Assisted migration aims to help forests – and people – adapt to a swiftly changing climate

By: - August 6, 2021 5:30 am

Dartmouth College’s Second College Grant in the North Country is participating in a study called “Adaptive Silviculture for Climate Change.” Standing at center is Samuel Zuckerman of UNH’s College of Life Sciences and Agriculture. (Courtesy of Jeff Hamilton)

A tree doesn’t just all of a sudden pull up its roots, pack its bags, and decide to set out for a more hospitable home when the climate changes.

But trees do migrate.

This can occur through processes like natural selection, where the tree species that are best adapted to a particular area are the ones that survive. When seedlings are cast off on the wind or carried by a bird, they land somewhere new, and if the conditions are favorable there, they will grow. 

Over long periods of time, that means a forest of a particular kind of tree can find an entirely new home. Take New Hampshire: The trees that currently grow in forests here arrived only after the last ice age receded, about 11,700 years ago. As temperatures gradually warmed, trees moved to the area.

That process takes a long time – and current models show that the rapid march of climate change is outpacing the speed at which trees are capable of relocating, a problem that researchers from UNH will study through a grant from Northeastern States Research Cooperative.

“Models predict that natural migration rates of tree species will likely not be able to keep up with the rapid pace of climate change,” said Heidi Asbjornsen, associate professor of natural resources and the environment at the UNH College of Life Sciences and Agriculture.

“Basically what we’re saying is that the climate is changing too quickly for trees to adapt through natural evolution,” Asbjornsen said.

While natural migration of tree species unfolds over hundreds of years, with global warming, drastic increases in temperature have become evident in the span of just one generation. In New Hampshire, this has led to concern that climate change may threaten forest ecosystems and rural communities, where forests are an economic driver through forestry and tourism.

Forests provide goods and services, writes Lori Tyler Gula for UNH: “timber, fuel, and other wood products; reliable supplies of clean water; habitat for plant and wildlife species; recreational opportunities; and a diversity of non-timber forest products, such as tree syrups, mushrooms, and forage production.”

That’s what researchers fear is at stake if climate change is left unchecked and measures toward adaptation aren’t taken.

“In this region in particular we rely on our forest ecosystems for a lot of important societal values, so not just timber, which is important but, you know, recreation is a big one, some of the non-timber forest products like syrup production and wildlife,” Asbjornsen said. “They have a lot of value to the residents in this region, and we don’t really know how or to what extent our forests will be able to adapt and survive.”

“As a society we need to be thinking about how to prepare for climate change in the future,” Asbjornsen said.

As a part of a new study, Asbjornsen will study that problem as well as one possible way that humans and trees can adapt to a rapidly changing climate: assisted migration, where researchers introduce species that don’t typically grow in New Hampshire. The researchers will study six to seven species of trees that could be a good match for the warmer and drier climate of the future.

The researchers also plan to track how trees acclimate to changes in their environment on a shorter timescale, like if they start producing smaller leaves, which are better adapted to drought conditions, for instance.

Some experiments are already underway in Durham and at the Hubbard Brook Experimental Forest, a 7,800-acre hardwood forest in New Hampshire’s White Mountain National Forest. In both places, scientists are diverting about 50 percent of rainfall from plots in the forest and monitoring the impact of drought on different tree species. In Durham, they’re studying the impact on red oak and white pine, while the Hubbard Brook experiment focuses on red maple.

“If we see that some species are more resilient to drought than others, those might be the best species to plan as part of assisted migration,” Asbjornsen said.

But while Asbjornsen has been creating drought conditions to study the impact, the naturally occurring drought this year has provided some insights as well. For one, it occurred in the early spring, which could have a greater impact on trees since that’s when they are growing vigorously.

A better understanding of how different species respond to drought could help inform decisions about which species should be planted to increase the resilience of the future forest, Asbjornsen said. But research on droughts in the Northeast – which tends to have a wet climate – is sparse at best.

Tony D’Amato, a silviculturist and professor of applied forest ecology at the University of Vermont, has been working to add to the data for the past several years through experiments. He and Asbjornsen are teaming up to complete the latest stage of research.

