MOUNT RAINIER, Md. — PITY our urban trees — wild, forest-dwelling species imprisoned in tight spaces, drawing water and nutrients out of poor, shallow soil and enduring endless assaults from vehicles, pollution and power company chain saws. Wouldn’t these trees be better off in a remote forest somewhere?
Surprisingly, not always. Down the street from where I live in this suburb of Washington is an Eastern hemlock that is the picture of health. This tree, at least 50 feet tall and perhaps 100 years old, bears a thick plumage of needles that shades the yard where the tree grows and an adjacent pocket park. In spring, the tree’s cascading branches were tipped with delicate light-green new growth and thousands of tiny seed-bearing cones.
Sadly, it’s getting hard to find a hemlock in such good shape in any forest in Maryland. Or, for that matter, in Virginia, North Carolina, New Jersey, Connecticut and large parts of the many other Eastern states that are infested with a tiny but deadly insect called the hemlock woolly adelgid.
Scientists believe this insect arrived in the last century on a shipment of nursery trees from Japan. Adelgids drain sap from hemlocks’ needles, which can weaken them and, in nearly all cases, eventually kill them. The insect weighs almost nothing and spreads easily by bird, mammal or wind. Each adelgid can have between 50 and 300 offspring, and with two generations per year, infestations proliferate quickly. Few if any Eastern hemlocks have shown resistance to the adelgid.
Fortunately for my neighborhood hemlock, an adelgid has to drift in from elsewhere and land directly on a tree to start an infestation — an event far less likely for an isolated tree than one in a vast hemlock grove. I sometimes wander up to the park and check the undersides of the hemlock’s needles for the telltale white fluff the adelgid makes to protect itself and its eggs. So far, I haven’t found any. One might say the tree has found safety in lack of numbers.
Human-dominated places have become, for some species, safer habitats than the unpopulated areas we’ve set aside to preserve nature. This would have been hard to imagine 150 years ago, when this area was farmland, and nearly every tree had been cleared. Today, the farms are gone, and many residents plant and nurture trees in their yards. City governments also plant trees — mostly native ones — in parks and along roads. In many ways, it’s a great time to be a native tree in an American city.
At the same time, life in the forest has, for many trees, become precarious. An invasive blight wiped out the great American chestnut decades ago; native elm, ash and beech trees also face grave threats. But the hemlock seems to have it worst. I recently stayed at a cabin in a valley by a small stream in southern Pennsylvania, a place that was once prime hemlock habitat. The valley is now a hemlock graveyard. Hundreds of gray trunks rise skyward with dead branches pointing every which way.
When an old hemlock dies, sunlight pours onto ground that may have been dark for centuries. Hemlocks can thrive in deep shade, and their needles create a canopy so thick that almost no other tree can grow. Now, vigorous saplings are shooting up from the forest floor by the stream. But these newcomers aren’t hemlocks; they’re birch or maple or tulip poplar. The forest is moving on; in a few decades there will be little sign of the ancient conifer that once dominated this place.
The insecticide imidacloprid can keep the adelgid in check if applied regularly. Because the chemical is typically applied tree by tree, treating a forest with many thousands of hemlocks is prohibitively expensive. But treatment can be affordable in areas where people want to preserve a few trees. The result is that cities and parks are becoming hemlock havens.
Unfortunately, cities will never give the hemlock an ecologically meaningful life. In nature, hemlocks support a complex and unique ecosystem that has evolved around them. Hundreds of species of insects and spiders and mites have been collected on hemlocks, and their fate is unknown if these trees disappear. Various birds and fish subsist on those invertebrates. A lone landscape tree can’t support such an ecosystem — it’s just there to look pretty and make a little shade.
The hemlock in my neighborhood was planted here, presumably by some early homeowner, long before the adelgid showed up in America. The tree has hung on remarkably well among hardwoods better suited to the mid-Atlantic’s warm (and warming) summers. But eventually, it will grow old and die; indeed, global warming may hasten the process. And that will be it. No children to carry on the line; no genetic future. If the hemlock is replaced, it will be by a tree grown in a nursery.
There is already a tree, the ginkgo, that continues to line our roads despite being nearly extinct in the wild. Is the Eastern hemlock doomed to become like the ginkgo, a living fossil preserved only because people like it? As much as I love my neighborhood hemlock, that would be a sorry fate indeed for such a majestic and ecologically important species. But it seems to be where we’re headed.
Gabriel Popkin writes about science and the environment.