As our climate heads south, trees can’t move north fast enough

At the edge of Thetford Center village stands a tree, tall and vigorous, but otherwise unremarkable. It’s been growing there quietly for more than half a century. Its leaves (now shed) are similar to ash, divided into seven or more leaflets on a slender midrib. Every few years the ground beneath the tree is carpeted in three-quarter-inch long, hard-shelled nuts. This is a bitternut hickory, cousin to the better-tasting and more desirable pecan that is, alas, not hardy this far north.

Hickory trees are at the northernmost edge of their range in southern Vermont, which is why they are not a common tree around here. But trees at the northern edge of their range have attracted the interest of biologists both at UVM and in the Landscape Change Research Group. The latter are assessing the vulnerability of our northern forests to the effects of climate change. They extrapolated future projections for climate into “low emissions” or “high emissions” scenarios to model how different tree species might fare in the future.

If you’re a balsam fir, black ash, or red pine the future doesn’t look good. These are among twelve species of northern forest tree that are predicted to have poor capacity to adapt to any climate change. The other species in this category are balsam poplar, black willow, eastern cottonwood, gray birch, mountain maple, pin cherry, tamrack, and striped maple. 

The Landscape Change Research Group projected the ability of various trees to establish and grow under two climate scenarios. The “low emissions” climate model predicts average warming of 1.9 to 3.2 degrees F relatively evenly across all seasons. The “high emissions” scenario predicts larger temperature increases with more than 6 degrees F of warming on average and 8.3 degrees F of warming in summer. In addition to warmer temperatures, there will be changes in precipitation, with the “low emissions” model predicting an increase of 0.9 inches and the “high emissions” model predicting 3 inches. Both models predict a precipitation increase in winter. The “high emissions” scenario also predicts wetter springs and drier summers by the end of the century. Models become less certain when it comes to extreme weather events like heat waves and intense storms, but it is clear there will be more of them. 

Some tree species will lose suitable habitat under both emissions scenarios and especially badly under high emissions. They are the above-mentioned balsam fir, black ash, balsam poplar, etc. High emissions will also be significantly inhospitable for quaking aspen, yellow birch, mountain ash, choke cherry, and striped maple. The beloved sugar maple will see a moderate decrease in suitable habitat by the end of the century under the high emissions model. Many other species, however, will experience an increase in suitable habitat until the end of the century under both emissions scenarios, while another group that includes the bitternut hickory will get more habitat only with the high emissions scenario.

Thus the bitternut hickory is likely to increase its numbers and spread northward. However, as it is not numerous, other trees that are both adaptable and abundant will likely outpace it. There probably won’t be Vermont hickory forests in the future without human intervention. 

There’s another group of trees that don’t currently grow this far north to any extent but will have the opportunity to exploit new habitat here in a warming climate. These include bald cypress, native persimmon, native pawpaw, osage orange, loblolly pine, shortleaf, pine, and sweetgum to name a few. 

How fast can trees move? From historical data, researchers at University of Vermont estimate trees in New England can disperse their seeds about one-tenth of a mile per year, maybe three-tenths for long-distance dispersers. 

But the northward march of climate change is happening at four to six miles per year. “The trees can’t keep pace,” says Steve Keller, a professor in UVM’s Plant Biology department and an expert on tree genetics. “Climate change is already causing stress. I'm talking about reduced growth, reduced carbon sequestration, more susceptibility to extreme events like drought and heat waves, less ability to fend off pests,” he says. “Our local forests will become more and more maladapted if we don’t do anything. So how do we help?”

A growing number of foresters and ecologists think that “forest assisted migration” can play a role, or at least help to hedge our bets. This approach consists of moving seeds of more southerly trees, or even their seedlings, to northern forests. The long-held tradition of just letting the forest grow back, for instance after logging, may soon not be viable with climate change. “It may become useful, even necessary, for landowners, timber companies, and conservation land trusts to start thinking about how they will introduce genes, trees, and even whole suites of species from farther afield that can keep forests healthy — or even forests at all.”  

Indeed, indigenous peoples cultivated trees valued for food such as oaks, butternut, and chestnuts and assisted their northward migrations. However UVM plant biologist Jane Molofsky cautioned, “Trees have a range for a reason. And it often has to do with the soil. There's a lot of interconnections between species, especially in the mycorrhizal (fungal) associations that occur under the plants. Unless you're migrating with your fungi it may not be successful.”

Indeed, if we waited 300 years Vermont could gain ten to twenty new species of climate-adapted trees. But changes that used to happen over hundreds or thousands of years are now happening in decades. UVM researcher Peter Clark believes that if we don’t explore some radical ideas to adapt our forests “we may be putting them at greater risk. Assisted migration is focused on keeping forests as functioning healthy forests.”

UVM biologists have included bitternut hickory in their assisted migration experiments, moving seedlings to a research forest in Jericho, Vermont. So far this species hasn’t taken off, but another tree species from southern New England, the black birch, appears to be thriving in its new location. 

Photo credit: Li Shen