Shotgun Sampling: Researchers Bag Leaves to Better Understand Trees’ Response to Drought

by Sydney Widell – Kevin Gauthier glances behind him and to both sides before he loads his shotgun and raises it carefully to his shoulder. His gaze follows the steel line of the barrel up into the treetops. Exhaling, he squeezes back gently on the trigger.
After the sound of the explosion fades, the forest air smells vaguely like gunpowder. A red squirrel flees down the trail.
“This isn’t my everyday shot,” Kevin says, neck craned back, studying the treetops.
I watch as a flurry of red pine needles tumbles from the upper canopy, and I try hard to trace their path to the forest floor. Kevin hit his mark.  

Brian breathes into a bag of pine needle samples. Once it’s fogged over with moisture, he seals it and puts it in a cooler. It won’t entirely prevent the samples from losing moisture on their way to the lab, but it slows the process. Photo: S. Widell

Kevin is an undergraduate researcher at Trout Lake Station. Shooting trees — with permission from the proper authorities, of course — isn’t part of his summer project, but he’s here in this small patch of woods between Trout Lake and Highway M today as part of a nearly all-station effort to help PhD student Dom Ciruzzi collect high canopy leaf samples from trees spread across almost 300 square miles of forest.
As his team of sharpshooters spreads out through the Northwoods for this flurry of activity, Dom admits that this is an unusual method of getting scientific samples. But it’s not without precedent. “This protocol has been used for leaf sampling for at least the past 60 years in forest research,” he says. “The first paper I found “shotgun” in relation to leaf sampling was in the 1960s.”
Not all fieldwork requires giving a heads-up to Wisconsin Department of Natural Resources (DNR) wardens and mandatory gun-safety training, but the particulars of this project are unusual.
Dom is studying the way access to groundwater impacts how different tree species respond to drought stress –  research that could offer insights into forest and water resource management.
Today, a plane has flown north from Madison to collect aerial imagery of his research transects.
If all goes well, the remote imagery will show Dom how much water the oaks, pines, aspens and maples he is studying are using.
The remote imagery is one of several components rounding out the broader scope of Dom’s research.
Dom examines a leaf sample inside a pressurizer. By measuring the amount of pressure it takes to force water out of a leaf, he can calculate how much water was in the leaf to begin with. Photo: S. Widell

In addition to collecting spatial data from the air, he is also working with trees on the ground, gathering information on sap flow, tree sway and water table depth — all of which can be used to measure the amount of water stress a tree is experiencing.
“The overarching questions are where, when and how much groundwater do trees use to reduce their vulnerability to drought,” Dom explains. “We observe tree growth and tree stress and we also observe where the water is — in the soil, in the tree, in the tree’s leaves. All of those things are connected.”
Dom likes to compare trees to giant straws. During the day, they constantly filter water out of the ground through their roots and exchange it for atmospheric carbon through their leaves. The entire process is called transpiration.
“A leaf is a pretty good indicator of the whole plant,” Dom says. “It is where transpiration occurs, and that interaction between the tree and the atmosphere is what we’re trying to pin down.”
During times of drought, trees have less water to trade for carbon. Since carbon is a building block for organic growth, less carbon uptake by the trees means a less productive forest.
But even during dry periods, trees in this region still have access to some groundwater. Dom wants to know just how much groundwater access can “buffer” trees against the effects of drought.
Dom hopes he will be able to scale up water status data from individual trees to the rest of the forest using the remote imagery from the flyover. In order to do that, he will have to calibrate it against leaves taken from the actual canopy.
When the leaves Dom needs to make those calibrations are located more than 50 feet above the ground, collecting samples can be a challenge.
Crazy as it sounds, Dom has found that shooting at the high canopy is one of the best ways to gather the leaves he needs.
After Kevin uncocks his 12-gauge, Brian Schlaff, one of Dom’s undergrad assistants, carefully scans the trail for the pine needles Kevin’s shot sent down. Brian inspects each set to make sure it is intact before he places it in a plastic bag.
“Time?” Brian asks, with a sharpie paused expectantly above the bag.
“11:35,” Kevin answers, glancing at his iPhone.
Back on station, students have taken time out of their own project to assure Ciruzzi’s samples are processed as quickly as possible. Photo: D. Ciruzzi

The timing is important. Dom only has a three hour window around the plane’s flight to collect and process his leaf samples. After that, the leaves may have lost enough water to render his calibration process inaccurate.
“It’s very important that all of these measurements are taken coinciding with the aerial imagery,” Dom says. “Once the leaf is separated from the tree, they are no longer in equilibrium, and it can’t say much about the tree if it’s left alone for too long.”
Brian slips the needles into a cooler and we hike out of the woods to our car. It’s time to drive the leaves back to the lab for processing.
The scene on station is organized chaos.
Fourteen researchers have paused their work to assist Dom in his massive leaf processing effort. In the next two hours, they will need to weigh, pressurize, scan and package almost 200 leaves taken from Dom’s sample trees.
Undergrads are shuttling leaves from lab to lab for different rounds of testing and analysis, and others keep arriving from the field, bearing more coolers full of samples.
“It’s an amazing, all hands on deck kind of vibe,” Dom says. “It’s like an assembly line.”
Dom is roving from room to room, checking in on his team’s progress. He’s been awake since 3:00 this morning, running predawn samples.
“I’m just so thankful for all the help,” he says. “This wouldn’t be possible otherwise.”

This is Part 1` of a 3-Part series on an ambitious summer research project exploring how trees manage their water supplies and respond to drought. Stay tuned for Part 2 (Wed. Aug. 1st) and Part 3 (Fri.  Aug. 3rd)!


TOP PHOTO: Kevin Gauthier, undergraduate researcher, aims his gun high into the canopy. He’s hoping to bag a few red pine needle samples for PhD candidate Dom Ciruzzi’s study on trees and groundwater. Photo: Sydney Widell