Assisted Migration: Facing Climate Change in Our Own Backyards

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From the very beginning of the world, the other species were a lifeboat for the people. Now, we must be theirs.
— Robin Wall Kimmerer, 'Braiding Sweetgrass'

It can be hard, I think, to have hope in a time of climate change and mass extinction. Oftentimes, seeing the diversity of trees that grow in my West Philadelphia neighborhood is what gives me more hope than anything else for the future. Like most people, for years I did not know or care about the urban forest stretching across the parks, streets, and gardens of my city. I only saw an indiscernible green wall, and like most I did not realize that these very trees and gardens can be one of the most effective tools in the fight against climate change. But these days, when I see the bald cypress, pawpaws, and willow oaks that people have planted in their yards and hell strips, I see trees that can survive climates as hot as those of Mexico and Florida. And in those moments I have hope because I know that we can prepare our communities, cities, and forests for climate change ourselves - that we don’t have to wait for our government or institutions to act. And moreover, we can create stunningly beautiful and fruitful landscapes in the process of preparing for climate change - even in the smallest or most challenging sites. Every gardener can be a guardian for the future, every garden a sanctuary for life, and every farm a forest of abundance. This blog will show you how to accomplish this at any scale in eastern North America, whether your growing space is as small as an apartment balcony or as large as a forest.

How bad could it get?

As climate change projections become increasingly dire, it is now apparent that global ecological systems are changing far more rapidly than our institutions are prepared for. Recent studies have found that mass extinction, too, is occurring at far faster rates than previously understood. Rapid urbanization and development continues to fragment the few natural areas that are left, rendering less than 2% of eastern North America sufficiently connected for species to be able to migrate in response to climate change.

To face these problems, an increasing number of ecologists and institutions, including the U.S. Forest Service, recommend Assisted Migration as one of the best means of proactively mitigating mass extinctions - particularly of native trees.

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What is Assisted Migration?

Assisted migration is the act of expanding the range of native species and communities northward faster than they could migrate naturally in response to climate change. Any gardener can become a guardian for species endangered by climate change and habitat loss.  While this is a new idea, there are many examples of humans successfully introducing native flora and fauna to new environments for other reasons.

Grey squirrels were introduced to urban areas in the mid-to-late 19th century, before which they were almost never seen in North American cities.  They are now a vital part of urban ecosystems, being one of the most effective dispersers of native trees. Umbrella magnolia (Magnolia tripetala) is another famous example of a southern native tree introduced far north of its range by horticulturists, and subsequently becoming naturalized in nearby native woodland. Cold hardy selections of American persimmon (Diospyros virginiana) were bred to grow in New Hampshire - 200 miles north of their northeastern most limit - by Dr. Elwyn Meader in the late 20th century. Prior to colonization the indigenous peoples of North America, more than anyone else, expanded the range of many native species - particularly fruit and nut trees.

Assisted migration is not only possible, it has already been achieved for some species, and a variety of methods exist for accomplishing it. Most recently, the Torreya Guardians project has been at the forefront of this movement in the U.S., and have compiled an incredible collection of resources for assisted migration. Two of its founding members - Connie Barlow and the late Paul S. Martin - have drafted a set of standards for assisted migration of threatened species, which includes criteria vital in choosing the best candidates. Plants like Franklin tree (Franklinia alatamaha), Florida torreya (Torreya taxifolia), and others meet multiple criteria, and will require assisted migration to ensure their survival.

 A  Franklinia  tree - one of the rarest woody species in North America - grows in this highly urbanized West Philadelphia garden far north of its former native range.

A Franklinia tree - one of the rarest woody species in North America - grows in this highly urbanized West Philadelphia garden far north of its former native range.

Other southern native species may not be threatened, but in fact will likely thrive far north of their current range in future climate conditions. These robust trees - like bald cypress (Taxodium distichum), pecan (Carya illinoinensis), southern magnolia (Magnolia grandiflora), and others - are also vital to plant in the Northeast and Mid-Atlantic now, as they could become the foundations of future forest communities there. Similarly, Deep Southern ecotypes of northern species like American beech (Fagus grandifolia) and sugar maple (Acer saccharum) can be planted in the North to introduce genetics more tolerant of extreme heat. Oftentimes cities are the ideal habitat for these southern plants. The 'urban heat island effect' makes cities up to 4°F warmer than the surrounding region, allowing less hardy plants to thrive far north of their native range.

 Bald cypress ( Taxodium distichum ) can grow well in even the most challenging sites, and is highly resistant to breakage in high winds and hurricanes

Bald cypress (Taxodium distichum) can grow well in even the most challenging sites, and is highly resistant to breakage in high winds and hurricanes

Peopled landscapes are the main barrier to native species moving northward, but they can also be where those plants are saved. University of Delaware professor Doug Tallamy has estimated that 95% of all land in the lower 48 is now human altered habitat between cities, suburbs, and farms - with 40 million acres of it lawn alone. If even a handful of gardeners on every street planted native species north of their range, any given neighborhood or backyard could become a Noah’s Ark for biodiversity. We can shepherd plants north, and make space for them even in our cities. Whether you have a balcony planter, a small community garden plot, or a whole forest - the plants you grow today can help create resilient and diverse forests for tomorrow.

Our gardens can be nature preserves - sanctuaries for native flora - in an increasingly volatile world. Just as classical Greek and Roman texts were preserved in the libraries of Islamic scholars and Irish monks in the Early Middle Ages, our gardens, parks, and farms can become places that preserve irreplaceable species today, and ensure the future of biodiversity in eastern North America.