Changes in the treeline zone
The term "treeline" is deceptive – there is not a sharp line where trees end, but rather a zone of transition from increasingly sparse trees to tundra. Treeline zones in Canada are both latitudinal, across the north of the country, and altitudinal, on the slopes of hills and mountains. The emerging picture is one of change, but not a uniform expansion of the treeline. In northern Quebec, trees in the forest-tundra zone have grown faster and taller since the 1970s19 but distribution of trees has not changed greatly.20 In Labrador, treelines have expanded northward and up slopes over the past 50 years along the coast, but not inland.21 In the mountains of northwestern Canada, tree growth and density have changed more than the position of alpine treelines.22
A study on the treeline in western Canada found only a small net increase in tree cover, but major changes in vegetation within the treeline zone. Tree cover increased in the northern half of the zone, but this was mainly offset by decreases in the southern half, especially west of the Mackenzie Delta – likely related to drier conditions due to higher temperatures.24 The biggest changes were an increase in shrubs and, in the northwest of the treeline zone, a replacement of lichen cover and bare land with small, non–woody plants (herbs).
Since 1900, treeline has advanced at 52% of the 166 sites examined around the world and has receded at only 1% of the sites.25
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