Fig. 3.

Basal area of dominant tree species of northern hardwood (A, C, and E) and boreal (B, D, and F) forests by elevation for 1964 and 2004. Sugar maple (A), red spruce (B), American beech (C), balsam fir (D), yellow birch (E), and paper birch (F). The elevational range of the ecotone is indicated by the underscoring. The shift in the ecotone has been driven both by increases in northern hardwood species at their upper elevation limit (e.g., A, arrows) and decreases in boreal species at their lower limits (e.g., B, arrows).