Across about 40 acres of forest land at Polly Hill Arboretum, some 40 per cent of the oak trees are dead. Just like tens of thousands of trees on other conservation, town and private land across Martha’s Vineyard.

Maybe enough trees to cover 1,000 acres, if you put them altogether, have died off in the past couple of years. That’s a big dying on a small Island.

The Polly Hill trees differ from all the other dead trees in only one respect: their passing will be more closely studied, even though the primary cause of their death is apparently clear.

Caterpillars.

“It’s the result of three years in a row of fall cankerworm defoliation,” said Tim Boland, executive director of the arboretum.

“And the last year that it was bad, 2007, the third straight year of defoliation coincided with a drought in July. The trees at that stage didn’t have enough energy to releaf.”

Mr. Boland’s diagnosis will come as no surprise to anyone who’s been on the Island over recent summers and experienced the caterpillar plagues. But big questions remain. Why are the oaks dying on some parts of the Island and not on other parts? What will regenerate where the oaks were? Why were there so many caterpillars, so many years in succession? Is the cycle of outbreaks now at an end, or was last year’s less-blighted summer just a hiatus?

To the first question, why more trees died up-Island, Mr. Boland theorizes a geological explanation. Most of the dead trees are on the morainal part of the Island, on the tops and sides of hills. They got less water as a result, and expired under the combination of caterpillar attack and water stress.

“As you get further toward Chilmark you have more clay and more moisture retention. But where we are it’s a moraine, essentially gravel mounds. And that’s where we saw a lot of damage,” he said.

Of more interest is the question of what will regenerate. Except for a few places where dead tress could present a hazard to people, the Polly Hill trees will be left where they are to rot and fall and decompose, while scientists study what comes up.

“What’s interesting is we’re not seeing a lot of oaks regenerating,” Mr. Boland said.

He believes over-abundant deer are eating them, but more detailed observations might take years.

“We just started a working agreement with the Harvard Forest of Petersham, in particular Dr. David Foster. We are looking at this as a chance to study the long-term ecological effects.

“And I mean long-term, because I’m an evolutionary ecologist. People say 10 years is a long time, but not in the forest.”

With Mr. Foster, Mr. Boland did a flyover of the Island in August to assess the damage.

“We have some pictures that are pretty dramatic. I think we may have the opportunity with Dr. Foster to do some more flyovers and more precise measurement,” Mr. Boland said.

The greatest concern is that the deaths will allow unwanted species to get established in areas where they previously were not. That is another reason why Polly Hill and other conservation organizations are not removing the dead trees.

Their advice to landowners who also have dead trees which must be cut is to remove them but leave the stumps intact. The less the ground is disturbed, the less likely that unwanted plants will sprout.

There are positive aspects to the dying of the oaks though, said Matt Pelikan, Islands program director for The Nature Conservancy. What’s bad for the oaks is good for the wild cherries, the woodpeckers, the blue-winged warblers, the various other native species which are moving in.

He nonetheless has concerns.

“There is a pretty good element of the unknown in how it will come back,” Mr. Pelikan said, “given the pests, pathogens and climate change.”

Starting with the caterpillars. Though Mr. Boland blamed one species, Mr. Pelikan said the defoliation was due to five or six.

“Mortality from caterpillars was probably always part of New England ecology, but the scope now seems to be greater, probably due to the multiple species of moths involved.

“There is the winter moth and gypsy moth, which are both introduced species, the fall canker worm, the forest and eastern tent caterpillar and the spring canker worm,” he said.

Between them, he said, they had killed perhaps up to 1,000 acres of trees across the Island. In some areas almost 100 per cent of the tree canopy was gone.

Of these pests, the winter moth is arguably the most problematic. As far as the experts can tell, the European species first turned up in Nova Scotia, sometime in the 1940s or 1950s, and with no natural predators, spread rapidly from there.

It arrived in Massachusetts about eight years ago and quickly became a major pest; in 2005, it was credited with having eaten its way through 34,000 acres of forest in southeastern Massachusetts.

Outbreaks have not been as big over the past year or two, but experts are predicting a bad year for caterpillars again next year, on the basis of exceptional numbers of adults mating now.

Robert Childs, an entomologist with the University of Massachusetts at Amherst, said the numbers were the biggest since 2005.

The good news is, there is a biological control, a parasitic fly, although it may be years before it succeeds in doing much about caterpillar numbers, Mr. Childs said.

“It took about eight or 10 years after its introduction in Nova Scotia,” he said. “They’ve only been introduced here for three or four.”

Perhaps a greater mystery though is why native species have suddenly appeared in such huge numbers in recent years.

Mr. Pelikan (who said he had not seen large numbers of winter moths yet this year), wondered about the role of climate change.

“It’s an interesting ecological phenomenon,” he said. “Maybe it’s involved with temperature or different patterns of rainfall. Maybe warmer winters will mean more moths survive to breed.”

Or maybe not. Maybe the oak forests will come back just as they were. Or maybe not.

Harvard Forest’s David Foster, an expert in the paleo-botany of the Vineyard, said his guess was that the forests would regenerate “just fine.”

On the other hand, he cited “good evidence that once, long ago, the oak died off and was replaced by beech. Five thousand years ago a great beech forest stretched from Falmouth out to Seven Gates.”

The reason, he said, was a change in the climate, a severe drought. Years upon years of dryer, warmer conditions.

Could that be happening now?

“Well, it happened in the past,” Mr. Foster said.