Why trees drop their leaves
Sure, we’ll mourn the loss of beautiful blooms and leafy, shade-giving trees. But what we get in return is that famous fall foliage and—when you understand the reason for the changing leaves—a really cool lesson in resourcefulness.
First and foremost, deciduous trees drop their leaves as a survival mechanism. If they didn’t, then wet, heavy snow could accumulate on them in winter, causing branches to break from the extra weight. No branches, no leaves. No leaves, no life.
And in another miracle of nature, trees know better than to let those nutrients in the leaves go to waste. So before dropping them, they recycle the nutrients into their roots and stems to feed the tree through winter. This process of reuptake is part of the color change we all know and love.
What’s chlorophyll got to do with it?
Green leaves are high in a pigment called chlorophyll. While other pigments are always present, chlorophyll dominates because it’s the powerhouse behind photosynthesis. This is the process of converting not just sunlight but also water and carbon dioxide into sugars and oxygen.
So what happens when the temps start dropping? Shorter days with less cumulative sunlight signal to the plant that winter is coming. Leaves produce a cork-like barrier around the stem to halt the flow of water and nutrients, and photosynthesis slows down (since there’s less sun to power it). Without a need for photosynthesis, chlorophyll production slows down. Finally, as chlorophyll wanes, other pigments become more visible.
These other pigments fall into two categories: carotenoids and anthocyanins.
- Carotenoids: Sound familiar? These pigments that give pumpkins and carrots their characteristic hue do the same for leaves, showing up in bright yellows and oranges. They’re also considered antioxidants, protecting leaves from the harmful byproducts of photosynthesis (a little like sunscreen). They’re present year-round but only become visible when chlorophyll fades.
- Anthocyanins: Unlike carotenoids, these pigments aren’t produced until chlorophyll has already begun to break down. They’re most active when warm, sunny days are followed by cool nights and present as shades of orange, red, purple, and blue. Leaves in direct light will generally appear brighter red, while those that are shaded will be less pigmented.
Fun Fact: Have you ever wondered why growing leaves appear green to the human eye? It’s all about light. Light can either be absorbed by, reflected from, or transmitted through a surface. Leaves absorb all colors except green, which is then reflected back to our eyes.
How weather affects leaf color
Beyond the basics of “more chlorophyll = more green,” there are several weather-related variables that can influence fall leaf color.
- Day length. As we get closer to winter solstice (the shortest day of the year), fewer and fewer sunlight hours will be available for photosynthesis.
- Cool, sunny weather. Sunny days followed by mild nights allow for optimal chlorophyll degradation and, thus, brighter-colored leaves.
- Hard freezes. When temperatures drop below 28ºF for more than a few hours, leaves can get damaged before chlorophyll is completely disassembled. This causes duller, more muted colors.
- Warm weather. Unseasonably warm temperatures tend to shorten the length of time that colorful leaves remain on trees, in addition to favoring oranges and yellows over red hues.
- Drought. Along with things like excess rainfall and nutrient deficiency, stress can either advance or delay color change, as well as leading to less intense colors or premature leaf drop.
Last but not least, there are a handful of non-weather variables that can determine when and how much leaves change their color. One is location, because lower-lying areas can collect cool air at night and change color earlier. The angle of the sun can also play a role, causing the side of the tree that’s exposed to more light to get brighter colors at a different rate than the side receiving less light. And if you see a tree that’s inexplicably displaying color earlier than those around it, you may want to check your tree for disease.
Are you ready to drop some knowledge next time the leaves start dropping? Arm yourself with these fun foliage facts! You can also visit The Shed to find more info on dealing with fall weeds and feeding your fall lawn.
Cited Sources
Changing Colors of Leaves. University of Tennessee Agricultural Extension Service.
Fall Foliage - The How and Why. University of Vermont Extension.
How weather affects fall colors. MSU Extension
Leaf Color Change in Autumn. University of Wisconsin-Madison Division of Extension.
Learn the Science Behind Fall Foliage. University of New Hampshire Extension.
Why Fall Color is Sometimes a Dud. Purdue University Extension Forestry & Natural Resources.
