A couple of weeks ago I had the brilliant idea of writing about why leaves change colors. Undeterred by the fact that now most of the leaves are on the ground, I’m forging forward, despite the fact that this has probably been explained on a million blogs by now.
A leaf’s green color is caused by chlorophyll, a molecule that the leaf uses to absorb energy from the sun to make food–the process called photosynthesis. Other colorful molecules exist in green leaves besides chlorophyll–carotene, an orange pigment; xanthophyll, a yellow pigment; and anthocyanin, a red pigment.
Not all trees have all pigments, though. Norway maples (Acer platanoides) have only green & yellow pigments; red maples (A. rubrum) have only greens & reds. What I would give to have one, just one, native red maple in my yard right now, instead of the sea of yellow-leaved invasive Norway maples!
During spring and summer, chlorophyll outshines all these pigments. Because spring and summer are the peak food-producing seasons, leaves constantly produce chlorophyll. As long as the tree is producing food, its leaves will be green (for the most part, realizing there are some trees with red or purple leaves or what have you).
But, as days get shorter and cooler, trees stop producing food. Chlorophyll production declines and the green color fades from the leaves as a result. The other pigments, which were there all along, now have a chance to strut their stuff.
Weather influences the intensity of fall colors. Cold temperatures slow chlorophyll production and boost anthocyanin production, as do bright sunshine and dry weather. Therefore, the most intense fall colors can be expected in dry periods where sunny days are followed by cool nights.
So, voila….better late than never: that’s why leaves turn from green to orange, yellow & red in the fall.