Eastern hemlock is an important component of the forest. Hemlock stands provide nesting habitat for a slew of bird species, such as the blackburnian warbler, and brook trout congregate in stream waters kept cool by the deep shade. And so the forests here have changed dramatically in the past three decades, as the woods have been exposed to and altered by the hemlock woolly adelgid, an exotic insect pest. The heaviest mortality has occurred on upper slopes and dry sites where hemlocks were historically limited due to drought and fire. Hemlocks in ravines and on moist sites have experienced more moderate mortality to date; it’s too early to tell if they’ll eventually succumb, but many of the remaining trees look healthy.
It’s fair to see the widespread hemlock mortality as a terrible thing: the brook trout, the dicranum moss, and all the other plants and animals associated with a dense hemlock stand certainly do. Dead hemlock is useless for lumber and can pose a significant hazard to hikers, hunters, loggers, and other forest users, so we can count ourselves among the affected. But as with everything in nature, kingdoms rise when other kingdoms fall, and while it may be hard to see a silver lining in all this destruction, it’s there if we look.
For starters, the adelgid infestation has added a lot of dead wood to a forest that needed it. Standing dead trees provide great habitat for woodpeckers and other cavity-nesting birds. When the snags fall, the downed logs make excellent cover for salamanders and good insect hunting for numerous small mammals and snakes. Large quantities of dead wood are the basic building blocks of a forest food chain, and are a characteristic of old growth forests. Connecticut only has a few small remnants of old growth that escaped the extensive agricultural land clearing from the 1600s to the early 1800s, and the wood cutting frenzy that lasted until the late 1920s. Thus, the dying hemlocks have created old growth structure that would not have occurred for another century or two (or until the next major hurricane). The dead trees also create canopy gaps of varying sizes, which add to the vertical structure of the forest – another old-growth characteristic.
While in some stands the adelgid is accelerating forest succession, in others, succession has been knocked back as the dead hemlock provides tinder for severe fires. Wildfires are relatively rare in Connecticut; only 1 out of every 4,000 acres burn annually. But pre-settlement fires probably burned at least 100 times that. Fire was used by Native Americans to promote habitat for game animals, help clear land for agriculture crops, improve ease of travel, and facilitate firewood and acorn collection. Since fire and its ecological role have almost been extinguished from the landscape, several disturbance-dependent ecosystems are not currently sustaining themselves.
On a parcel I manage in Guilford, one six-acre fire killed more large oak trees than a normal spring fire because the large amount of dead hemlock increased its intensity. The fire created a large patch (another old-growth attribute) in the forest by killing a concentration of larger trees. This paved the way for a new generation of sun-loving oaks and aspens that would not have been able to survive in the shade of an intact forest. Large new patches often host an increasingly uncommon suite of birds and mammals such as blue-winged warblers, chestnut-sided warblers, and the New England cottontail, currently a candidate for the endangered species list. These animals require dense young seedling and sapling forests that develop after most of the existing trees are killed or farmland is abandoned. These species are declining as the events necessary to create their habitat have long been in shortage.
Insect epidemics, severe windstorms, and frequent fires are among the historic disturbances that have helped sustain the biological diversity of southern New England for thousands of years. Increasingly, foresters are turning to natural disturbance management models to help restore some critical components of biodiversity. Harvests of small and large trees are designed to mimic the effects of natural disturbances at their historic frequencies, sizes, and intensities. Harvests can be planned to greatly speed up the accumulation of old growth structure.
The hemlock woolly adelgid mortality, and subsequent forest fires, have shaken the tranquil ecological trajectory of Connecticut’s forests, but the loss of many majestic hemlocks and oaks has opened another chapter of forest succession. It can be hard to watch the old trees go, but death is a requisite part of nature. Ecologically speaking, when foresters in southern New England initiate unnatural disturbances (i.e., harvests) in certain stands, they’re simply standing in for insects and fire. Where natural processes are available to adequately conserve biodiversity, it’s wise to stand aside and let nature reclaim its realm.