Boreal Forest Has Retreated Southwards

Evidence from ancient charcoal indicates the boreal forest historically extended much farther north than it does today.

Areas in northern Quebec that are now covered in arctic tundra were once forested in black spruce.

At lower elevations in the Rivière Boniface watershed, the undulating topography grows stands of spruce trees. Farther up, these give way to spruce krummholz, a distorted form with matted branches that seldom rises more than a couple metres off the ground. Hill tops above 165 metres elevation only support tundra dominated by lichen and heath. This is as far north that trees grow along the eastern flank of Hudson Bay.

Scientists from University of Laval pieced together the forest history on 40 square kilometres in the watershed. They used clues such as fire scars on stems and buried charcoal from wood that burned as long as 3500 years ago.

Though fires in the region are infrequent, only one-tenth of the area has escaped burning during the last two millennia. Remnant unburnt patches harbour old-growth forest relicts that have been around for perhaps 3000 years.

The rest of the spruce stands regenerated after wildfire, repeatedly on some sites where fires revisited. But all the patches of trees and krummholz that came back after fire did so 2250 to 950 years ago.

During the last thousand years, tree seedlings have rarely succeeded in establishing after a fire. Most burns in the last 950 years destroyed old-growth forest, and none of it has grown back to more than 10 percent tree cover. Overall, the landscape has lost 70 percent of its forested area in the last 1800 years.

Fires were especially frequent during a period from AD 1550 to 1750, and the land they swept across still remains deforested. Spruce trees and krummholz occupied 35 percent of the hilly terrain 2250 years ago, but today take up less than 1 percent. In place of spruce grows lichen-tundra vegetation.

The study's authors attribute the lack of new trees to a change in climate beginning 950 years ago. Around that time mean annual temperature likely dropped by at least 1 °C. Chilling of the north prevented trees from re-establishing there after a forest fire.

All the woodlands and krummholz now in the area got started when temperatures were higher than they have been for the last thousand years. Although the region has warmed since the mid-1990s, the trend isn't yet long enough for the boreal forest to recover from centuries of fire and retreat.

The history unveiled at Rivière Boniface indicates that a large expanse of forest tundra has vanished from eastern Canada over the last thousand years. These days, one has to travel about 170 kilometres (105 miles) southwards to find any spruce forests re-establishing after fire.