Madison, USA — Far removed from streams of gas-thirsty cars andpollution-belching factories lies another key player in global climate change.Circling the northern hemisphere, the conifer-dominated boreal forests – one ofthe largest ecosystems on earth – act as a vast natural regulator of atmosphericcarbon levels.
Forest ecologists at the University of Wisconsin-Madison are studying howenvironmental factors such as forest fires and climate influence carbon levelsin this forest system. Their most recent findings, reported in the Nov. 1 issueof the journal Nature, offer insight into the balance of carbon uptake andrelease that contribute to atmospheric carbon dioxide levels worldwide.
Second in size among forests only to the tropical rainforests, the borealforests form a massive green band spanning the higher latitudes of Canada,Alaska, Siberia, China, and Scandinavia. Their sheer size, coupled with the factthat they are expected to experience the greatest warming of any forest biome asglobal temperatures rise, means that climate-related changes here are likely toresonate well beyond the forest boundaries, says S. Tom Gower, UW-Madisonprofessor of forest ecology and management and primary investigator of theproject.
In the new study, Gower and his colleagues used a computer model to simulatethe carbon balance of one million square kilometers of the Canadian forest overthe past 60 years, to determine the relative impacts of climate and disturbanceby wildfire.
The group found that the effects of carbon dioxide and climate – temperatureand precipitation – varied from year to year but generally balanced out overtime and area. Instead, forest fires during the 60-year period had the greatestdirect impact on carbon emissions from the system.
However, “because fire frequency and fire intensity are directlycontrolled by climate change, it doesn’t mean that we shouldn’t be focusing onclimate change,” Gower says. “Climate change is what’s causing thefire changes. They’re very tightly coupled systems.”
The researchers believe that fires shift the carbon balance in multiple ways.Burning organic matter quickly releases large amounts of carbon dioxide. After afire, loss of the forest canopy can allow more sun to reach and warm the ground,which may speed decomposition and carbon dioxide emission from the soil. If thesoil warms enough to melt underlying permafrost, even more stored carbon may beunleashed.
A trend toward hotter and drier conditions is likely to exacerbate theeffects of fire by increasing the frequency, intensity, and size of burns.”All it takes is a low snowpack year and a dry summer,” Gower says.”With a few lightning strikes, it’s a tinderbox.”
Historically, scientists believe the boreal forest has acted as a carbonsink, absorbing more atmospheric carbon dioxide than it releases, Gower says.Their model now suggests that, over recent decades, the forest has become asmaller sink and may actually be shifting toward becoming a carbon source.
“The soil is the major source, the plants are the major sink, and howthose two interplay over the life of a stand really determines whether theboreal forest is a sink or a source of carbon,” he says.
Though the model is not currently designed to forecast future conditions,Gower says, “Based on our current understanding, fire was a more importantdriver (of the carbon balance) than climate was in the last 50 years. But ifcarbon dioxide concentration really doubles in the next 50 years and thetemperature increases 4 to 8 degrees Celsius, all bets may be off.”