Every autumn, forests in temperate regions turn yellow, orange and red within a few weeks. It looks like the trees are adding colour, but much of it was there all summer, hidden under green.
Key Takeaways
- Leaves are green because chlorophyll dominates during the growing season.
- As days shorten, trees stop making chlorophyll and break it down, revealing yellow and orange carotenoids.
- Reds and purples come from anthocyanins, which many trees produce fresh in autumn.
- Sunny days and cool, but not freezing, nights tend to produce the most vivid reds.
Green: chlorophyll
Chlorophyll captures light for photosynthesis, and it is constantly being broken down and replaced during the growing season. There is so much of it that it masks other pigments.
The trigger: shorter days
As nights lengthen in late summer and autumn, deciduous trees begin preparing to drop their leaves. A layer of cells called the abscission layer forms at the base of each leaf stalk, gradually cutting off flow between leaf and branch. Production of chlorophyll slows and stops, and the tree breaks down and reabsorbs valuable nutrients such as nitrogen from the leaves.
Yellow and orange: carotenoids
Carotenoids, the same family of pigments that colours carrots and egg yolks, are present in leaves all year. They help capture light and protect leaves from damage. When chlorophyll fades, carotenoids show through. Birches, aspens, hickories and many maples turn yellow this way.
Red and purple: anthocyanins
Anthocyanins are different. Many trees produce them in autumn, from sugars trapped in the leaf. Scientists have proposed several reasons, including protecting leaves from light damage while nutrients are being reabsorbed. Red maples, sumacs, sweetgums and some oaks are known for reds.
Brown: tannins
When other pigments are gone, tannins can leave leaves brown. Many oaks hold brown leaves into winter.
Why some years are brighter
- Sunny autumn days help leaves produce sugars.
- Cool nights above freezing slow the flow of sugars out of leaves, encouraging anthocyanin production.
- An early hard frost can kill leaves before colour develops.
- Drought in late summer can cause leaves to brown and drop early.
- Heavy rain and wind can strip leaves before peak colour.
Why evergreens stay green
Most conifers have needles with a waxy coating and a shape that reduces water loss in winter. They keep their needles for several years, replacing them gradually.
Why do trees drop leaves at all?
Broad leaves lose a lot of water and are easily damaged by snow and freezing. In winter, frozen ground limits water uptake. Dropping leaves helps deciduous trees conserve water and avoid damage.
Frequently Asked Questions
Do leaves change colour in the tropics? Many tropical trees are evergreen, though some drop leaves in dry seasons.
Does colour vary by location? Yes. Species mix, climate, elevation and weather all affect colour.
Why do some trees in the same area peak at different times? Species differ, and so do individual trees’ conditions, such as sun exposure and soil moisture.
The bottom line
Autumn colour is chlorophyll leaving the stage. Yellows were waiting underneath, and reds are made new each autumn, with the weather deciding how bright the show will be.
