So What Makes Leaves Change Colors to Begin With?
Chances are you don't remember this lesson from grade school science class, so here's a quick refresher. To understand why leaves turn red, orange, yellow, or brown, you first have to understand why leaves are green. It all starts with photosynthesis and chlorophyll (maybe Billy Madison was wrong). Specifically, leaves appear green, thanks to the millions of chlorophyll cells that are generated during photosynthesis in the tree's chloroplasts, the organelles that carry out the process. And here's the thing: Without all of that green chlorophyll, leaves would always look like they do in autumn.
The constant creation of chlorophyll throughout spring and summer gives trees and other plants their vivid green hues because it masks other compounds in the leaves that would otherwise make them appear yellow and orange. In other words, those are the leaves’ true colors. In fall, photosynthesis slows and eventually stops due to the significant decrease in sunlight trees receive -- allowing the leaves to return to their "natural" orange, yellow, and red colors.
As the University of Vermont explains, each of the colors comes from specific pigments in the leaves: carotenoids and anthocyanins.
“‘Carotenoids’ are leaf pigments responsible for yellow and orange colors in leaves, particularly in fall. Think of orange, as in carrots. They are present during the season but masked by the green chlorophyll, except in plants that may be stressed or with yellow leaves normally,” per the university. “… ‘Anthocyanins’ are the pigments responsible for red and purple fall leaf colors. These are only produced in the fall when sugars are trapped in the leaves. They function similar to the carotenoids, and help the leaf use up any remaining energy as chlorophyll disappears. Abundant dry weather and sunlight lead to more sugars in leaves, which in turn leads to brighter fall reds. More red in leaves, and earlier reds, can come about too from plant stresses, low nutrition, and near but not freezing temperatures.”
The colors you see also depend on the type of tree. Some trees -- beeches, hickories, tulip poplars, and birches -- have mainly yellow and orange fall leaves. You’ll find red leaves on trees like sumacs, sweet gums, sourwoods, mountain ashes, scarlet oaks, red maples, and some sugar maples, according to UV.
When the nutrient-producing process finally ends, the trees will essentially seal the connections to the leaves, an as a result, the leaves will die, turn brown, and fall to the ground as fertilizer for the growth season in spring and summer. So, if you missed out on peak fall foliage this year, let the brown leaves crunching under your shoes serve as a reminder: the process will happen all over again next year.