Last Rites
Site managers leave dying trees to natural cycle

by Pamela Zerbinos

Where do trees go when they die?

In most cases, it depends on where they lived. If they lived here at Fermilab, chances are they don’t go anywhere at all.

In the southern end of the Tevatron ring, near DZero, there is a veritable forest of dead trees. Their number is uncertain, but they definitely outnumber live ones.

“Not only is it unsightly,” said Dmitri Denisov, who works at DZero and gives about one tour of the facility each month, “but it also raises some uncomfortable questions.”

Those questions, Denisov said, come from visitors who seem to be concerned that the trees—which are mostly inside the accelerator ring—are being killed off by radiation.

Of course, this is not the case. The trees, mainly cottonwoods, are being killed off by the Ecological Land Management Committee. Some of the trees have been killed by the annual prairie burn, and some have been girdled, which involves making a cut around the circumference of the tree to prevent nutrients from circulating. A girdled tree usually dies within a year or two.

Those trees were never supposed to be there in the first place. The land inside the ring is prairie land, or at least it’s supposed to be, and prairies don’t have trees. But cottonwoods are a weedy, aggressive species, and they’ve invaded the southern end of the ring.

“There are good places for trees and bad places for trees,” said Peter Kasper, a Fermilab physicist and ELM bird monitor. Grasslands and prairies are bad places for trees.

Birds such as the Grasshopper Sparrow or the Meadowlark, Kasper said, won’t nest anywhere near a tree for fear of predators. If a tree ends up in the middle of prime grassland, that grassland will be ruined for many animals that otherwise would like to live there.

“There are GOOD PLACES FOR TREES and BAD PLACES FOR TREES”

Beginning in the mid-80s, ELM started paying careful attention to where trees were planted in the lab, and now there is a detailed plan—online at www-esh.fnal.gov/ELM/ELM_Plan_2002.htm —for each of Fermilab’s 6,800 acres. The northwest corner of the lab is being converted to woods— trees are regularly planted in those areas, and valuable trees like oaks are relocated there (rather than killed) whenever possible. The plan for the ring calls for getting rid of the cottonwood grove and converting the land there to restored prairie.

If that grove of trees—or any grove of trees— is instead left to its own devices, it will eventually kill the prairie, as the trees grow and choke out the shorter grasses. Fires traditionally kept the woody areas at bay (if you look at pre-settlement habitat distributions, you’ll notice forested areas generally developed around rivers and other natural fire breaks). But natural prairie fire is a rarity these days, and land managers interested in prairie restoration have had to find other means of controlling the spread of trees.

The most familiar method is probably the controlled prairie burn, but others include mowing and tree removal. These are part of an overall strategy known as “ecosystem management,” or EM, adopted by ELM in the last five to 10 years. It was a broad strategy until recently, but now a four-person subcommittee makes decisions on a tree-by-tree basis to help the overall plans along.

The goal of this strategy, said committee member Rod Walton, “is a full-scale reconstruction of a functional ecosystem that is as close to pre-settlement conditions as possible.”

And that means that prairie needs to be prairie, and woods need to be woods.

Which brings us back to that forest of dead trees, and why it’s not going anywhere.

“Tree removal is always dangerous,” said Mike Becker of Fermilab’s Roads and Grounds department, which does the actual work of cutting them down. “You’re using chainsaws and heavy equipment.”

Despite the risk, the trees would be removed if they were near roads or buildings, or someplace where people go regularly—”basically anywhere they’d threaten infrastructure,” Becker said. This approach isn’t unique to Fermilab. Spokespersons for both the Morton Arboretum and the DuPage County Forest Preserve said they have similar policies. But since the trees in question are in a natural area, there’s no real reason to cut them down.

“The only drawback to leaving them there is looks,” Walton said. “They provide food and shelter for insects and birds, and when they decay they provide humus for the soil.” Humus is dark, rich, fertile soil produced by the decay of organic material. Many people buy it to use as fertilizer for their gardens. In the meantime, Kasper said, the dead trees keep woodpeckers out of the power poles and provide convenient perches for hawks.

“It’s not so much a matter of educating the public,” Kasper said, “but of educating ourselves. A lot of people interact with the public...They should know we’re actually managing the land intelligently and with a purpose and—importantly for this place—in a way that has some science behind it.”