Many state-based professional organizations have their own certification programs as well. “A lot of companies are formalizing their training,” said Trumbull Barrett, president of the Massachusetts Arborists Association.
More arborists are incorporating climate change into their decisions. “We are seeing things on the horizon that are very disconcerting, very unnerving,” Mr. Yaple said. “So when people ask us to recommend tree plantings, we suggest that people plant trees that are very happy in the Mid-Atlantic states.” The range of some tree species is expected to shift north, following warming temperatures. So Mr. Yaple has been eying redbuds and scarlet oaks.
Alexander R. Sherman, city forester for Springfield, Mass., is doing the same: growing more southern trees in a nursery and seeing how they fare on the streets. At the moment, he is experimenting with a type of tulip tree, a Kentucky coffeetree, and a sawtooth oak.
This spring, Ms. Bezanson planted over 300 trees in the mostly hemlock-and-white-oak woods around her home. “Some people baked bread. Other people planted trees. I did both,” she said. “What I have tried to do is brace for climate change and plant trees that are going to be more resilient for the future.” Ms. Bezanson planted species that do well in warmer, drier climates, such as black gum, pawpaw, and persimmon. And she planted balsam fir because hemlocks are being wiped out by an aphid-like pest, the hemlock woolly adelgid.
There are chemical treatments for some diseases and pests like the woolly adelgid. And this summer, the Massachusetts Department of Conservation & Recreation continued releasing wasps whose larvae feed on the emerald ash borer — an approach called biocontrol. But biocontrol and pesticides can be expensive and virtually impossible to deliver at the scale of millions upon millions of trees.
Which is why many experts, from arborists working with individual trees to foresters working with vast woodlands, are increasingly managing for diversity. “If you have 12 species of trees in one forest and now the ash is dying, that is terrible, but at least you have 11 other species,” said Michael Mauri, a consulting forester based in South Deerfield, Mass. “Protecting and maintaining diverse species is kind of our best defense against all the stuff, known and unknown, that is going to be visited upon us.”
Foresters often do that not by planting, as Ms. Bezanson did, but by cutting. Ten years ago in the 103-acre Fox Valley Wildlife Sanctuary, for instance, white pines were removed from five acres where the species was dense and all nearly the same age.