The following text was the September 2002 sidebar content in our Boundary Waters & Quetico calendar. Photo credit: Paul Sundberg. Lake Agnes.
In the BWCAW and Quetico, peak leaf color is usually from the middle to the end of September. As early as July, single red maple leaves add bits of color to the portages. Entire red maple trees begin turning in lake August and, since they are slow to drop their leaves, they add brilliance to the landscape until snow or a hard freeze. The ash trees often turn yellow before the end of August and are the first to lose their leaves.
The yellow, orange, and brown colors of autumn leaves are pigments called carotenoids (like carrots, which they also color). These pigments are present in the leaves all summer, but are masked by green chlorophyll. When deciduous trees stop making food in preparation for winter dormancy, they also stop making chlorophyll. This lets the carotenoid colors show.
Reds and purples — in leaves, berries, apples, and other fruits — are a different kind of pigment called anthocyanin. These are formed from the metabolism of sugar in bright light. Warm days promote sugar production, cold (but not freezing) nights trap the sugar in the leaves, and sunshine provides bright light, so the most dramatic leaf colors occur during falls with warm, sunny days and cool nights. Cool, rainy falls reduce anthocyanin production, and we must be satisfied with carotenoid yellows and oranges.
