Rising waters, waning forests: How scientists are using tree rings to study how rising sea levels affect coastal forests

Guest editorial by Dr LeeAnn Haaf, assistant director of Estuary Science, Partnership for the Delaware Estuary and author of a new Frontiers in Forests and Global Change article

Frontiers

Ghost forest — image:
When trees in coastal areas die a graveyard of dead trees—known as a ‘ghost forest’ is left behind. Salt-tolerant marsh plants take root and form a green carpet below the remains of the once-thriving forest. Image: LeeAnn Haaf.
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Credit: LeeAnn Haaf

Sunlight filters through the canopy of pines, holly, sweet gum, and red maple while bird calls echo in the distance. These coastal forests may seem like others in the Mid-Atlantic, but a hidden challenge looms. Standing tall next to their salt marsh neighbors, where the wind carries the sharp scent of sulfidic seawater, these trees are more than just part of the landscape—they are living monuments to a rapidly changing environment. As sea levels rise, the future of these forests is uncertain. While the adjacent salt marshes can adapt to encroaching waters, the trees, vulnerable to the increasing frequency of saltwater flooding, face a grimmer prospect. Additionally, temperatures are increasing, and rain patterns are shifting. How long can the forest withstand the pressure of a changing climate? When will they finally succumb to a rising tide?

Rising tides

Coastal forests occupy low-lying land just above sea level, situated beside tidal marshes. Being low and close to tidal channels, these forests can flood with saltwater, which may happen a few times a year or only during the most severe storms. However, as sea levels rise, the boundary between land and sea pushes upslope, leading to more frequent flooding. Tidal marshes dynamically build elevation or migrate upslope, maintaining their positions relative to flooding. Forests, however, are far less adaptable. Along the lower edges, individual trees begin to die, forcing the forest to retreat until what remains is a graveyard of dead trees—known as a ‘ghost forest.’ Here, salt-tolerant marsh plants, such as smooth cordgrass (Spartina alterniflora), take root and form a green carpet below the remains of the once-thriving forest. This shift is beneficial for tidal marshes, allowing them to expand even in the face of erosion or other threats, but it comes at the expense of the coastal forest.

The stark reality of this transition is already apparent in many coastal areas, where acres of dead trees stand as a testament to the encroaching saltwater. Retreating coastal forest can result in a loss of biodiversity, and perhaps carbon sequestration; if nothing else, it represents the loss of critical buffer space between the land and sea. Land slope plays a role in determining where these forests retreat, but the variability is enough to leave land managers questioning: Where will forests retreat and where will tidal marshes take their place? Proactive management is paramount, as once the trees begin to die, it is likely too late to alter their fate. To anticipate these changes, it is essential to understand the subtleties that occur before tree death. Signals of stress can be gleaned from how well trees are growing as flooding increases, temperature rises, and precipitation patterns change. These signals point towards what conditions may eventually lead to tree death, and depending on other characteristics of the forest, where coastal forests are more vulnerable to retreat.

Tree rings show highly specific effects of sea level rise

Our study delved into this using dendrochronology, the analysis of tree growth rings, to explore relationships between flooding, climate variables, and site-specific conditions. Dendrochronology allows us to understand the conditions under which trees thrive or struggle, with narrower growth rings indicating periods of stress. Traditionally, simple correlations have been used to study these relationships, but we employed a different technique: gradient boosted linear regression. This machine learning approach can uncover complexities that correlations might miss, such as non-linear growth patterns across a spectrum of environmental conditions. We applied this method at four sites, with three tree species common to coastal forests in New Jersey and Delaware: loblolly pine, pitch pine, and American holly.

Our hypothesis was that rising sea levels would lead to reduced growth across species. However, the results were far more nuanced. The effects of sea level rise on tree growth varied depending on temperature, precipitation, and the site. At one site, we found that American holly grew better when winter water levels were higher. Loblolly pines appeared vulnerable to autumn water levels. We also observed frequent non-linear growth responses, painting a more complicated picture of how these forests react to rising sea levels and climate change. We also analyzed whether the gradient-boosted results indicated that trees would fare better or worse under predicted changes in temperature, precipitation, and water levels. Our findings revealed few consistent patterns, highlighting the influence of species and site-specific factors on overall vulnerability.

Learning to manage coastal forests

Before trees reach the point of no return, the impacts of environmental changes on their growth are anything but simple. In some cases, climate change might even enhance resilience to increased flooding. For example, loblolly pine, situated at its northernmost distribution in our study sites, could benefit from warmer winters, perhaps offsetting some stress caused by flooding. Similarly, American holly showed markedly different results between two sites, possibly due to variations in moisture availability. These and other factors likely contribute to the variability in how and when specific coastal forests will retreat in response to sea level rise.

Overall, the effects of climate change and increased flood frequency on coastal forests are complex and often non-linear, highlighting the need for nuanced forest management strategies. In the future, similar dendrochronological studies could serve as valuable tools for assessing coastal forest vulnerability to climate change and sea level rise. Our findings aim to inform land management efforts, helping to strike a balance between conserving coastal forests and tidal marshes given the growing pressures of climate change and sea level rise.