How Coastal Trees Adapt Amid Rising Seas

With rising seas and worsening droughts, coastal forests face an uncertain future. Scientist Riley Leff dives deep into the hidden strategies trees use to survive extreme stress, from strategic growth spurts to surprising water-handling tactics. Could understanding trees’ secret survival playbook help protect entire ecosystems, and even our planet?

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Balancing Act at the Water’s Edge

You stand tall on the shores of the Atlantic Ocean. With the sun beating down on you, it's hard to catch your breath. Breathing faster only makes you thirstier, and you wonder when you will get a much-needed break. This is the life of a coastal tree: rooted in place, exposed to the elements, and constantly balancing the need for water, sunlight, and growth. A tree’s response to its environment changes throughout its life, allowing it to meet its needs at any time. Its ability to respond to the needs of its environment is crucial, as it can determine whether it thrives or is outcompeted by its neighbors. Trees employ different structural and physiological tactics to match their local conditions at each developmental stage, but that careful balance can be greatly disrupted by events like climate change, putting whole ecosystems at risk. 

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Timing is Everything: Growth Strategies

Early in life, spending energy on growth is a tree’s most vital investment. Young trees receive small amounts of available light, and without the ability to move, this resource — in addition to soil nutrients — is fixed. If a tree doesn’t develop enough early on, it is at risk of being dominated by larger neighbors and losing available resources. Investing more into upward growth for canopy height or downward growth for root spread helps a tree outcompete its neighbors and monopolize its local resources. Strategic phenological differences show up early on, with some juvenile trees budding and leafing out earlier than adult trees, according to the U.S. Forest Service. 

A tree’s strategy throughout a given year can vary greatly as well, especially in temperate biomes. During the wet, resource-rich growing season, some trees add “earlywood”: thinner, porous wood. As conditions grow harsher and drier later in the year, trees add denser, thicker-walled cells, creating the dark bands of the recognizable tree rings. The thickness of a ring can offer clues about how favorable growing conditions were in a given year. Tree rings are vital to the science of dendrochronology, or the science of studying the history of trees through their rings. 

Tree rings reveal annual climate conditions, with thick rings marking wet years and thin rings signaling drought. (Image: science.org)

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Unlocking the Tree’s Inner Workings

Riley Leff, a PhD candidate at the George Washington University (GWU), studies how coastal trees respond to sea level rise. He has spent the past four years studying saltwater-inundated forests, observing that while there has been a significant change in vegetation composition, with shrubs and the invasive perennial grass Phragmites expanding through the forest, the trees have barely changed. Despite living in adverse conditions with high salinity and multiple floods a year, there hasn’t been significant tree mortality in the short term, Leff said. To uncover how these trees persist under extreme stress, Leff is studying how these trees respond to harsh conditions and what adaptations allow them to be more resilient. 

Early in his PhD, Leff utilized the Sperry model when studying reverse sap flux events in trees, a process in which water travels downward through a plant's root system, opposite to the typical upward movement that occurs during transpiration. He observed that trees can survive floods with little to no health deficits by dispersing the load across different aspects of their physiology. 

Leff wants to understand how the different internal systems of trees are coordinated at the organismal and ecosystem levels. 

“From the physiological level to the whole system level is where we’re zooming to, asking where the incentives are,” he said. 

Leff explained that he finds coastal forests specifically fascinating for studying tree decisions due to the high variability across a site, where some areas are dominated by older trees, while others affected by salinity form canopy gaps that young plants must rapidly compete to fill. These variations in vegetation lead to different types of tree competition, and studying how these dynamics change over time under the stress of sea-level rise provides a unique perspective on tree physiology and strategy. 

Although there is well-established knowledge of chemical and physiological aspects of tree growth, there is more to be learned about how these processes scale up when trees are under stress. Leff explained that his research studies different aspects of tree mechanics under hydraulic stress. These aspects are displayed in prominent modeling, with models like the Sperry model, which helps predict trees' responses to drought, and the FATES model, which outlines how individuals or groups of trees live under different resource stresses and competition dynamics. Advanced modeling offers clues into the science of these processes at multiple levels. 

“The challenge we face as ecologists is taking what we can be really sure of and scaling it,” Leff said.

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Forest Royale: Learning Tree Survival Through Play

Leff said he was inspired by the April 8th Bio Grad Student Council science fair at GWU and wanted to create an educational supplement to help share his ideas with a wider audience, including kids. He created Forest Royale, a game where the player is a tree with the ability to make decisions about carbon allocation and water uptake to survive and grow. Leff said a game seemed like an excellent idea to complement his poster and get his audience more excited about the tree physiology he is studying. 

Forest Royale, an interactive game exploring tree survival strategies under environmental stress.

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Why Tree Strategies Matter for Our Future

It is urgent to understand how trees adapt to environmental stressors, especially in an era of rising sea levels and increasing droughts. Trees, with long life spans and immobility, are especially vulnerable to these environmental changes that upset their delicate balance. 

Predicting how trees respond to adverse conditions can shape how humans manage forests around the world. According to the United Nations, forests make up 31 percent of the world, providing habitats for 80 percent of terrestrial organisms that live on, in, or around them. Trees’ response to environmental stressors can propagate through ecosystems, and their important role in carbon sequestration can affect the whole planet, making it extremely important to understand and protect them.