Tree death toll from climate change likely underestimated

Climate change is likely killing more trees in the boreal forest than predicted because many scientific assumptions about them are wrong, a new Canadian study suggests.

Younger forests, drought-sensitive species more vulnerable

A 2004 photo shows aspen killed during the 2001-02 drought near the southern edge of the boreal forest in the South Saskatchewan River valley. A Lakehead University study suggests that younger forests tend to be more vulnerable to drought than older forests. (Michael Michaelian)

Climate change is likely killing more trees in the boreal forest than predicted, a new Canadian study suggests.

Models that predict the impact of climate change typically assume that older forests are representative of all forests. But older forests are less vulnerable to the effects of climate change than younger forests that make up the vast majority of Canada's boreal forest ecosystems, suggests a study of Alberta and Saskatchewan forests by Yong Luo and Han Chen, forest ecologists at Lakehead University in Thunder Bay, Ont.

The research published Wednesday in the journal Nature Communications also found that some tree species were far more vulnerable than others. Specifically:

  • Species that need more water, such as balsam and poplar, tend to be hit harder than drought-tolerant species such as jack pine.
  • Species that colonize forests later after a fire, such as black spruce and white spruce, are more sensitive than the "pioneers" that grow back first, such as aspen and jack pine.

That surprised Chen, who assumed, as most models do, that climate change would hit all species roughly equally.

Effects on other animals, plants

The fact that it doesn't could have implications about how climate change will affect forest ecosystems, as a decrease in the relative abundance of those species — or their disappearance altogether — will impact other plants and animals that rely on them for food, shelter, or other needs, Chen said.

"I think that's going to be a huge challenge for society as a whole," he added.

The differences in vulnerability of forests by age and species seems tied to their ability to withstand drier conditions that are linked to warmer temperatures.

"It really surprised us that younger forests were more sensitive to drought," Chen said. "We don't really understand what the real mechanism is."

4 out of 5 forests are young

The age-related sensitivity has significant implications because roughly 80 per cent of forests in Canada have an average tree age of less than 80 years, while just 20 per cent are "older" forests with an average tree age over 80 years. That's because forest fires typically occur in a given forest an average of once every 50 years, Chen said.

Chen and Luo analyzed tree mortality patterns in 887 sample plots in boreal forests in Alberta and Saskatchewan between 1958 and 2007 and related them to local climate change over that period to test the assumptions made in models predicting the effects of climate change on forests. Those models typically use data from older forests to represent all forests because researchers assume that they are more stable and the mortality of older trees depends less on other factors in the environment compared to younger forests, making it easier to tease out the effects of climate change, Chen said. The models also usually assume that similar effects are seen in all species.

The results of the new analysis suggest that current predictions underestimate tree mortality due to climate change, especially in forests with younger trees and those with more drought-sensitive species. They also imply that climate change will cause changes to the proportions of different species within a forest.

Chen thinks that the results can be generalized to all boreal forests, but he noted that local impacts will depend on the extent that climate change causes drier conditions in a given area, which is not known.

Nevertheless, he suggested more effort may be needed to maintain or conserve more climate-vulnerable species and to reduce climate-induced drought overall.

He added, "We really have to control CO2 emissions to reduce the greenhouse effect."