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Natural Detectives

Field Naturalist trip challenges graduate students to read the land

By Joshua Brown Article published November 2, 2005

naturalists
Graduate students in the Field Naturalist program begin unlocking a mysterious landscape in Bristol with detailed observations. (Photo: Joshua Brown)

There’s a strange patch of forest outside the village of Bristol. Just east of Route 116, at the base of the Bristol Cliffs, you can suddenly leave the sea of rich hardwoods that cover most of the state — sugar maple, beech and birch — and step into vegetation found near the top of Camels Hump or far to the north in Canada. Here, a spongy half-acre of sphagnum moss grows with blueberries and Labrador tea, shaded by cold-loving black spruce trees.

What is the boggy place and why is it here? Write “cold-air talus woodland” and you’ve got the answer. But, like any cheater on a test finds out, getting the answer is nothing like finding it. That’s why the eight graduate students on the Field Naturalist and Ecological Planning programs' Oct. 21 field trip are spending the entire day trying to figure out what caused this odd bit of boreal habitat to grow on a low-elevation site in the Champlain Valley.

Their instructors, Alicia Daniel and Matt Kolan, have not told them the name of this rare natural community — but they have armed them with soil thermometers and geologic maps.

“The purpose of this course is to give students the skills to interpret any landscape,” Kolan says. “Obviously most of their sites are in Vermont, but if they were dropped somewhere in Africa by helicopter they’d know how to start making sense of the place.”

As they prod the ground, squint up the trunks of trees, peer at rocks with a hand lens, and thumb through plant guides, the students are practicing a method of inquiry that they call “pieces, patterns, process.” Like any good detective, they are trained to start with the details of the evidence. Into waterproof notebooks go comments like: “soil temperature, 38 degrees Fahrenheit,” “pH: 3.5. Wow. Very acid soil!” “bedrock: Cheshire quartzite. Breaks conchoidally (i.e., doesn’t break in clean planes. Results in irregular chunks).”

They’re looking for how these details resolve into patterns — and trying to build a case to explain what process could cause this ecological island. “I love to see the ‘ah-ha!’ moment,” says Kolan, “where they come to the answer on their own.”

Cliff notes
But at 11:00 in the morning, none of the students are saying “ah-ha!” Instead, they are looking warily up a slope of naked boulders that rises hundreds of feet directly above their study site to another line of trees high above.

“It looks like a road project,” says first-year field naturalist Corrie Miller, tipping her head back.

“More like the moon,” says another student from back in the trees.

Though it may look like lunar highway construction, this jumble of sun-pounded rocks is entirely natural. It’s “open talus” — a deep and steep layer of piano-sized boulders broken off the cliff face.

Leaving the shadows of the spruces, some of the students start to climb, picking their way from rock to rock. Reaching the trees at the top, they bring out their thermometer again. Soil temperature: 44 degrees Fahrenheit. It’s 6 degrees warmer than the soil at the bottom.

By three in the afternoon, small groups of students have explored in all directions. They re-gather at the study site and sit huddled in a chilly circle, trading notes. “OK, now what do you think is happening here?” says Daniel. “Let’s get multiple working hypotheses going.” The students speculate about soil types and vegetation gradients and elevation. The instructors ask: Why is it colder at the bottom than the top?

“Well, it is like a giant solar panel out on those rocks,” says a student. They’re getting close.

But this is not a Sherlock Holmes tale and so the students don’t suddenly whip from their cloak a neat package explaining exactly why this strange woodland has formed. It takes a (sometimes stumbling) half-hour of discussion, with prodding and diagrams by their instructors, to bring the dozens of disparate observations into a coherent story.

Here’s the summary: the rocks on this west-facing slope heat in the sun, and, like a summer-time parking lot, the air immediately above them gets blazing hot. This hot air rises up the steep slope like a chimney, while cold air settles down the slope and sinks into the deep spaces between the rocks. In this honeycomb of boulders, the coldest air is protected from stirring winds and also chills the groundwater that runs underneath. This package of cold air and cold water wends its way downhill, until it reaches the place where the students sit feeling, well, cold. This effect is so strong that ice can be found here in July and only hearty boreal plants survive.

But a crucial question remains. Why did a cold-air woodland form on this one small terrace and nowhere else along the base of the wide talus slope? “Remember the geology maps we looked at this morning,” says Kolan. To prevent the cold air and water from simply draining out into the valley, something has to stop it, to make it pool here and only here.

“Is there a bowl in the bedrock that is catching the water?” asks Field Naturalist Katie Pindell.

Exactly. Directly below where the students sit, there is a dip in the underlying rock layers that geologists call a parasitic fold. The cold pours in and stays there.

“So,” says Sarah Bursky, a first-year master’s student in ecological planning, “we’ve been sitting in a natural freezer for the last hour.”