In many ways, planting a tree is an act of faith and hope. As we firm the soil around the tree’s roots, we may imagine a future in which generations to come will picnic in its shade. They may gather its fruit or colorful leaves, and think kindly of us. So, of course, we want to choose the right kind of tree.
These days we would do well to consider more than beauty, soil, water, sunlight and resistance to disease and insects — we should also think about our changing climate.
Weather records show that average air temperatures in New England rose by 2.4 degrees Fahrenheit between 1901 and 2011. More important for plant survival, average winter temperatures rose in our region by 4.2 degrees.
One might question the accuracy and relevance of instrumental measurements and temperature averages. Yet plant phenology in New England shows that something is indeed different.
Autumn comes a little later. The green canopy duration of trees at Hubbard Brook Experimental Forest in New Hampshire increased about 10 days during a 14-year period. And spring arrives a lot earlier. Henry David Thoreau noted that highbush blueberries began blooming in Concord on May 11 in 1853. Last year, blueberry bushes began blooming on April 1.
Average temperatures are higher, frost comes later, complete winter snow cover is less frequent and spring begins sooner. Longer growing seasons may sound great for gardeners and farmers, but the extreme scenario also comes with a greater chance of drought, certainly more heat stress, and a fair chance of increased insect and disease problems. What kinds of trees are going to make it through? How might we discover what types of trees will adapt and thrive in a changing climate?
Here’s one way get a sense of that: Walk across the Holyoke Range from north to south. On the Amherst side, you’ll find tree communities adapted to colder, wetter climates dominated by northern hardwoods and hemlocks, common a hundred miles or so north of here. A warmer climate is not likely to be favorable for these trees. The hotter and drier southern side calls to mind the oak-hickory forests of southern Connecticut and New Jersey.
For a more detailed analytical approach, I recommend the Climate Change Atlas developed by the U.S. Forest Service. It provides regional summaries and detailed pages on the climate response of 151 kinds of Eastern trees.
The data for the atlas come from measurements of millions of trees on thousands of plots throughout the lower 48 states, re-measured every 10 years beginning in 1930. By correlating temperature, precipitation, soil conditions and elevation of plots with the growth of individual trees, researchers have created response curves that are remarkably detailed for the more common species.
The atlas data suggest that trees with significant range to the south and already growing here may do fine. Red and sugar maple, northern red oak and white oak — now extending down into the Smokies and the hills of northern Alabama, Georgia and Arkansas — are adaptable enough to tolerate the cooler end of the extreme scenario and should hold their own.
Winters may not be cold enough for maple sap to run well enough for sugaring before bud break, but maples will persist. Those species with a predominantly southern distribution, but with some populations in our area, will probably increase in frequency, such as tupelo (black gum), sweet gum, sassafras, black locust, basswood and tulip poplar. Other species in our area, but at the southern end of their current range — such as hemlock, red and white pine, spruces, larch, serviceberry and mountain ash — will probably only persist much farther north.
But there’s a problem. We can’t just consider the conditions now and even likely changes when choosing a tree to plant. Our chosen tree must survive and (we hope) thrive even as temperatures rise and our Plant Hardiness Zone changes from 5b down here in the Valley, to as warm as zone 7 or even 8 on some southern slopes.
Persimmons and pecans might well thrive in the 2090s, but they almost certainly will be set back by the frost in the 2020s. It’s also likely that individual trees, grown from seed sources in a specific region, will be locally adapted, even though the species as a whole covers a broad climactic range.
I’m afraid we can’t escape the uncertainty of both climate change and tree response. So plant the trees you love best and leave a time capsule explaining your choices. Future tree lovers will understand.
Lawrence J. Winship is emeritus professor of botany at Hampshire College and a former board member of the Hitchcock Center for the Environment. The Climate Atlas may be found at www.fs.fed.us/nrs/atlas/tree.
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Earth Matters, written by staff and associates of the Hitchcock Center for the Environment at 845 West St., Amherst, appears every other week in the Daily Hampshire Gazette. For more information, call 413-256-6006, or write to us.