Spaceborne lidar observations reveal impacts of inundation on coastal forest structure across the U.S. Mid-Atlantic Estuarine, Coastal and Shelf Science

The impacts of accelerated sea level rise (SLR) on coastal ecosystems due to climate change has yet to be fully realized. SLR, combined with an increasing intensity of storm surges, are driving significant regime shifts in vegetation across coastal landscapes, leading to marsh migration and upland forest mortality. However, the specific effects of tidal inundation, stemming from elevated water levels and soil salinity, on forest vertical structure remain poorly understood. In this study, we use spaceborne light detection and ranging (lidar) data from the Global Ecosystem Dynamics Investigation (GEDI) to explore the response of vertical forest structural dynamics in areas highly vulnerable to increased inundation across the U.S. Mid-Atlantic coastal region. We assessed the impact of inundation on three forest structural traits derived from GEDI data. We identified the threshold position where forest structure is no longer impacted and investigated the environmental factors influencing these positions across watersheds to determine the forest's vulnerability to transitioning into marshes. We discovered that watersheds with a high proportion of area below Mean Higher High Water (MHHW) tended to increase vulnerability to forest conversion into marshes whereas watersheds characterized by steeper slopes and drainage densities tended to have positions reflecting lower vulnerability, suggesting an overall increased resistance to marsh migration. These findings highlight the importance of monitoring forest structural dynamics for early detection of upland marsh expansion, with lidar technology offering a potentially valuable tool to enhance our understanding of ecological shifts in coastal environments. Such insights may be essential for evaluating ecosystem responses to SLR and may foster a more comprehensive understanding how SLR and other climate change-induced disturbances will affect the coastal carbon sink.