The vast northern forests of Canada and Alaska are moving northwards as a result of climate change. Also, the shift is faster at the southern boreal boundaries than the northern ones. The discrepancy indicates that boreal forests are contracting on a continental scale, according to a new study. The team used 20 years of satellite data.
Boreal forests are expected to shift northward due to climate warming as trees may be able to grow into areas that were previously too cold for them. At the same time, the climate may become too warm and dry in the southern regions of the boreal forest distribution where boreal forests have evolved under cold conditions. “The rates at which these two processes happen is key. the southern boreal may lose trees much faster than the northern boreal gains trees through expansion,” says lead author Ronny Rotbarth. “Such a mismatch would indicate a contraction of the biomes.”
The team used 20 years of tree cover data from satellites across the North American boreal biomes to study the change in tree cover along a south-to-north gradient. During this period, forests became denser in the central boreal and thinner in the southern boreal. The researchers found a discrepancy between high tree cover losses in the south and low tree cover gains in the north.
The team also compared how forests changed under the influence of fire and timber logging. Both disturbances increased tree cover losses more significantly in the southern boreal, where forests are already stressed due to warming.
Boreal forests are of great global importance, as they store large amounts of carbon in the biomass of trees and in the soil. A long term contraction of the region may release a part of this carbon pool, which could further increase global warming. “We do not expect that the forest dynamics we observed will slow down. On the contrary, warmer temperatures will increase forest fires and insect outbreaks, intensifying the loss of forests in today’s southern ranges,” says co-author Milena Holmgren.
Knowledge about the future health of the boreal forests could help us predict the impacts of forest changes on global climate. Jane Jepsen, a researcher in the study said, “Boreal forests are sentinels of change also affecting arctic and temperate ecosystems. Continental-scale biome shifts, such as the one documented by us, highlight the urgent need for long-term monitoring, particularly at the edges of our major forest ecosystems.”