The Washington Post - 01.08.2019

9

MD

the washington post

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thursday, august

1

,

2019

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.”

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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

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

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

As you dodge

sunbeams on a

hellaciously hot

and humid

summer’s day, it’s

worth

remembering that

you have a

constant friend

willing to take a

photon to save

you.

We refer of course to the

humble tree, so seemingly

passive and yet so instrumental

in getting us through high

summer in Washington. If its

beauty were not enough, or its

ability to mitigate greenhouse

gases, the shade the tree

provides is a real measure of

relief from excessive summer

heat. It can feel 15 degrees cooler

beneath an old oak or maple,

and a stand of them can create a

breeze as they forge their own

microclimate.

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.

Trees can’t escape the heat, but they’re pretty good at handling it

Adrian

Higgins

GARDENING

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KATHERINE FREY/THE WASHINGTON POST

Trees stand between us and the sun to provide welcome relief from summer’s heat. Scientists in Switzerland have identified 2 billion acres

of land worldwide that could be forested to mitigate the effects of climate change. Trees release oxygen and capture carbon as they feed.