological regimes during the early growing season (March) and peak biomass season (August). Vegetation structure was measured and aboveground biomass (AGB) and necromass (AGN) were evaluated at each site by harvesting vegetation inside duplicate plots (0.25 m²), located 5 m apart at each sampling station. In addition, this study assessed the accuracy of different vegetation indices, such as Normalized Difference Vegetation Index (NDVI) and Enhanced Vegetation Index (EVI), in estimating AGB and carbon stocks from PlanetScope images across the study area.

Results/Conclusions

Total AGB increased from freshwater to saline sites along both basins and was on average 3 times higher in peak biomass relative to early growing season. Total AGB ranged from 530 to 1300 g m^-2 and AGN from 10 to 770 g m^-2 in the peak biomass season, with the highest AGB estimates in saline sites across all zones and basins. Species diversity increased with seasonality in freshwater sites across basins. Significant changes in species composition were observed in the saline sites during peak biomass, with S. alterniflora dominating other salt resistant species like S. patens. Species composition in the freshwater site in the Atchafalaya Basin was consistent in the supratidal zone during both seasons. In contrast, the intertidal zone was dominated by S. nigra and Polygonum spp. during the peak biomass season. NDVI derived from PlanetScope images of the study area had relatively consistent accuracy for estimating AGB and carbon stocks when compared to other vegetation indices. The results show the importance of assessing vegetation in response to seasonality and salinity gradient in the MRDP. With a better understanding of vegetation dynamics and zonation, our results will be used to generate landscape-level vegetation maps to identify indicators ecosystem vulnerability.