Carbon has been a hot topic as of late, and one detail very relevant to forest ecosystem research is the way in which trees act as CO₂ storage banks by sequestering it within their physical structures. Calculating the carbonsequestering ability of trees requires measuring their overall mass, which is relatively easy when it comes to trunks and stems. Easy, when compared to the task of determining the rest of a tree’s mass—the subterranean tangle of roots.

That’s where Butnor’s invention comes in. His “skateboard” is designed to drag a radar antenna along the ground. If you’re used to seeing the sweeping radar screens used to scan the sky, this may seem a little strange. Ground Penetrating Radar, or GPR, differs in that, as the name suggests, a radio transducer directs waves into the ground and then “listens” for reflections. GPR has been used in archaeological, military, and civil engineering applications for years, but Butnor’s work is somewhat of a departure.

Traditionally, GPR used very longwavelength, low-frequency signals to penetrate tens of feet into the ground. Resolution was very poor, but it didn’t necessarily matter. “If you were using GPR equipment to clear a minefield,” says Butnor, “you’d say ’oh, a reflection, I’d better put a flag here.’ As opposed to saying ‘I want to see what’s down there. What’s the diameter of this root? How heavy is it?”

To get such hard data, Butnor’s system uses a much higher frequency signal to achieve finer resolution. This allows him to map out and measure the size and distribution of roots down to a diameter of a little under a quarter of an inch. Such high-frequency signals can only penetrate a few feet of soil—a perfect fit for this application since that’s where the vast majority of roots are located. The single survey wheel on Butnor’s skateboard sets off a radar pulse every few hundredths of an inch as it’s moved along a number of straight paths through a plot. Afterwards, the data is processed to form a grayscale picture of the roots.

The system still has its limitations. Rocky soil, for instance, tends to “confuse” the radar. A second limitation is logistical: Too many obstacles on the forest floor can be a hindrance when trying to drag such a large object around.

For these reasons, Butnor’s GPR system has been most successful in the more controlled, sandy, well-drained soils of southeastern forest sites, where the technology allows researchers to take repeated measurements in situ. The more traditional methods of taking soil cores and digging up, drying, and weighing roots are more destructive and don’t allow the type of long-term studies the team is doing. “You can get really good information when you dig up roots,” says Butnor, “but you can only do it one time.”

Sniffing Out the Soil

While Butnor’s GPR system helps to analyze the amount of carbon that trees are converting into mass and sequestering away, his Automated Carbon Efflux System (ACES) does the opposite, measuring the CO₂ being emitted from trees through transpiration and other processes.

While the mobile GPR platform looks somewhat odd, ACES looks almost alien. Resembling an electromechanical octopus, it consists of a central “brain” housed in what looks like a large red tool box connected up to 16 sensors designed to measure the CO₂ diffusing from the woody stems of trees or from the soil near their roots.

Butnor designed ACES from the ground up to provide more accurate carbon readings than similar systems of the past. Such systems used closed sampling chambers, which could skew CO₂ readings. In contrast, air in the sampling chambers of the ACES is recycled whenever it isn’t being actively sampled.

Butnor and his colleagues have constructed a total of 22 ACES units, which are currently in use at sites all across the country.