Why do tree leaves change color in fall? If you have forgotten this stuff from your school days, it’s time for a refresher. Plus, more is known today than years ago. For starters, did you know the colors aren’t actually changing but—to put it more accurately—different colors are revealed? I’ve kept this simple so you can master the facts and impress your friends.
Also, if you are raking fall leaves, find out why you should not send them away. There’s a lot of value for your garden in there!
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Why Tree Leaves Change Color in Fall
Here’s how it works
This is generalized, and there are exceptions, but overall, here’s how it works.
- Tree leaves contain chlorophyll, which, as you may recall from school, enables photosynthesis.
- Photosynthesis (very basically speaking) converts the energy in sunlight to carbohydrates (starches and sugars), which provide fuel for trees.
- Chlorophyll is naturally pigmented green.
- Other pigment colors—hues of yellow, orange, and brown (xanthophylls and carotenoids) are also present in leaves, but the green of the chlorophyll is so abundant, that’s all we see in spring and summer.
- Lots of chlorophyll means nice, green leaves.
- Trees ‘know’ the tender leaves cannot survive freezing weather temperatures so they make as much use of them while they can.
- By early summer, many trees have already formed their buds for next year’s leaves, and start storing fuel (from the leaves) in their roots, branches, and buds for next year’s growth.
So, at this point we have green leaves with lesser amounts of yellow, orange, and brown pigments hidden inside.
Summer solstice marks the time when days become shorter and nights are longer.
The trees sense this, of course.
If you love learning about the inter-relationships between nature and the seasons—see phenology for more.
- As days become shorter and temperatures start to cool, chlorophyll production slows and gradually stops.
- Less chlorophyll means less green pigment in the leaves, which is why the existing yellows, oranges, and browns start to show. As we said, they were there all along but now they become visible. A-ha!
- Separately, anthocyanins are produced, giving red and purple pigment colour. Why some trees use valuable energy to produce red pigments in fall so close to leaf drop remains a mystery.
- Once the tree has made as much use of the leaves as possible, storing the carbohydrates in the roots, branches, and buds, the veins seal off and leaf drop occurs.
So that’s it for the colours. Did you remember it from school? Keep reading to learn why the intensity and timing of colours seems to vary each year.
What Affects the Quality and Timing of Fall Leaf Colours?
Lots of things.
Light levels and the presence of chlorophyll have the greatest influence on leaf colours. The timing of autumn leaf colours and which colours appear also vary by tree species, temperatures, and moisture levels throughout previous and current growing season.
Here’s some examples of why fall leaf colors vary.
- A cool fall season with good moisture levels gives the brilliant reds and purples.
- Warm, dry fall days weaken the color show.
- A late spring or summer drought can delay fall leaf colour changes by weeks.
What is the Value of Fallen Leaves?
Nature is one smart whippersnapper. Nothing goes to waste.
- As leaves decompose, they provide nutrients to the soil and the many organisms that reside there. They also absorb and hold moisture, slowing releasing it into the ground.
- This is why it is so important to let nature do its thing and not remove fallen leaves from your yard. Yes, clear leaves off grass lawn (thick, damp leaves will smother grass), but hang onto them. I use a mulching lawnmower to speed up the process and add the leaves to my garden beds and leaf mold pile.
- Chlorophyll gives leaves their green colour.
- Other color pigments (yellow, orange, brown hues) are also present in leaves, though not visible due to the abundance of chlorophyll during spring and summer.
- As chlorophyll production drops off, the green colour disappears, revealing the yellows, oranges, and browns.
- As fall nears, some trees also produce a red leaf pigment.
- Sunlight levels have the greatest influence on leaf colours.
- Moisture levels during the growing season will also affect the brilliance: better moisture provides brighter colors.
- Tree species type and weather patterns also play a role.
Unless diseased, keep your fall leaves for mulching your garden and making compost.
~Melissa the Empress of Dirt ♛
- The Science of Color in Autumn Leaves | The United States National Arboretum
- Why Do Fall Leaves Change Color | National Geographic News
- Why Leaves Change Color | USDA Forest Service
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