Traveling trees: Assisted migration for climate resilience


Leslie Brandt, Forest Service climate adaptation specialist, stands in front of an adaptive forest worksite planted with cottonwood seedlings from trees adapted to warmer climate conditions. Her team is checking whether these trees are more resilient to the changing climate conditions there. (Photo courtesy of Leslie Brandt).

Wandering trees look like freaks from the sci-fi or fantasy genres – like JRR Tolkien’s Ents or Groot from Marvel’s Guardians of the Galaxy. But tree species do migrate in their own way, although it may take decades or centuries.

“Natural migration is very slow for most tree species,” said Leslie Brandt, a climate adaptation specialist with the USDA Forest Service. “They just don’t have a very large seed dispersal distance, and they just can’t keep up with the rapid pace of climate change.”

The progressive and predicted climate changes exceed the natural ability of many tree species to migrate and adapt. This can result in impacts of climate-related dieback on the entire forest and landscape.

When North American native trees don’t fill the space, it creates even greater problems as invasive tree species can quickly take over. Or the ecosystem could change rapidly, like from hardwood to pine forest or from pine forest to prairie. These dramatic changes can have detrimental effects on wildlife and people who depend on these landscapes to survive and thrive.

This is where assisted migration comes in. Assisted migration is the human-assisted movement of populations or species in response to climate change and could be a proactive, pragmatic tool for building climate resilience in our landscapes.

Assisted Migration 101

An illustration demonstrating the three types of assisted migration.  Assisted population migration, assisted range expansion, and assisted species migration.

This graphic shows each type of assisted migration using the dark green conifers as an example; The three types of assisted migration are applicable to all systems. Different populations are represented by individual gray areas. Orange arrows represent human-assisted movement of plant matter from the dark green conifers to new sites. The historical climate bar on the left shows the movement of plant matter from warmer, drier climates (red) to historically cooler (blue), wetter climates. Graphic first published in USDA Forest Service Seedlot Selection Tool Guidebook.

“Historically, humans have moved species around the globe for centuries, if not millennia. This is nothing new for forestry,” said Brandt. “What’s new is the intentional use of climate data to inform these decisions.”

There are three types of assisted migration: assisted population expansion, assisted range expansion, and assisted species migration.

Assisted population migration involves tree movement to new sites within their established historical range, typically as a seed or seedling. For assisted population migration, researchers are collecting seeds from a slightly lower elevation or latitude of trees of the same species that likely have slightly different genetic material that makes them adapted to warmer conditions.

Assisted range extension is the movement of trees from their current range to suitable areas just outside their current range.

Forest Service scientists are also researching assisted tree movement beyond range extension to higher elevations and latitudes that it is unlikely to reach without human intervention in the near future. This movement of trees beyond their historical species range and typical range expansion is referred to as assisted species migration.

“Assisted migration of species can be a bit more controversial, but many scientists argue that it is necessary given the speed of changes we are seeing in the landscape, particularly due to exotic insects and diseases. For rare or threatened species, assisted migration may be the only way to save a species,” Brandt said.

Assisted migration in action

Three Forest Service volunteers implement assisted tree migration while planting young trees at an adaptive silviculture site in Saint Paul, Minnesota.

Forest Service volunteers implement assisted tree migration while planting young trees at an adaptive silviculture site in Saint Paul, Minnesota. This project aims to restore the canopy after the site’s ash trees died from emerald ash borer infestation and threatened to take over invasive plant species. (USDA Forest Service photo by Leslie Brandt).

When tree species are declining in an area, it is sometimes advisable to incorporate additional species to fill that space. In these cases, rangers look for North American species that would still fit in that ecosystem but may not have been native to that particular location in the past.

For example, a wooded area in Saint Paul, Minnesota suffered extensive canopy loss due to the emerald ash borer. The emerald ash borer is a non-native invasive insect species that is wiping out North American ash trees.

“When these ash trees die, we don’t see any natural regeneration of other tree species,” Brandt said. “Invasive grasses and herbaceous species are crowding out native trees.”

To restore the treetops, Brandt’s team planted 19 different tree species. Some of these species, such as silver maple, are native to the area. Other species planted there — such as sycamore and tulip poplar — come from warmer environments in southern Minnesota, Missouri, and Illinois. These are examples of all three types of assisted migration.

“We hope to establish a canopy that is both resistant to the emerald ash borer because we don’t plant ash and includes other species native to the Mississippi Basin,” Brandt said.

The aim is to build up the forest ecosystem in such a way that it can be maintained over time as a result of the predicted climate change.

“In the forest service we speak of forestry-supported migration,” said Brandt. “Our work is about sustaining ecosystem functioning over the long term to provide services that people rely on — like timber products, clean water, wildlife habitat, recreational opportunities — without fundamentally changing the nature of that system.”

Creation of an assisted migration plan

An illustration depicting assisted tree migration with five trees carrying baggage and arranged by height, with the smallest on the left and the tallest on the right.This cartoon depicts assisted tree migration, a practice being evaluated by the Forest Service and its partners as a way to improve forest resilience to climate change. (Forest Service illustration by Savannah Halleaux).

The Forest Service released its reforestation strategy in July 2022 with a vision to “grow and nurture resilient forests for tomorrow”. The strategy describes the need for adequate reforestation in the face of climate change to ensure the development of healthy, resilient forests of the future. Assisted migration is a reforestation tool to ensure the right seedling is planted in the right place at the right time, the right species and in appropriate scales.

The agency’s Office of Sustainability and Climate recently convened the Forestry Assisted Migration Technical Assistance Team to develop tools and resources to help managers decide if, when, where and how to facilitate the assisted migration of tree species to the Forest Service managed areas is to be implemented.

With support from the Forest Service and its Office of Sustainability and Climate, the Superior National Forest of northern Minnesota is developing an assisted migration plan for its landscapes in collaboration with over 100 tribal, state and local government representatives, and partner organizations. When completed, this plan will serve as a pilot project for the National Forestry Assisted Migration Technical Assistance Team to test methodologies and processes to develop field assisted migration plans for other forests across the country.

The forest service also participates in the Adaptive Silviculture for Climate Change Network, a collaboration of scientists, land managers and key partners that is exploring how planning and adaptation to climate change can be integrated into forest planning and management.

The goal is to understand where and how assisted migration can help maintain healthy, thriving landscapes across the country.

“We find that some tree populations alone cannot keep pace with changing climate patterns,” Brandt said. “With assisted migration, we’re just helping the trees out somehow.”