Evolution among tree and bug species is something of a cat-and-mouse spy game: One side discovers a strategy that earns it the upper hand, at least temporarily, then the other side counters with improved defenses or perhaps a new breakthrough of its own. In the forests of Vermont and New Hampshire, this shifting evolutionary struggle is continuous.
For example, pollen and fossil evidence shows that the eastern hemlock was beset by an insect outbreak nearly 5,000 years ago that all but eliminated the tree from our local forests. The embattled hemlock persisted in isolated pockets until, nearly 1,000 years later, it adapted to the insect and returned to reoccupy its former range.
In recent years, the cat-and-mouse game has taken a rapid and decisive turn in favor of the bugs and other bad guys. Trees across the north woods are in trouble; no fewer than four of our familiar trees – ash, sugar maple, hemlock, and butternut - are in danger of vanishing from our woods soon, joining another, beech, that’s on the ropes and two more, chestnut and elm, that are, for all intents and purposes, gone.
The reason? Insects and fungi have been ferrying around the planet, taking advantage of global trade and international travel to colonize tree species that had previously evolved without them.
Eastern hemlock is under siege again, this time from the hemlock wooly adelgid, an aphid-like insect that arrived from Asia in the 1920s and has been on the march ever since. Much of the hemlock in southern New England already has been devastated; the adelgid was identified in southern New Hampshire and Vermont just in the past few years.
A third insect, the Asian longhorned beetle, has been spreading from Brooklyn, N.Y., since it arrived there in the 1990s, also hidden in shipping pallets. Though the beetle feasts on numerous tree species, sugar maple is a preferred host, and the beetle was discovered in Worcester, Mass., this past summer.
These three tree species beset by insects are joined by two others under siege from disease: the beech and the butternut – the latter now largely absent from the forests of Vermont and New Hampshire thanks to butternut canker, a disease that appeared locally in the 1960s. The beech has been grappling with beech bark disease since a European insect and an introduced fungus arrived on the scene over a century ago, enabling the fungus to penetrate the bark; and though the beech seems in no imminent danger of extirpation, it’s becoming a shadow of its former self. Finally, there are the chestnut and the American elm, two species barely hanging on in isolated pockets, having succumbed to an Asian and European fungus, respectively.
In fairness, it isn’t just the bugs and diseases that have fared well in the globalized world and caused harm. Some tree species, when moved from their home ground to exotic locales, have also tipped the ecological scales. Californians imported the Australian eucalyptus, and Australians imported the Californian Monterey pine in efforts to improve timber production, only to see both species become invasive plants in their new locales. Closer to home, in New Hampshire and Vermont, Norway maple and European buckthorn, having been introduced as ornamental plantings, have now become pesky species as they have begun displacing local species.
But in recent years it has been the bugs, not the trees, that have had the upper hand, largely because of international travel. It is the bugs that travel around the world alive, stowed away in packing materials or elsewhere, while most trees, in transit, are already dead – cut into logs or boards. In this form they are unable to colonize new shores upon arrival.
If the ongoing struggle between trees and their tormentors is a natural part of evolution, why fight it? When trees and their attackers struggle over time, at a “natural” pace, each making incremental gains in offense and delivering setbacks in their defense, the upshot can be a more diversified forest of species that is more resilient – which seems all for the good. In this sense, the “bad guys” aren’t really that bad; they’re helping trees improve their game.
But when insects and diseases are artificially aided in their efforts, as they currently are by international trade and travel, forest diversity and resilience can suffer, perhaps for thousands of years. Unfortunately for those of us alive today who appreciate the hemlock, ash, sugar maple, beech, and butternut, not to mention the chestnut and elm, that’s longer than we’re apt to be around.