Wisnewski, today Harvard Forest’s woods crew supervisor, told Foster that loggers need a flat area to stage removal of logs from the forest. So instead of cutting trees, which would leave stumps behind, they simply pull them down with a winch and cable, lop off the trunks, and pile the roots to one side.
An experimental path cleared, Foster turned to the forest’s archive to plan the simulated storm In 1938, graduate student Willett Rowland recorded the Great Hurricane’s damage at the forest, showing that about half of the large trees came down and that some species, such as white pine, were more susceptible to wind damage.
With that knowledge in hand, Foster laid out an east-west plot 50 meters by 160 meters in the Tom Swamp tract and marked the trees to come down. Preparations complete, they drove in the skidder and hauled the cable into the forest, pulling down tree after tree, all oriented so their crowns pointed northwest, as if felled by a hurricane’s prevailing winds.
Most trees came right over, Foster said, but some broke and were left to regrow as they were. Only one tree — a large old oak — resisted the skidder’s tow.
“We decided that, well, in a hurricane that tree wouldn’t have fallen,” Foster said. “We went and found one of equal size downslope and pulled it over.”
Then began the lengthy task of monitoring. An early revelation was the stability of key indicators like soil temperature, overall productivity, and carbon dioxide and nitrogen gas cycling among the trees, the soil, and the atmosphere.
Another was that the trees didn’t die right away. Ninety percent of trees damaged by the winch leafed out regardless, photosynthesizing, drawing water from the earth, and contributing to the forest ecosystem even though they were flat on the ground. As they slowly died, the understory took over. Saplings that had been awaiting their chance shot upward, sprouts grew from the fallen trees’ roots, and newly seeded trees got started. Lost production — measured in the amount of leaf litter each fall — recovered in just six years.
“Despite the fact that this looked like a destroyed forest, because it was physically altered in such a major way, it was functioning as an absolutely intact ecosystem,” Foster said.
In trying to understand the forest’s unexpected stability, researchers realized that most experience with disturbed sites was at places subjected to the common practice of salvage logging, where fallen trees are cut and dragged out using soil-churning heavy equipment. In some cases, as after the 1938 hurricane, the piled debris left behind is burned.
“We’re used to looking at sites that were subsequently disturbed after a major wind storm or ice storm by people going in and logging,” Foster said. “The 1938 hurricane was the biggest salvage logging exercise in U.S. history. And it pretty comprehensively turned the 1938 hurricane into one great big cutting operation.
“In almost every case you can think of, if your intent is to encourage the recovery of the forest and ecosystem function with minimal change … doing nothing becomes a viable alternative.”
When Audrey Barker-Plotkin arrived at the site eight years after the pulldown, just walking around was a challenge. Today a senior researcher and the author of several studies on the site, she recalled having to weave through tangled branches and wrestle with wiry new growth that all seemed to be at “face level.”
“It was like walking through a jungle gym. The plot seems a lot smaller now that you can see through it,” Barker-Plotkin told the visiting scientists. “Just the changes I’ve seen in 20 years have been really remarkable. … [The site] was different every single year.”
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Another thing scientists didn’t expect, Barker-Plotkin said, was the stability of the tract’s species makeup. Researchers thought that more pioneer species like cherry and paper birch — usually fast-growing colonizers of disturbed sites — would take hold. But the stability of even the damaged ecosystem didn’t provide much of an opening. While those species did appear in disturbed soil around upturned roots, that was less than 10 percent of the forest floor. Invasive species, another threat at disturbed sites, were also absent, she said.
Today, she said, the experimental plot has largely recovered structurally, but is still struggling to catch up with the surrounding forest’s growth. Tree volume has reached about 80 percent of what it was before the pulldown, but measurements of the nearby control plot show that the surrounding forest has grown 25 percent over the intervening decades as part of New England forests’ continued recovery from Colonial-era clear-cutting.
New leaders and a landscape full of questions
Like the long-term processes they measure, the hurricane pulldown and other experiments continue to produce data even as their original investigators’ careers come to a close. A smooth transition to new leadership will be essential in maintaining both research continuity and excellence, Foster said. At several sites the group visited, experimental founders handed off presentations to younger researchers, as Foster did to Barker-Plotkin at the hurricane site and Melillo did to University of New Hampshire Professor Serita Frey, a soil microbe expert, at the soil warming experiments.
LTER’s new principal investigator, Thompson, spoke of the importance of ensuring the continuity of key long-term experiments even as researchers move on from work that has run its scientific course.
“In some ways, the experiments they set up in the ’80s just look so prescient now,” he said.
An important question still to be explored is how long recovering forests will keep absorbing carbon, Thompson added. That answer has potentially crucial implications for climate change, since global forests absorb roughly 20 percent of the excess carbon humans emit.
Part of the problem, Thompson said, is that though remnant old-growth patches exist, they may not be good models for understanding forests regrowing on former farmland, since they’re typically in poor growing locations, which is why they weren’t cut in the first place.
“We know how much [carbon] is in the forests,” Thompson said, “but we don’t know how much carbon can be in these forests.”