It looks like autumn paints the leaves, but mostly it does the opposite: it strips a colour away. All summer a leaf is packed with green chlorophyll, the stuff that catches sunlight to make food. Chlorophyll is such a strong green that it hides the other colours sitting right beside it. Those other colours, the yellows and oranges, are there the whole time. When the days get shorter and the nights turn cold, the tree knows winter is coming and stops making chlorophyll. The green it already has crumbles away, and suddenly the hidden yellow and orange show through. Some trees go further and make a brand-new red colour from sugar that gets trapped in the leaf. Slide the simulator toward autumn and watch the green fade out while the gold appears and the red builds up.
Most people think autumn paints new colours onto green leaves. In fact the yellows and oranges were there all summer, hidden under green; autumn removes the green to reveal them, and only the red is genuinely new.
What's actually happening
For most of the year a leaf is a small green factory. Inside it, chlorophyll soaks up sunlight and uses that energy to turn carbon dioxide and water into sugar, the food the whole tree runs on. Chlorophyll is a vivid green, and there is so much of it that it drowns out every other colour in the leaf. We look at a summer tree and see nothing but green, and it is easy to assume that green is simply what a leaf is.
It is not. Tucked in alongside the chlorophyll, all summer long, are other pigments called carotenoids, the same family of yellows and oranges that colour carrots and egg yolks. They are quietly present the entire time, doing helpful work catching light the chlorophyll misses, but we never see them because the green sits on top. When autumn comes and the days shrink, the tree gets a signal that the growing season is ending. Holding onto leaves through winter is not worth it, so the tree begins to shut them down. It builds a corky layer across the base of each leaf stalk, stops topping up its chlorophyll, and lets the green it already has fall apart. As the green drains away, the yellow and orange that were hiding underneath finally show. Nothing new was painted on. A mask was simply removed.
Red is the exception, and the most dramatic part of the show. Trees like maples and some oaks do something extra: as that corky layer seals the leaf, sugars made on warm sunny days get trapped inside, unable to flow back into the branch. With bright light and cool nights, the leaf turns those marooned sugars into anthocyanins, brand-new red and purple pigments that were not there in summer at all. This is why the fieriest autumns follow a run of crisp, clear, frost-free weather, and why a maple can blaze one year and barely turn the next. So the gold of a birch and the scarlet of a maple come about in completely different ways: one is a colour revealed, the other a colour freshly made.
Autumn gold is the green fading to reveal colours that were always there, while red is a fresh pigment some trees brew from trapped sugars.
- 1Tear up a few green leaves (spinach works), put them in a jar with a splash of surgical spirit, and stand the jar in warm water until the liquid turns green.
- 2Dip the bottom edge of a paper coffee filter into the green liquid and leave it to creep up the paper for half an hour.
- 3Look closely: above the green band you will often see faint yellow, the carotenoids that were hidden in the leaf all along, separated out for you to see.
Common questions
Yes. Carotenoids, the yellows and oranges, are present in the leaf the whole growing season but are hidden by the much stronger green of chlorophyll. Autumn simply removes the green and reveals them.
Yellow is unmasked when chlorophyll breaks down, but red is a new pigment, anthocyanin, that only some trees make from trapped sugars. Maples and some oaks make it; birches and many others do not, so they only turn yellow.
The brightest reds need bright sunny days and cool but frost-free nights to trap sugars and drive anthocyanin production. A warm, cloudy or early-frost autumn gives a duller display, so the same tree varies year to year.