Every now and then some saucy commenter on a beaver article who thinks he’s seen the big picture remarks that beaver dams release carbon into the atmosphere and contribute to global warming – there by undoing all the good things beaver dams do. Since Ellen Wohl is very beaver-savvy researcher from Colorado State University and the one who wrote definitively about beavers shaping America, I think her study was motivated by finer impulses, but parts of it still makes me a little anxious.
Now, it appears that beavers play a complex role in climate change, too. A new study suggests that beaver dams and the sediments corralled behind them sequester carbon, temporarily keeping greenhouse gases containing the element out of the atmosphere. But when the animals abandon these sites, the carbon leaks back out, contributing to global warming.
Did you get that? When beavers are actively maintaining the dam they are sequestering carbon. But when they abandon the dam – OR ARE TRAPPED OUT – those broken dams release carbon.
In recent fieldwork in Colorado’s Rocky Mountain National Park, Wohl studied the wetlands and floodplains upstream of beaver dams—areas collectively called “beaver meadows”—along 27 streams draining watersheds covering more than 700 square kilometers. She analyzed the carbon content of 29 samples of sediment collected along two of the larger waterways (one of which included remnants of 148 beaver dams, and the other had 100). Then, she combined these data with results from previous research to estimate the carbon content of beaver meadows throughout the region. Altogether, beaver meadows occupied about one-quarter of the total length of major streams in these watersheds, she notes.
Because the water table is elevated behind an intact beaver dam, oxygen can’t get to much of the wood and other organic matter buried in sediments there, so it decomposes more slowly. In fact, Wohl says, wood buried in soggy beaver meadows can last about 600 years—longer than a typical log that falls in the forest. But when the water table drops and the soils dry out, decomposition begins to release carbon dioxide to the atmosphere.
In work published last year, Wohl and her colleagues found that sediment upstream of active beaver dams in the park contained about 12% carbon by weight, most of it locked in wood. But in research to be published in a forthcoming issue of Geophysical Research Letters, Wohl reports that, on average, only 3.3% of the sediments in abandoned beaver meadows is carbon.
Hmm what’s the moral of the story? Beaver dams trap way more carbon than they release and KEEP THE BEAVERS THERE! I’m sure she meant to add at the end of her scholarly work. Come to think of it here’s an example of why,
John and Janice Collett were hiking up Thomas Creek early on the morning of July 4, stopping to admire the waterfall above the campground before setting out for the top of the canyon. “A main attraction of this hike for many is the huge beaver dam and pond about a half-hour up,” Janice Collett said.
“We stopped 15-20 minutes into our hike for a breather at a favorite swimming hole where the creek winds through granite walls and drops in a waterfall … “John and I were about to continue our hike when I heard an ominous roar and glanced upstream to see an exploding chocolatey wall of mud, entire trees and debris rushing toward us.”
I guess that washout released a lot of carbon, but I’m more worried about the beavers themselves. Being as we’re talking about eastern Nevada in July new water will be hard to come by. It’s a long walk on webbed feet to anyplace resembling home. Grimly best case scenario, those beavers were already predated by mountain lions or coyotes and that’s why the dam washed out in the first place. Sigh.
Beaver ponds can indeed be large sources of potent planet-warming greenhouse gases such as methane and nitrous oxide, says Jennifer Edmonds, an aquatic ecologist at the University of Alabama, Tuscaloosa. (Over the course of a century, methane traps heat in Earth’s atmosphere about 25 times as effectively as carbon dioxide; nitrous oxide does so almost 300 times as effectively over the same interval.) But considering the whole landscape, she adds, “if I had to bet, I’d bet that [the beaver meadows] are storing more carbon than they’re producing.”