16 December 2014
Growing forests with fire
Posted by kwheeling
By Nicholas Weiler
In the summer of 2012, Heather Alexander traveled to a remote larch forest in eastern Siberia, gathered together piles of dry twigs and branches, and lit a match. Alexander, a biologist at the University of Texas at Brownsville, is a fire starter. Her work aims to understand whether increasingly common fires in the boreal larch forests of north-eastern Siberia are unleashing more carbon into the atmosphere or, paradoxically, helping the forests capture and store atmospheric carbon by promoting the growth of new stands of trees.
Over the past 30 years, as the climate has warmed, forest fires around the world have become more frequent and more intense, Alexander said at a poster session Monday at the American Geophysical Union’s Fall Meeting. Boreal forests like the Siberian larch stands where Alexander set her fires contain a significant proportion of the world’s carbon: approximately twice as much as currently floats in the atmosphere.
A worrying possibility is that as atmospheric carbon warms the planet, increased forest fires will release even more carbon into the atmosphere, producing a vicious cycle that could significantly worsen the warming climate. But fires also have a positive role in the health of boreal forests. They clear land for new growth and thaw the permafrost to allow young trees to take root. Alexander wondered how this new growth changes the overall impact of forest fires on atmospheric carbon.
“If they accumulate and store as much carbon as fires release, then the total effect is neutral,” she said.
In the summer of 2012, Alexander and her colleagues traveled by river to the remote Northeast Science Station in Cherskii, in far eastern Siberia. At the time there was no way for her and her team to access any naturally occurring forest fires, and their Russian hosts were nervous about letting her spark a large fire of her own. So the team made do with setting controlled fires in small plots.
“We took a campfire approach,” Alexander said.
The plots with the least fuel barely scorched the moss and organic matter that makes up the top layer of soil in these forests, but the most intense fires burned deep into this organic layer, leaving it grey and ashen.
These severe burns enhanced the summer thaw of the underlying permafrost, Alexander said, leaving the soil warmer and moister all summer. And once the permafrost was disrupted, it stayed disrupted. When the researchers returned the following two summers they found that the permafrost in the heavily burned sites still thawed faster and deeper.
The enhanced thaws are good news for larch seedlings. The researchers planted larch seeds in each plot at the end of the first summer, and over the following two years found that the most severely burned plots produced the most successful saplings.
Alexander says it’s pretty clear that increasingly severe fires in Siberia’s boreal forest could ultimately offset the carbon they release by promoting denser larch stands. On the other hand, she said, there are many other factors at play that weren’t captured in the current study. For instance, changing how the permafrost thaws could also destabilize the landscape, leading to erosion that could expose carbon that had been buried for millennia.
The next step, she said, is to examine how fires actually change the density of larch stands over time, and to project how this affects the carbon cycle of boreal forests under a warming climate.
Nicholas Weiler is a science communication graduate student at UC Santa Cruz.