Food for Thought: Assisted Migration in an Age of Climate Change

The sugar maple (Acer saccharum), one example of tree species potentially vulnerable to climate change.


By Thomas Christopher

It is impossible to be an active gardener and not notice changes occurring in our local habitat. Chief among these is the on-going warming of our climate. In the northeast, cities from Portland, Maine to Hartford, Connecticut reported record-breaking heat last July, and Boston has experienced 6 of its ten warmest-ever months in the last decade. Dr. Bethany Bradley, who spent her childhood in Massachusetts and returned ten years ago to teach ecology at the University of Massachusetts Amherst, has a more personal observation. Fall and cooler weather used to start at Labor Day when she was a girl; now, she adds, it seems to kick off at Columbus Day.

Dr. Bradley has been researching plant communities’ reactions to climate change and on November 10th, she will be speaking about one possible response to a room-full of gardeners at Rooted in Place, the Berkshire Botanical Garden’s annual ecological gardening symposium. She granted this writer a preview, leaving him with plenty to think about.

By the middle of this century – in just thirty years – Dr. Bradley noted, climate change is predicted by to make Massachusetts’ climate more like that of present-day Maryland. This will have a profound impact on the local flora and fauna.  Indeed, some sensitive species are already responding the warming trend. I mentioned that I’ve read studies finding that sugar maples are not thriving as the weather turns hotter. They may be subjects, Dr. Bradley responded, for “assisted migration.”

What is assisted migration? It’s a process by which plants and animals are moved from more southerly areas northward or upward in altitude in the expectation that the areas receiving them will become warmer and hence prove suitable homes for the southerners in the future.

Why bother? This is intended as a proactive response to problems, such as localized extinction of sugar maples, (Acer saccharum) that climate warming may cause. Such losses are likely to tear big gaps in local plant and animal communities, and if no action is taken, invasive and undesirable plants may move in to fill the vacated ecological niches.

Certainly, the plant and animal life of our future fields and forests is going to be different from what we find around us now if the climate continues to change. Assisted migration seeks to sow seeds, metaphorically and literally, of native North American plants and animals that will be better adapted to a warmer future.

Dr. Bradley is a scientist, not an advocate, and she was quick to note that proposals for assisted migration are controversial in the ecological community. This is because the local ecology responds to changes in ways that are often difficult to predict.  We cannot be sure that the plants we introduce may not prove too successful, perhaps crowding out some of what we would like to preserve. In the past, this has often occurred with exotic species introduced by gardeners, such as Japanese knotweed and Japanese barberry.

It’s not impossible that something similar could happen with natives introduced from more southern regions. In fact, black locust (Robinia pseudoacacia), a tree thought to be native from Pennsylvania southward, has escaped from landscape plantings to become something of a pest in central New England and upstate New York.

“Do no harm,” the motto of the medical profession, has been quoted by ecologists opposing assisted migration.  However, by warming the climate, humanity is already doing harm to local ecosystems and Dr. Bradley proposes that a well-thought-out program of assisted migration would be likely to yield more benefits than problems.  Fast-spreading species such as black locust, and those that respond enthusiastically to disturbance of the habitat, could be avoided.

Anyway, she adds, government agencies are already experimenting with assisted migration of plants on a modest scale. When the Forest Service revegetates western national forests after wildfires, it commonly plants the pre-existing tree species, but gathers seed from the southern, warmer end of the trees’ ranges.

There’s a role in this for responsible gardeners.  When you plant, Dr. Bradley urges, consider that you may be sowing the seed for the future of surrounding wildlands. Vote with your dollars: select native species at the nursery and inquire wherein their native ranges the plants came from.

For more information, come hear Dr. Bradley and the other experts speak about ecological gardening at Rooted in Place, from 9:00 am to 5:00 pm on November 10th, at the Berkshire School in Sheffield, MA. Admission is $95 for Berkshire Botanical Garden members, $110 for non-members. Registration includes lunch and refreshments. More information at

  Thomas Christopher is the co-author of “Garden Revolution” (Timber Press, 2016) and is a volunteer at Berkshire Botanical Garden. Be-a-Better-Gardener is a community service of Berkshire Botanical Garden, one of the nation’s oldest botanical gardens in Stockbridge, MA. Its mission to provide knowledge of gardening and the environment through 25 display gardens and a diverse range of classes informs and inspires thousands of students and visitors on horticultural topics every year.  Thomas Christopher is the co-author of Garden Revolution (Timber press, 2016) and is a volunteer at Berkshire Botanical Garden.