Werner Kurz of Natural Resources Canada in British Columbia and colleagues have calculated that Canada's managed forests could become a source of between 30 and 245 Mtonnes of carbon dioxide "equivalents" per year from 2008 to 2012. (A carbon dioxide equivalent, or CO2e, includes non-carbon dioxide greenhouse gas emissions converted to units of carbon dioxide). This represents 4 to 33% of Canada's greenhouse gas emissions in 2005.

Kurz and co-workers say that the forests, which were carbon sinks for two years in 2000 and 2001, became a source for greenhouse gases in 2002 because of projected fires and widespread insect outbreaks. The insects, which include pine beetles and spruce budworms, infest forests and kill trees. The researchers predict that the forests will remain a net carbon source until 2022.

Fire, insects and other natural disturbances, such as wind and drought, could turn other managed forests around the globe into carbon sources, say the scientists.

Forests remove carbon dioxide from the atmosphere and store it in wood, dead organic matter and soil carbon. Forests also provide timber, fibre and energy. "Management can affect how much carbon is stored in forests and how much is available for harvest," Kurz told environmentalresearchweb. "How society responds to the threats and impacts of future natural disturbances on a forest's carbon balance will also affect future climate change."

For instance, logging trees killed by fire or insects rather than chopping down living trees for fuel and fibre is an option. "Helping forests rapidly re-establish after natural disturbances and thus accelerating carbon removal from the atmosphere is another example of how forest management can help contribute to solutions for climate change," said Kurz.

Another implication of the work is that future greenhouse gas accounting rules need to separate the impacts from human activities (like felling and land-use changes) from natural impacts (such as wildfire and insects). If this is not done, countries with forests that act as carbon sinks in response to climate change could be rewarded for reasons not directly related to human activities, while countries losing carbon due to natural disturbances could be penalized, explained Kurz. In either case, the rules do not encourage improved carbon management.

The researchers obtained their results by simulating future natural disturbances between 2008 and 2022 and quantifying the annual average greenhouse gas balance of Canada's managed forests during this period. They used a computer simulation model to predict the probability distribution of the balance. This Carbon Budget Model of the Canadian Forest Sector takes the state of the forests – such as their age, the species of trees present, projected harvest rates, areas annually burned and projected outbreaks of several important forest insect species – into account.

The work was published in PNAS.