In an age of universal air conditioning, the sheltering value of a tree has become less obvious, along with the phenomena that allow it to ride out the heat wave in a way that we could not. Our forebears understood the value of getting to leafier, higher ground, even before expanses of asphalt and concrete created the heat islands of the modern city.
Chip Tynan, horticulturist at the Missouri Botanical Garden, said St. Louis has removed trees from once-leafy boulevards in advance of their slow death by the emerald ash borer. “It has created a whole lot of very hot streets,” he said.
Trees are, among other things, great columns of water, drawing moisture from the soil and exhaling it through the leaves. It has been estimated that a single apple orchard can lift 16 tons of water a day.
This is not to say that trees are not stressed by this heat or have not had to adopt mechanisms to cope with it.
As temperatures climb into triple digits and humidity raises the heat index to insane levels, trees adopt two basic and related strategies, said Todd Forrest, vice president for horticulture and living collections at the New York Botanical Garden.
The first is to wilt. Prolonged wilting in drought-stressed plants, especially young ones, can be deadly, but temporary wilting on established trees and shrubs is a defense mechanism and can occur even if soil moisture is adequate. By folding its leaves, the plant reduces its foliar surface area to sunlight and reduces the evaporative effects of the wind.
The second stratagem is to close the microscopic pores — stomates — found mostly on the undersides of the leaves. This shuts down transpiration and the gaseous exchanges needed for photosynthesis, in which the tree takes in carbon dioxide and releases water and oxygen.
When the heat is prolonged and the rain dries up, our temperate hardwoods react in progressively drastic ways, Forrest said. First they wilt, then the leaves show signs of scorching and then the leaves drop prematurely. He likens it to getting a tan, then sunburn, then heat stroke.
Tony Aiello, of the Morris Aboretum in Philadelphia, is less worried when trees go into stress mode after early August because by then they have made most of their growth and carbohydrate stores for the year. Late July’s extreme heat — the heat index was in the triple digits — was mitigated by its brevity and the abundant rainfall of recent weeks. “If we hadn’t had the rain I think we would see a lot more leaves falling, a lot more browning,” he said.
That applies to established trees with extensive roots sytems, but trees planted in the past three years still need help when it’s hot and dry.
They should get an inch of water a week during the growing season; some experts say two inches during heat spells. Casey Trees offers rain gauges and electronic alerts to help folks water young trees.
At the State Arboretum of Virginia in Blandy, Va., arborist Chris Schmidt spent much of the first day of the recent heat wave getting water to young trees there, including rare franklinia trees she has grown from seed. She recounted this while sitting under the shade of an old white pine.
We can retreat indoors if we have to. But Forrest, who has been around trees his whole career, still marvels at what they can endure.
They stand out in freezing weather, in blistering heat, in hurricanes, during droughts, and yet they soldier on while giving us (and other, more furry creatures) shelter and sustenance. We wouldn’t last long as trees, he muses. “Particularly if you had to get food from sunlight and carbon dioxide and water, with nobody feeding us steaks or raspberries.”
On a micro level, trees shelter us from the infernal summer sun. On a macro level and in an age of global warming, the ability of trees to cool the environment while exchanging carbon dioxide for oxygen makes tree planting a no-brainer.
A study published July 5 by scientists in the Crowther Lab in Zurich identified more than 2 billion acres of land worldwide that is not densely settled or used for agriculture and could be forested to buffer climate change. Once mature, such forests could capture 200 gigatonnes of additional carbon. I’m not sure what a gigatonne is, but it sounds like a lot.
Human societies are complex and fractious and we live in the age of mammon, so there will be obstacles to such an idea. But we can plant trees individually, or see to it that our street trees are planted and cared for.
This is my standard advice about selecting and planting a tree: Be patient and plant a small tree, which establishes better than pricey big ones; give it space to grow; and pick a tree that works in your soils and climate but is a medium to slow grower. It will have stronger wood and stay in bounds. There are many beautiful trees that are of this continent, but a tree doesn’t have to be native to be virtuous.
As increased temperatures and extreme weather events become more frequent, the challenge is to pick tougher trees. Peter Del Tredici, a lecturer in ecology at MIT, recommends bottomland species — pin and willow oaks, for example — and trees that are inherently adaptable, such as the ginkgo. Most conifers find it too hot in low-elevation urban environments, with the exception of the bald cypress. “That can take a tremendous amount of heat,” he said.
Del Tredici said the recent heat wave “is a precursor of coming attractions. We have to think about future conditions and which trees are more tolerant of heat and drought.”