NATURE’S VIEW

Attracted by the habitat and food they provide, the author experimented with planting black walnut trees on his woodlot in Sutton with varying degrees of success.

Imagining a Future Forest While Experimenting with Assisted Migration

By Dave Anderson

Over the past couple of years, I have collected a few dozen black walnuts from the Forest Society’s Creek Farm Reservation in Portsmouth and planted them on our family woodlot in Sutton. In this experiment with assisted migration—moving a temperate hardwood tree north, but still within its natural range stretching into northern New England and Canada—the walnuts leap-frogged some 75 miles from a former seaside estate to the rugged foothills of the Sunapee-Kearsarge region, 1,000 feet above sea level.

Why plant walnut trees in the first place? Because they will grow here, produce shade and valuable lumber, and provide nutritious, edible nuts for wildlife. I have spotted relict American chestnut trees sprouting from blight-resistant 100year-old roots on my woodlot. In the early 1900s, a chestnut blight killed billions of native chestnuts growing in the eastern United States. The loss of the chestnut tree negatively impacted wildlife living in this country’s original hardwood forest. Animals rely on nuts and seeds for sustenance, and when a food source disappears, it may take a while for nature to backfill that void. Hence, one motivation to plant walnut trees.

To begin my walnut-planting foray, I removed the husks and dropped them in a bucket of water to determine their viability. I discarded the floaters and kept the sinkers. Walnuts germinate best after an exposure to cold temperatures for 75–125 days in a process called stratification. Some people stratify walnuts for planting in a refrigerator over winter. It’s easier to direct sow them in late autumn and have nature do the chilling, but squirrels are the issue. These foraging phenoms consumed 100 percent of the first round of walnuts I tried planting. I even caught squirrels inside my garage plucking walnuts from a 5-gallon bucket of water as if they were bobbing for apples at Halloween. To thwart their walnut thievery, I tried pre-sprouting the nuts in springtime, planting them in 4-inch-deep pots. My plan almost worked, though just a measly 2 percent germinated because I hadn’t checked the viability of the walnuts and the pots I used should have been at least 6- to 8-inches deep to accommodate the taproot. To add insult to injury, the squirrels also dug them from the pots in the summer. It was another epic fail.

This past autumn, I acquired 1,000 walnuts from a property manager who collected them from lawns to prevent damaging his lawnmower’s mulching blades. I peered into one of the 50-gallon trash barrels containing an estimated 500 walnuts. The tart, citrus-scented walnut husks oozed a brown liquid that stains skin and clothing. The husks contain the chemical juglone, which inhibits the growth of plants looking to set up shop underneath a walnut tree or within their root zone. It’s a classic example of allelopathy: chemical warfare to control competition. Save yourself some time and grief by not composting the walnut husks in compost destined for

vegetable or flower gardens. The brown liquid ooze acts like a broad-based herbicide that will wreak havoc on your veggies.

I planted these walnuts before snowfall in November using a heavy stone bar to auger holes in the soil and widen each to create a pocket. After hitting too many rocks, I began simply tossing and scuffing walnuts under the red oak leaves in sunny openings amid old logging slash for concealment. Even if squirrels and mice consume 95 percent of the walnuts before spring germination, I thought my effort might generate 50 seedlings. Or maybe that’s just wishful thinking?

The “wishful” part is envisioning the enhancement of tree species composition in a future forest in this era of rapidly changing climate and extreme weather. Some climate warming scenarios suggest New Hampshire will have the same climate as present-day Georgia. Climate change is already resulting in warmer, rainier winters and hotter, drought-prone summers punctuated by extreme rain and flooding events. With this in mind, does it make sense to encourage temperate zone flora by planting trees that formerly reached a northern range limit south of New Hampshire? I could probably go even further with my assisted migration experiment by planting a few more Central Appalachian trees and shrubs that are growing successfully in southern New Hampshire, including bitternut hickory, shagbark hickory, sassafras, red mulberry, and possibly tulip poplar. By increasing tree diversity, could it help our woodlot adapt more quickly to climate change impacts?

Black walnut, shagbark hickory, and sassafras are native eastern forest tree species. I planted native walnut tree nuts a little further north of their existing natural range. A cautionary tale: it is worth considering the potential downsides of planting haphazardly. The planting of ornamental trees and shrubs as part of horticultural experiments have introduced some pervasive non-native plant species, which have escaped to colonize New Hampshire forests. Examples include Norway maple, autumn olive, winged euonymus (aka burning bush), bittersweet, barberry, Japanese knotweed, and more. Imported nursery stock may have contributed to the more rapid spread of hemlock wooly adelgid and hemlock elongate scale diseases in the state.

Forest Society Field Forester Gabe Roxby clarifies that tree planting is not necessarily building forest resilience but more an example of transition, a term used in a framework developed by the Northern Institute of Applied Climate Science. “Assisted migration—planting trees outside their current ranges—is an example of a climate adaptation strategy called transition. Management techniques that fall into this strategy seek to actively alter forest composition to make forests better adapted to predicted future climate conditions, ” Roxby says. “Two other climate adaptation strategies are resistance and resilience. Management techniques to promote resistance are those that make forests better able to stay the way they are, resisting occurring changes to stay in their current state. Resilience strategies are those which seek to make forests better able to withstand future climate stress and perhaps be changed by them, but are resilient enough to return to their original tree species composition and ecosystem function. ”

“Suitable habitat for given tree species will move northward. Whether the trees themselves move northward is uncertain, ” Roxby says. “Individual trees at the very northern edge of their range become relatively more important. They are the most cold-hardy genotypes of their species and can be expected to do better as the climate warms. ” Changing forest habitats could also host a changing suite of mammals, reptiles, amphibians, fish, and birds. Conservation biologist Chris Martin at the Audubon Society of New Hampshire says bird range expansions are already underway. “As more temperate vegetation gradually moves in, it will create conditions that host more Appalachian species, ” he says. in central New Hampshire in the last few years. That’s new! And black vultures are appearing with greater frequency, too. ” Martin says it makes sense to look at permanent residents rather than the strongly migratory birds. “For example, up in Pittsburg and Errol, our Christmas Bird Counts have only begun to find wild turkey flocks, including hens with young, in the last 15 years. In the North Country, there are examples of new surprise residents that have moved in over the last 10 years, including wintering red-bellied woodpeckers and tufted titmouse. Thanks to milder conditions, one mid-state bird that has increased in numbers [up north] over winter is the Carolina Wren, a vocal species folks might readily recognize by call. ”

As I planted the walnuts on my woodlot, I saw myself in the role of a seed-dispersal vector, a human equivalent of the mice, squirrels, and jays that originally moved the nut-bearing trees north. The hardwood forest we might recognize growing hickory, butternut, chestnut, oak, and beech arrived relatively late during the forest colonization of the post-glacial New England landscape. The oak-hickory forest type of the central Appalachian region appeared in the southern New England pollen record approximately 4,000 years ago compared to the boreal tree species, including willow, birch, spruce, and fir, which established themselves 9,000 years ago. These nutbearing trees moved northward relatively quickly as birds and mammals transported nuts faster and further.

Pondering our warming planet, receding glaciers, and my opportunities to plant a few trees, I vote for growing more trees that will produce fruit, seeds, and nuts. I’m sure my resident squirrels will agree. Perhaps establishing walnut trees is merely “playing squirrel” in a warming forest landscape. More accurately, it is my attempt to find some thin silver lining in the dark thundercloud of predicted negative climate change impacts.