The band of deciduous forest which extends roughly from the Blue Ridge Mountains to northern Quebec is unusual in the world for its autumnal color. The only other extensive swathe of forest that rivals it stretches across parts of East Asia. Deciduous trees elsewhere do not exhibit the spectacular mixture of reds and yellows. In Europe and midwestern United States, trees typically turn only yellow.
The reason for this geographic difference remains a mystery to scientists, though there are intriguing theories. The appearance of yellow leaves is understood. With the diminished light of fall, the green chlorophyll in leaves departs, allowing existing yellow pigment to appear. Red autum leaves are the consequence of a different process. With the drop in chlorophyll, anthocyanin, a red pigment, is produced in the leaf. This is a relatively new discovery and has led to several theories about why trees use energy to create red pigment when they are shedding their leaves.
According to an hypothesis published recently in the journal New Phytologist and discussed in this ScienceDaily article, the different climatic conditions during ice ages 35 million years ago are responsible. The bands of forest in North America and Asia developed red pigments to ward off insects that were unable to survive the harsher conditions in Europe and other parts of the world where deciduous trees also evolved.
A recent headline in a Vermont newspaper said, They Came, They Peeped, They Left. As the “leaf peepers” who crowd New England’s backroads know, these trees are beautiful in panorama. But the greatest effect is from inside, from within the woods or even a small stand of maples and poplars. The golden pillows, Persian carpets, and glowing lanterns of leaf have a singular effect on the human heart. They create a mixture of nostalgia and anticipation. They speak to the fugitive within, as Robert Frost noted in this famous fall poem:
I have been treading on leaves all day until I am autumn-tired.
God knows all the color and form of leaves I have trodden on and mired.
Perhaps I have put forth too much strength and been too fierce from fear.
I have safely trodden underfoot the dead leaves of another year.
All summer long they were overhead, more lifted up than I.
To come to their final place in earth they had to pass me by.
All summer long I thought I heard them threatening under their breath.
And when they came it seemed with a will to carry me with them to death.
They spoke to the fugitive in my heart as if it were leaf to leaf.
They tapped at my eyelids and touched my lips with an invitation to grief.
But it was no reason I had to go because they had to go.
Now up, my knee, to keep on top of another year of snow.