Growing the Forests of the Future

While the epic migrations of salmon, humpback whales, and wildebeests are world-renowned, animals aren't the only species on the move. Trees, it turns out, can migrate too. Of course, an individual tree can't pick up its roots and travel south for the winter, but trees do move—in their own way. 

Katie Nigro, a postdoctoral fellow with the Oak Ridge Institute for Science and Education (ORISE), explained that trees expand their range in response to environmental changes. If the climate gets warmer, a tree species may no longer be able to grow in the southernmost parts of its range, but, through wind or animal-dispersed seeds, it can expand farther north over time into areas that were previously too cold.

However, that process might be too slow for human-caused climate change. To keep pace, projections suggest, seeds would need to be dispersed at least 10 times faster than their historic migration rates of about 20 to 40 kilometers (12 to 25 miles) per century. Given the quickening rate of environmental changes and the slow rate of tree growth, researchers believe trees will struggle to adapt.

To help, scientists are taking seeds from habitats they expect will no longer be suitable to a species and planting them in climates that may favor their growth in the coming century. For example, an ecosystem in Colorado that is currently ideal for a Douglas Fir could become too warm to sustain it in a few decades, but that tree might thrive somewhere farther north. 

Assisted migration is about planning for the future, explained Mike Battaglia, a research forester with the US Forest Service. His work in silviculture—the growth and development of forests—involves figuring out which species to plant in reforestation efforts. 

“Trees live a long time and take a long time to get to reproductive age,” said Battaglia. Because they have such long lifespans, trees have less opportunity than faster-growing plants and animals to evolve and naturally select for the current climate. So, considering the climate of the future is important when choosing what to plant today. 

The research station where Battaglia works sits along the base of the Rocky Mountain foothills, a semi-arid climate home to Ponderosa pine and Colorado blue spruce. While not very far away from one another, a Ponderosa near his station will be adapted to different conditions than another tree of the same species growing only 20 miles away up in the mountains. Foresters like Battaglia and Nigro use climate models to foresee the likely future conditions of a location. The models predict factors such as temperature, humidity, and frequency of natural disasters. They then match the location with the seeds of trees—sometimes even within the same species—currently adapted to those climatic conditions in other places. 

There are many different factors to consider besides temperature, though. Reduced snowpack in the mountains will mean drier summers and longer growing seasons; more frequent fires will mean that silviculturalists need to consider fuel loads on the forest floor and the ability of trees to regenerate after fire. The goal is to help the ecosystem withstand such disturbances while still meeting the desired outcome for that forest, such as reforestation, climate adaptation, or even logging. 

The seedings selected for those future conditions are usually planted during reforestation efforts after a disturbance, such as wildfire, an insect or disease outbreak, or the mechanical harvest of trees. Robert Slesak, a research forester working with the Pacific Northwest Research Station in Olympia, Washington, said that from a practical standpoint, it's a good practice to do this climate-adaptive planting when reforesting is happening anyway. He works on a project across research stations called the Experimental Network for Assisted Migration and Establishment Silviculture (ENAMES) that incorporates climate adaptation into reforestation. 

“So much land is forested, and the amount that gets reforested every year is so small, and that’s why we push it,” said Slesak. “It’s kind of a no-brainer to just at least do what you can on the ground.” 

There are at least three forms of assisted migration, and they all come with risks. The foresters with ENAMES are mostly using assisted population migration to move trees. In other words, they’re taking a species’ seeds genetically adapted to warmer environments, for example, and planting them in areas expected to get warmer. Another form of tree movement includes assisted range expansion, which is intended to mimic the incremental movement that a species might make on its own. Using this strategy, researchers plant seedlings just outside of their current range. 

The “riskiest” form is generally considered assisted species migration, which entails moving plant species really far away from their current range. A recent example is the introduction of California’s coastal redwoods and giant sequoias to Washington state. Proponents of the idea hope the effort will help save the species from possible extinction as their already-limited range contracts. With range expansion and species migration, on the other hand, there is the risk of adding bad genetics to a local population or introducing pests and disease. However, Slesak noted that there is also a risk in doing nothing. 

Another program aimed at learning more about how trees shift their habitat is Adaptive Silviculture for Climate Change (ASCC). It includes a network of research installations that allow researchers like Nigro and Battaglia to measure how trees are responding to climatic changes. Crucially, the research also includes local land managers and other stakeholders. In total, there are 14 study sites across the US and Canada, all of which help researchers learn what reforestation tactics suit tree migration best. In doing so, they hope to uncover different options for land management and study how the trees interact with droughts, wildfires, and other environmental conditions.

The broader aim of ASCC is to help forest managers decide what route to take with their own lands. "It really comes down to what the managers or what the community values in the landscape,” said Nigro. “There's a lot of opportunity to decide what you want the next forest to look like.” 

Managers might want trees that grow the tallest, produce the most cones, or are the best-surviving in the face of disturbances. After learning what works and what doesn’t at these installations, foresters can create new management plans accordingly for forests that sustain themselves in the long term. Slesak noted, however, that there are still significant barriers to implementing these practices on a broader scale, especially for private landowners who might not have access to the necessary seeds or other resources.

While assisted migration is seen pretty favorably today by foresters, it wasn’t always. Battaglia said that when ASCC began work on their installation in Colorado’s San Juan National Forest more than a decade ago, “there was no stomach to do assisted migration. Fast forward five years even, and the conversation has changed.” 

Practices that worked 20 years ago, such as replating the same trees in the same location, are no longer successful and have led to reforestation failures. Consequently, foresters are warming up to newer reforestation techniques, including assisted migration. “Local is best” has long been the dominant seeding mentality among foresters, but as local conditions change, that long-held belief must as well.

“There's a tremendous amount of uncertainty,” Slesak said. “One of the things with forestry is we have to make decisions now that are going to have implications 50 or 100 years down the road.”