John Denver sang, “Sunshine on my shoulders makes me happy.” Apparently, sunshine shining on a maple tree can make it red.
Take a drive on a sunny October day and look closely at the trees along the road. On some maples, especially leaves on the sun-facing shoulder may be brilliant red, while those on the shaded side are still yellow or even green.
And that isn’t an illusion.
We’ve learned a lot about fall color since I was in school. I was taught that green chlorophyll masked the pigments in leaves. When chlorophyll was no longer being produced and began breaking down, those colors were revealed.
And that’s still basically true of the yellows and oranges.
But red is different.
The red pigments, anthocyanins, often weren’t hiding beneath the green at all. Many trees actually manufacture anthocyanins in their leaves during autumn. And one thing that encourages their production is sunlight.
Which raises an interesting question: Why would a tree spend precious energy making red pigment in a leaf it is about to throw away?
Researchers are still debating that. But one well-supported explanation is photoprotection.
An autumn leaf is shutting down, but it isn’t simply dying. The tree is dismantling its chlorophyll and reclaiming valuable nutrients, especially nitrogen, before sealing off the leaf and letting it fall.
Meanwhile, sunshine keeps shining.
As the leaf shuts down, it can absorb more sunlight than it can safely use. Anthocyanins seem to act somewhat like sunscreen, helping protect the leaf while the tree finishes salvaging what it can.
So on a bright autumn day, leaves receiving the most sunshine may manufacture the most anthocyanin. That’s why the exposed outer canopy, and often the sunny side of a roadside maple, may blaze brilliant red while shaded leaves remain yellow or even green.
John Denver probably wasn’t thinking about anthocyanins when he sang “Sunshine on My Shoulders.”
But it makes me happy to take an October drive and realize that while sunshine may not make a maple happy, sunshine on its shoulders can make it red.
