HOEY — Responding in faith to a warming world: My little hemlock


Ten years ago, I bought an Eastern Hemlock and planted it at the edge of a wooded ravine in my backyard.

I knew I was tempting fate. Tsuga canadensis is an important species in New England and the Great Lakes region and ranges south into the Appalachian Mountains, but the tree likes cool, moist uplands and is not native to most of the Midwest and is certainly no fan of Missouri’s hot summers.

I mostly plant native trees, meaning I normally exercise some common sense, but I couldn’t resist the little hemlock. Visions of Henry David Thoreau walking in the deep shade of a cathedral-like stand of Hemlocks led me to throw caution to the wind.

Hemlock groves have always fascinated me, and I am not alone. Henry Wadsworth Longfellow sang their praises long ago: “This is the forest primeval. The murmuring pines and hemlocks/Bearded with moss, and in garments green, indistinct in the twilight.”

Despite my foolish romantic impulse, my little hemlock has somehow managed to survive, probably because I planted it in the shade of some large oaks and hickories, which has protected it to some degree from Missouri’s hot summer days.

Most conifers don’t like shade, but hemlock is very shade-tolerant and can grow slowly in almost complete shade. They can bide their time in the dark understory of a forest for a century and then, after some disturbance, such as lightening or fire, pick up their growth rate and race toward an opening in the forest canopy.

My hemlock is now 8 feet high and about the same width. Last year, the utility company cut down a few of the old oaks which were too close to the power lines, giving my hemlock more sunlight. It seems to be growing faster but I will never live to see it as some feathery giant as in a New England forest.

Of course, the little hemlock may not even survive my lifetime. The white oaks I have planted have a better chance of doing that; they’re native to Missouri, adapted to our state’s hot and humid summers.

Within its range, however, hemlock can become the dominant species, forming a forest canopy that sheds deep shade. Light received by the forest floor may be only 1 percent of the light received at the top.

This deep shade suppresses undergrowth and creates a micro-climate where summer temperatures can be 10 degrees cooler than the surrounding countryside. The forest floor becomes a deep, soft, almost spongy place favored by many insects adapted to these unique conditions.

It offers an ideal habitat for red-backed salamanders that thrive in the cooler environment amid fallen logs.

Shaded hemlock streams feature cold water, and trout like that; one study found that brown trout and brook trout are two to three times more prevalent in heavily shaded hemlock streams than in hardwood streams.

In winter, deer take refuge in hemlocks to avoid cold winds and deep snow; hardwoods can’t compare in providing this kind of shelter.

Old growth stands of hemlock also take up carbon dioxide and store it for centuries, reducing the greenhouse gas emissions.

But hemlocks are in trouble. The wooly moth adelgid, a tiny insect, just a half a millimeter in size, can kill an entire grove of giant hemlocks (some over 120 feet in height) in five years.

The wooly moth adelgid was accidently introduced to this country in 1951 when a Japanese hemlock was shipped to a backyard garden in Virginia. Since then, the tiny assassin has spread to 18 states and killed millions of our native hemlocks.

Bearing witness to this unfolding tragedy and trying to understand it are foresters and scientists associated with Harvard Forest, a 3,750- acre experimental forest owned by Harvard University.

This forest, located 70 miles west of Boston in north-central Massachusetts, features a mix of white pine and hemlock stands, along with deciduous trees such as birch and red maple.

For over a century, the staff of Harvard Forest and visiting scientists have studied this forest and other New England woodlands for changes in forest composition.

They have also looked at the deep past of these forests. Taking core samples from lake sediments, they have traced the history of these woodlands back thousands of years, mapping out the rise and fall of dominant tree species.

Hemlock has often dominated in New England forests, but the tree suffered an abrupt decline about 5,500 years ago, which lasted 1,500 years. This was a period when a warmer climate prevailed.

Some scientists theorize that drought-weakened hemlocks became more susceptible to attacks from insects such as the eastern hemlock looper, a voracious inchworm that can infest and defoliate hemlocks.

Today, history may be repeating itself. Hemlock decline is most catastrophic in the warmer, southern portions of its range, in the Appalachians.

Now, as hotter summers and milder winters visit the Northeast, the wooly moth adelgid is spreading further and faster.

David Foster, the director of Harvard Forest, has edited a handsome volume — Hemlock: A Forest Giant on the Edge — that focuses on the hemlock tree as a foundation species and how climate change and infestations of the wooly moth adelgid may be interacting to bring about another abrupt decline in hemlock stands.

The book includes black-and-white photos dating from the 1920s of scientists conducting studies in Harvard Forest.

Just as medieval monks sought to preserve ancient manuscripts for the edification of future generations, these scientists are trying to read the forest, its past and present, in order to uncover clues as to what the future may hold.

Foresters refer to hemlock as a foundation species. Hemlocks are like the stone foundation to a large cathedral, taken for granted at times, but essential to the food web of this very unique eco-system.

What will happen if they disappear? Loss of hemlock stands may reduce habitat for snowshoe hare, porcupine and bobcat.

Bird species that specialize on hemlock woods, such as the black-throated green warbler and the hermit thrush, may see steep population declines.

Yet overall, the loss of hemlock groves — and conservationists don’t like to hear this — will lead to more biodiversity as yellow birch, red maple and red oak take over these forests, letting in more sunlight, leading to the more understory trees and bushes that deer and other wildlife can browse and thrive on.

Forests are dynamic and have tremendous reserves of resilience; one species declines but another succeeds it. At the same time, the loss of the old hemlock groves is a great loss for anyone who appreciates the unique ecosystem created by these great and noble trees, the cool, dark and still understory lauded by American poets since colonial times.

In his encyclical, Laudato Si’, Pope Francis declares: “Everything is interconnected.”

The pope’s declaration highlights how human beings are part of the web of life on earth. We are no island set apart.

As our climate changes, there are winners and losers. Hemlocks are one of the losers, at least for now.

Some small stands of these trees are showing resistance to the wooly moth adelgid. Maybe hemlocks will make a comeback. But it is sobering to know that it took about 2,000 years for hemlocks to recover after their abrupt decline 5,500 years ago.

Harvard Forest, meanwhile, is carefully monitoring this transformation from hemlock to oak, maple and birch forests.

It is sad in many ways, but much can be learned by listening to what the hemlocks have to say. In 1859, Emily Dickinson penned a verse worth pondering today:

I robbed the Woods -

The trusting Woods -

The unsuspecting Trees

Brought out their Burs and mosses

My fantasy to please -

I scanned their trinkets curious -

I grasped — I bore away -

What will the solemn Hemlock -

What will the Oak tree say?