Session 1B: Landscape Evolution

June 11
12:15 - 1:45 pm

Chesapeake Salon EF

Saltwater intrusion in the real world: Observations from South Dorchester
Matt Kirwan
Virginia Institute of Marine Science

Sea-level rise is driving the landward movement of seawater into terrestrial portions of the coastal landscape, leaving behind a mosaic of marshland, dead trees, and salt-stressed agricultural fields. The impact of saltwater intrusion on coastal forests and farmland is typically understood as sea-level driven inundation of a static terrestrial landscape, where ecosystems neither adapt to nor influence saltwater intrusion. Yet, new process-based research into the hydrologic, geomorphic, biotic, and anthropogenic mechanisms involved suggest at least some resistance of coastal ecosystems to salt water intrusion. Here, we use observations from Dorchester County, MD and the greater Chesapeake Bay region to illustrate the rates and drivers of sea-level driven land use change. We find that 100,000 acres of forest and farmland have been lost in the Chesapeake since the 1850’s, with rates that depend principally on sea level rise and topographic slope, and the salinity and tidal range of adjacent rivers. Surprisingly, rates of coastal forest retreat are about half the rate expected by SLR propagating across a static topography, which we suggest reflects the resistance and slow death of large, mature trees. Unprotected agricultural fields actually retreat faster than forestland, though modifications (levees, ditching) slow their conversion to marshland by approximately 50%. Together, these findings suggest that assumptions of instantaneous conversion of uplands to wetlands will overestimate future land conversion, but also imply that the full effects of historical sea-level rise have yet to be realized.

Mapping Delmarva's Changing Landscape: Monitoring Saltwater Intrusion Effects Through Remote Sensing
Manan Sarupria
University of Delaware

Saltwater intrusion (SWI), in coastal areas, is movement of salt water from ocean to inland areas, a problem expected to worsen due to climate change, unsustainable farming practices, and increasing demand for crops and water resources. This phenomenon, causing soil salinization, impacts an area approximately 1.1 million km 2 globally, larger than the continental United States. SWI is closely tied to the expansion of marshlands, which are increasingly affected by rising sea levels, posing a complex challenge. To tackle this issue, our research utilizes machine learning to perform spectral unmixing of Sentinel-2 satellite data to accurately map the spatio-temporal evolution of salt patches and marshlands. Focused on 14 coastal counties in Delaware, Maryland, and Virginia (the Delmarva Peninsula), our study generates annual maps spanning 2019-2023, detecting salt-impacted farmlands and marshlands. Our findings reveal a considerable increase in salt patches across Delmarva, rising from 10 km 2 in 2019 to 26 km 2 in 2023—a 2.5-fold surge in five years. Similarly, marshland extent grew from 1,149 km 2 in 2019 to 1,173 km 2 in 2023. These insights provide crucial guidance for landowners in managing their properties. Furthermore, we're exploring the relationship between various hydroclimatic factors—including storm surges, precipitation, temperatures, sea-level rise, farmland elevation, coastline proximity, and soil types—and the observed changes in vegetation health (NDVI) of farmlands, serving as an indicator of SWI severity. This analysis aims to deepen our understanding of the spatial and temporal dynamics of salt patches and marshlands, aiding in effective mitigation strategies.

Where are the ghost forests: mapping and understanding tree mortality of the North American Coastal Plain
Xi Yang
University of Virginia

Ghost forests are both extensive and highly heterogeneous along the coastal regions of the North American Coastal Plain (NACP). Understanding the drivers and potential impacts of ghost forest formation are important to both the scientific community and the local communities. Yet, the spatio-temporal variations of ghost forests are still intractable at the regional scale. In this talk, I present some of our team's effort to map the extent of ghost forests in NACP using mid-resolution Landsat datasets and the high resolution NAIP images. Using Landsat, we tracked the changes in Coastal Forested Wetland (CFW) between 1996 and 2016. We show that the compounding effects of sea level rise salt water intrusion, tropical storm, and local topography drive the spatial variations in the loss of CFWs. Using NAIP images, we mapped over 6 million dead trees (2020) in the coastal regions (<3 m) from South Carolina to Maine, allowing us to find local hotspots of tree mortality. We show that these hotspots exist in both protected and non-protected areas. These datasets shed light on the distribution and drivers of ghost forests, and provide the base layer for analyzing the impact on carbon cycle, biodiversity, and ecosystem resilience.

Global changes in coastal wetlands and what becomes of them
David Lagomasino
East Carolina University

Coastal wetlands, including mangroves and marshes, are critical ecosystems providing numerous socioecological services that support communities all around the world. These transitional landscapes thrive at the boundary between the land and sea, but rapid changes in land use and climate have altered hydrologic processes affecting the timing, distribution, and movement of water. These changes in water lead to a complex evolution of the landscape that can reinforce resilience or exacerbate ecosystem degradation. These compound pressures are important to understand so that more effective management strategies that balance economic development and conservation can be implemented. Through a combination of “big” satellite data, we can delineate the spatial and temporal patterns of coastal wetland degradation and loss, but also recovery and gains. The unique information provided by satellite and other global spatial data can help resolve not just when and where coastal change is occurring but also how that change is happening. Specific structural and functional signatures exist on the landscape that can provide insight into how the landscape has changed which can be captured from optical, lidar, and radar sensors. Understanding why coastal wetland loss occurs is important, as it provides insights into the underlying causes of change, such as human activities, natural disturbances, or environmental factors. This information is critical for tracking trends in coastal wetland losses and for informing the decision-making processes around wetland management. This presentation will focus on how coastal wetlands around the world have changed over the past three decades and the reasons behind those changes. From the direct impacts of human reclamation and storms to the indirect influences of geomorphology and climate, this talk will highlight how culturally and ecologically important wetland complexes have changed over time, provide predictions on what we may see into the future, and discuss what that means for the communities that rely on them.