The Quest for Shade

Maine, it appears, is no longer exempt from the warming patterns that made five of the last seven years the hottest on record.

Two summers ago, I wrote about staying cool without air conditioning but failed to mention the importance of trees, which offer a tonic of shade in the dog days of summer.

Perhaps that’s because our home only has aspiring shade, small ovals on the yard cast by two sugar maples we planted 12 years ago as bare-root saplings. We placed the slender sticks to the west of our house, the optimal side to shade windows from the beating afternoon sun. One tree will soon crest the roofline, but we’re not yet gaining the full benefit they may offer in coming decades.

Well-positioned trees can reduce household energy consumption by 25 percent, cutting cooling costs in summer and heating costs in winter. The Department of Energy estimates the annual savings for three well-sited trees at $100 to $250.

Trees that offer shade make good neighbors, there when you need them but not interfering when you don’t. In winter, their bare outlines don’t obstruct the view and let the long, low light in to warm and brighten the house. In summer, especially in urban settings where abundant pavement soaks up heat, they can lower ambient temperatures by 20° F. or more at peak temperatures, according to the US Environmental Protection Agency.

Trees further moderate hot spells through the evaporative cooling of water vapor released through their leaves. A single tree offers cooling equivalent to 10 room-size air conditioners operating 20 hours/day. But instead of generating carbon dioxide emissions like those appliances, trees absorb and store carbon.

Trees also help buffer against the deluges associated with climate change. Their leaf canopies protect underlying soils and minimize erosion while their roots help soil absorb and hold rainwater. The Forest Service suggests that a single tree can prevent 4,000 gallons of stormwater runoff per year.

To reap all these arboreal benefits, assess your site for the best place to locate new trees. University of Maine horticulturalist Brad Libby suggests starting with a soil test. Make sure the planned site is far enough from your home to shade it when the tree is mature but not to overhang the roof. It is easy to underestimate a tree’s mature size.

Consider the microclimate that the tree will experience, but that’s no longer enough; now you must also factor in the climate writ large and how it’s changing. In 2012, the USDA shifted most of coastal Maine from Zone 5A (based on average annual extreme minimum temperatures of -20 to -15° F.) to the milder Zone 5B, with portions of coastal York County and the Midcoast even entering Zone 6A.

Which tree species suited to these zones might best withstand the stressors associated with climate change? You don’t want to get a tree to soak up carbon and solar heat only to find that it’s not particularly resilient in the face of climate change.

As it turns out, our sugar maples may not have been the best choice. Choosing two different species would have been smarter; your “key defense is diversity,” Libby says. The vision we’d had of grandchildren someday tapping the trees seems less and less likely as climate change is disrupting the sugaring season. A better choice would be red maple, Jeff Tarling, Portland’s city arborist, suggests.

I compared the two trees on a new USDA Forest Service “Climate Change Tree Atlas,” which lists for 134 species the likelihood of disturbances they’ll face – including drought, flood, deer browse, pollution and disease, and ice. Those data are combined with biological characteristics and likely climate scenarios to create for each species an “adaptability score” between 1 and 10. The site’s not especially intuitive (unless terms like “edaphic specificity” trip lightly off your tongue) but there’s useful information (some extrapolation is needed when considering landscape specimens, though, as data is based on native trees in forest settings). The red maple has an adaptability score of 8.5 whereas the sugar maple attained only a 5.8.

Species-wide forecasts can be challenging, Tarling notes, because so much depends on a tree’s site. Sugar maple, a personal favorite of his, is increasingly “in the wrong situation,” he admits. It’s susceptible to icing salt so should not planted near roads or drives, and the Atlas identifies it as more vulnerable to flooding than red maple.

Native red maple may be more resilient, Tarling thinks, than hybrid cultivars like “Autumn blaze” that are marketed for their rapid growth. He’s found that fast-growing varieties fare poorly during storms that hit when leaves are still on – like last year’s Halloween storm. Libby echoes this caution about fast-growing species, citing the horticultural maxim that “the best trees are worth waiting for.”

Libby and Tarling both note that warming could make some species native to the South – like sweet bay magnolia, Carolina silverbell and sorrel (or sourwood) tree – reasonable options in some coastal Maine settings. But Libby cautions that it’s hard to “trial” woody plants even in stable conditions, and now our climate is anything but stable.

Taking a risk in planting diverse trees – which may or may not withstand the climate shocks ahead – may be easiest if you buy bareroot stock, typically an investment of just $10 to $30 per tree (although not all species can be readily found in bareroot form). The time frame for planting is short, but several Soil and Water Conservation Districts (SWCD) around Maine offer them each spring (with the widest selection from the Knox-Lincoln district), along with Maine’s Fedco Trees. Tarling says it easy to plant bare root saplings, and he’s found their “success rate is really good; they can catch up.”

Bareroot trees offer another benefit when it comes to climate change, one I hadn’t considered until talking with Hildy Ellis, district coordinator for the Knox-Lincoln SWCD. Most bareroot stock is seed-grown, she notes, which means “you’re getting genetic diversity. So there’s a chance they will be tolerant of the changing climate.” Maybe there’s hope for our sugar maples after all; I’m rooting for them.

© Marina Schauffler, 2018. All Rights Reserved. Column reprints available upon request.