Be a Better Gardener: Food for thought: Assisted migration in an age of climate change

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 six of its 10 warmest-ever months in the last decade.

Dr. Bethany Bradley, who spent her childhood in Massachusetts and returned 10 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 around Columbus Day.

Dr. Bradley has been researching plant communities’ reactions to climate change and on Nov. 10, 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 me a preview, leaving me with plenty to think about.

By the middle of this century — in just 30 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 to 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 where in 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 a.m.-5 p.m. Nov. 10 at the Berkshire School in Sheffield, Mass. Admission is $95 for Berkshire Botanical Garden members; $110 for nonmembers. Registration includes lunch and refreshments. More information at berkshirebotanical.org.

Be-a-Better-Gardener is a community service of Berkshire Botanical Garden located in Stockbridge, Mass. Its mission to provide knowledge of gardening and the environment through a diverse range of classes and programs both informs and inspires thousands of students and visitors each year. Thomas Christopher is a volunteer at Berkshire Botanical Garden and is the author or co-author of more than a dozen books, including Nature into Art, The Gardens of Wave Hill (Timber press, 2019). His companion broadcast to this column, Growing Greener, streams on WESUFM.org.