D’Amato said when he first moved back to the Northeast from the Midwest, people were largely unconcerned about how to manage forests for drought. But he’s seen that change quickly over the past few years. One complicating factor is that projections for the next hundred years forecast a lot of moisture, but, D’Amato said, “what is often missed in terms of a key detail is that a lot of that moisture is going to come in a few events.”

“We’re going to get these episodic, you know, high-intensity rainfall events, and then we might have two to three weeks with no rain,” he said. “And so even though on average we are getting wetter, it’s still going to create these stressful conditions for certainly tree seedlings and other organisms like salamanders that really rely on fairly predictable moisture.”

That combination makes it more complicated to understand how trees in the Northeast will respond. D’Amato and other researchers hope assisted migration will help the forest be more resilient amid all kinds of stressors – not only climate change but the emerald ash borer and Asian longhorn beetle, for example, invasive insects that can take a toll on trees as well.

The goal, D’Amato said, is to “really sustain the values we care about, like clean water, carbon sequestration, local wood products.”

In some places, assisted migration means planting a tree species that already grows in a forest and is expected to do well in the future to increase representation. In others, an out-of-state species is introduced into an area where they’re expected to do well in the future.

Northern red oak is one example. There’s not much of it in northern New England, but it’s expected to do well. Eastern white pine is another contender. Trees with larger seeds, like oak, hickory, and beech, have more reserves that can help them survive drought from a young age, providing the reserves for the tree to send a deep tap root down into the ground early on and access water. Researchers look to trees that grow naturally in a climate that matches the predictions for New Hampshire by looking to places like Pennsylvania and the Mid-Atlantic. D’Amato said they are also evaluating red spruce and trying to restore it to the landscape. In spite of some skepticism about its chances for the future, D’Amato said he’s seen evidence that it’s recovering.

D’Amato said a common misconception around this work is that scientists are trying to re-engineer the forest. “We’re trying to work with the ecosystems and not against it to accomplish these goals,” he said.

Some in the timber industry are skeptical about the efforts. Jeff Eames, who studied forestry and now owns a logging company, said that in his experience trees that regenerate naturally grow best. He said that at the end of World War II, a lot of red pine was introduced to the region, but now Norwegian red pine is dying off. It’s less resistant to problems like scale, an invasive insect that can harm the trees. 

“​​They’re finding out now that introduced species don’t do as well as natural,” Eames said. He said he wouldn’t consider planting seedlings; he prefers to rely on natural regeneration and, for now, drought isn’t a concern.

Still, research takes time, and Asbjornsen said now is the time to learn more about how to adapt. The earlier we figure this out, she said, the better position we will be in to promote resilient forests in the future. 

Sen. Jeanne Shaheen praised the efforts, saying the research would “spur economic growth and job creation.” She also cited climate resilience and said that protecting New Hampshire’s forests and prioritizing the rural economy were top priorities for her in the Senate.

And Sen. Maggie Hassan called the Northern Forest Region “an essential part of our state’s economy and way of life.”

D’Amato said that in the current political environment there is broad support for addressing climate change. 

“It certainly helps empower this idea that we’re addressing climate change,” he said.

A big barrier to these efforts is that the nurseries in the region aren’t equipped with seedlings, which is largely because trees most often regenerate naturally. This not only affects adaptation efforts like assisted migration but also makes it more difficult to curb climate change through efforts like the Trillion Tree Initiative, which aims to plant trees as a way of storing carbon and removing it from the atmosphere.

“We really have a limited range of places where we can actually buy seedlings from,” D’Amato said.

“If people are going to get serious about planting trees both for carbon but more importantly for adaptation, you know, we really don’t have the current infrastructure to do that in terms of nursery capacity in the region,” he said.

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Amanda Gokee
Amanda Gokee

Amanda Gokee is the New Hampshire Bulletin’s energy and environment reporter. She previously reported on these issues at VTDigger. Amanda grew up in Vermont and is a graduate of Harvard University. She received her master’s degree in liberal studies, with a concentration in creative writing, from Dartmouth College. Her work has also appeared in the LA Review of Books and the Valley News.