Coastal spruce-fir dynamics in the face of sea-level rise and salt marsh migration

The diverse ecosystems of Acadia National Park include spruce-fir forests, tidal creeks, and salt marshes. In some places, these systems are adjacent to each other, creating a unique interface that is sensitive to climate-related changes including sea-level rise.

Park managers generally want to enable salt marshes to migrate inland as sea levels rise because marshes filter water and provide important habitat, among many other functions. And yet, the inland migration of salt marshes may encroach on coastal spruce-fir forests, which are also vulnerable to rising temperatures. Low-elevation spruce-fir forests are at the southern edge of their range in Acadia, and the forest type is broadly expected to retreat northward as the climate changes.

How these forests have changed over recent decades is the subject of 2020 Second Century Stewardship Fellow Jay Wason, but Wason’s research does not include the edge or boundary of the forest. On coastal forest edges in the mid-Atlantic and further south, exposure to salt water and salt spray has been implicated in forest decline, and rising sea levels have created “ghost forests” at salt marsh-forest boundaries. Such dynamics have received little attention in the Northeast, in part because the rocky, steep topography creates an unusual salt marsh-forest ecotone.

In this project, Littlefield plans to examine the shifting boundary between marsh and woods: how salt marshes may be advancing, and how spruce-fir forests may be retreating. She will evaluate conifer health, mortality, and the emergence of seedlings at increasing distances from the salt marsh interface and measure indicators of seawater in soils.

Working with colleagues at University of Vermont (Jane Foster) and Schoodic Institute (Peter Nelson), Littlefield will use satellite imagery to determine if patterns observed in the field have been unfolding for years and if the patterns are widespread. This information will inform the creation of predictive maps of salt marsh migration and spruce-fir forest retreat.

“This study will directly inform a wide range of important work taking place in salt marshes in and around Acadia,” said Jesse Wheeler, plant ecologist with Acadia National Park. “For example, it will help us plan vegetation restoration at the upland edge of Bass Harbor Marsh, and may inform our work with Wabanaki scientists and harvesters to preserve sweetgrass populations, which grow near upland edges of saltmarshes and are culturally significant.”

The National Park Service is challenged to manage resources “for continuous change that is not yet fully understood.” Soon managers will face decisions on how to support salt marsh migration and simultaneously protect spruce-fir forests, Wheeler noted, “Managing that change will be difficult and we managers can use all the information we can get to help us make informed decisions.”