Watching the trees: Mark Raleigh studies snow and forests

By Abby Metzger
May 2022

Trees have many stories to tell us about snow, as they uniquely affect the ways that it accumulates, melts and moves. Assistant Professor Mark Raleigh is listening to those stories, or watching them unfold rather, sometimes from within the towering trees and sometimes from far, far away using satellite images. His varied observations are giving him a bigger picture of forest hydrology, specifically the connections between snow, trees, climate change and water availability. Because snow is a significant portion of the water supply in places like Oregon, understanding these complex interactions may ultimately improve management of this precious resource—whether for farming, recreation or fish.

One fundamental problem Raleigh and others have been trying to solve is understanding just how much snow there is within a forest, which can tell us about water availability. We can’t simply stick a ruler into the snowpack in a single location, because snow doesn’t develop uniformly on forested ground. Some snowfall is intercepted by the canopy, where it may melt or fall as clumps to the forest floor. Some canopy snow never makes it to the ground because it is lost directly to the atmosphere through sublimation. And once on the ground, the timing and rate of snowmelt may be altered due to shading and heat from trees, depending on climate and forest characteristics. On top of these spatial complexities, direct snow measurements have been sparse.

“Historically, we’ve had at most a few locations in a given watershed where we measured snow, and these were typically away from trees, in forest clearings or meadows. But snow varies so much over different scales and landscapes,” Raleigh says. “The equivalent would be surveying five people randomly through the U.S. about their view on something. You’d have questions about the sample representativeness. And that’s what we have with snow.”

Laser focus

Remote sensing is one of the many approaches to quantifying snowpack, using satellites, drones or airplanes. One challenge in forests, however, is that thick stands make it difficult to discern the snow below. “In these forested environments, some remote sensing technologies can’t even see below the canopy,” Raleigh says.

An early breakthrough came almost a decade ago when NASA launched the Airborne Snow Observatory. The program flew planes over mountain ranges and mapped the depth of snow in a particular basin with LiDAR—a technique that uses pulsed lasers that can penetrate even vegetated areas and identify surface characteristics. These airborne LiDAR surveys can yield meter-scale maps that provide a comprehensive picture of variations in snow depth across a watershed.

The technique has been a boon for both hydrology research and water management. “It was unprecedented. We never had that kind of information,” Raleigh says. He participated in several NASA-funded studies, mostly while at the University of Colorado, that helped test approaches for integrating these remotely sensed observations into models that would fine tune snowpack assessments and help resource managers make better decisions toward a sustainable water supply.

Treetop sway

It is hard to hear the north wind again,
And to watch the treetops, as they sway.

They sway, deeply and loudly, in an effort,
So much less than feeling, so much less than speech
–Wallace Stevens

In addition to remote sensing, Raleigh keeps his eye on forest hydrology through on-the-ground observations. Since joining the geography faculty at Oregon State in 2020, he has gained national and international recognition for an innovative, six-year field study in Colorado that monitored how much snow gets intercepted by trees. Measuring interception is a big piece of the cryo-puzzle: Up to half of snowfall ends up in the canopy, depending on the forest. The fate of that canopy snow in turn affects snowpack on the ground and water availability as snowmelt.

Yet, directly measuring snow in the canopy is difficult, and current models that aim to predict snow interception are limited. To address these issues, Raleigh and colleagues at the University of Colorado Boulder devised an innovative solution. They strapped accelerometers to trees to measure their sway, similar to how your Fitbit measures your movement. The method was inspired by the work of John Selker and others at Oregon State who used tree sway to measure rainfall in canopies. Raleigh was interested in whether the technique could also work for canopy snow.

Mark Raleigh and colleagues strapped simple accelerometers onto trees to measure sway as a proxy for snow mass.

A confounding factor in Raleigh’s study was freeze-thaw cycles in trees, which also influence tree sway. After accounting for these cycles, Raleigh discovered that when trees are saddled with snow, they sway more slowly. The pattern of tree movement, therefore, could provide an estimate of canopy snow mass.

“The whole setup was a couple hundred bucks. We used duct tape and zip lock bags to keep the sensors dry and fixed to the tree boles, so it was low impact and low cost. And you can leave it unattended in a harsh environment,” he says.

Future vision

Outfitting trees with accelerometers was a proof of concept that Raleigh would like to take further. He imagines coupling the accelerometers with video cameras to measure sway at the larger stand scale. He recently purchased a drone with video and LiDAR capabilities to enable additional remotely sensed observations.

“Forests have so much complexity in terms of the shadows, the lighting of surface, which can make it difficult to see,” he says. “So, a LiDAR-enabled drone will fire lasers and get through the canopy. They may also provide another way for quantifying snow amounts within the canopy.”

Regardless of the technology he is using to measure forest snowpack, Raleigh takes a 30,000-foot view of the topic. “The snow that I’m measuring is the same snow I ski on. It’s the same water that feeds our rivers and helps grow our crops,” he says.

“There’s so much you can learn about these systems. It’s almost Zen in some ways, but trees really do tell us things if we’re there to listen.”

Photos courtesy of Mark Raleigh

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