Wild Side : Caterpillar damage
By Matt Pelikan
Published: July 17, 2008
Having leafy trees at this point in the season is a refreshing novelty for the Vineyard, after four or five years of voracious caterpillars, dangling silk, and a rain of frass. The outbreak this year is confined to isolated patches, a welcome relief. But especially in up-Island oak woodlands, thousands of trees slain by caterpillars memorialize the severity of the event.
Defoliated several years running, some of them already weakened by drought or disease, the trees simply ran out of reserves to invest in a new set of leaves. Now their trunks show like bones among the leafy canopy. Among the dead are many of the grand old trees of the Vineyard woodlands, sprawl-limbed ancients that cleft their acorns in pastures of a previous century.
One of the best places to get a sense of the extent of tree mortality is from the hilltop overlook at Waskosim's Rock, from which you can scan much of the West Tisbury moraine. Some areas survived the caterpillars unscathed, but some patches acres in size lost nearly all of their canopy trees. Across large swaths of the moraine, it looks like oak mortality ran around 10 percent; in a few substantial areas, it's more like 30 or 50 percent.
Mature woodland has dominated West Tisbury and Chilmark for decades now. But of course most of the Vineyard's morainal forest was cut to the ground by European settlers, and the opened land grazed by tens of thousands of sheep. Today's woodlands have regrown, similar though surely not identical to their previous form, from almost nothing. Compared to that centuries-long cycle of disturbance and regrowth, a little moth damage barely registers. But the impact of the moths approximates what one might get (in, trees knocked over instead of dead ones standing) from a major hurricane - a rare event over the long run.
Caterpillar outbreaks (like fire and storm damage) are a natural part of forest ecology, though human introduction of new insects may have made them more frequent. And the boom-and-bust cycle is as central to the biology of the moths involved as it appears to be to Wall Street. The outbreak of the past few years, though, was unusual in scope, duration, and diversity of moths - at least five divergent species, some native and some exotic, were out of control at once.
I have no compelling explanation for the severity of the outbreak. But its scale amazed me: those few species of caterpillars diverted most of the productivity of an entire forest into self-replication. Huge quantities of nutrients morphed from oak leaf into moth flesh. And then, just as abruptly, something - disease among the moths accelerating predation and parasitism self-defense, (in the form of increased tannin production) by the trees? - brought the outbreak to an end. For now.
The loss of so many canopy trees creates a new kind of woodland. Through each hole punched in the forest canopy, sun and air enter, producing brighter, windier, and less humid conditions. With more light and less competition for water, understory vegetation will flourish, growing into shrubby woodland clearings, attracting a new mix of insects and birds (good for blue-winged warblers, bad for wood thrushes).
The dead trees themselves provide a resource. Some will run Island wood stoves. A host of beetle species that live by burrowing under bark, or into dead wood, have hit the jackpot. So have birds, like woodpeckers, that eat wood-boring beetles. Standing dead trees, as they start to decay, will begin to offer nesting cavities to many kinds of birds and social bees and wasps.
As limbs drop and, eventually, wind-heaved trunks crash down, the downed wood will shelter snakes, salamanders, ants, and still more beetles, while feeding hosts of wood-loving fungi. Eventually all the nutrients locked up in wood will find their way back into the soil, feeding whatever vegetation is growing there at the time.
What will that be? Probably a new set of oaks, which, as before, are likely to triumph through height, longevity, and shading out competitors. But several factors could alter that trajectory. For one thing, the decades of forest regeneration will take place in a warming climate, with more extreme weather, which could reduce tree survival or stress trees and make them more vulnerable. And additional exotic insects or diseases may arrive and flourish out of control, producing more frequent defoliation.
If so, today's altered woodland may be a hint of a new regime for our forests, more open and shrubby and with a distinctly different mix of wildlife as a result. What is certain is that the caterpillars have launched a vast experiment, which will play out over a human life span or more, and some species will win and others lose.
