Autumn: when trees prepare for winter

Science

A magical transformation

Leaves are green - you'd think that was obvious. But in fact, it's only true during spring and summer. In autumn, leaves turn brown, red and yellow. Where do these colors suddenly come from?

Science

Only green leaves can photosynthesize

The deep green pigment in leaves is called chlorophyll. It absorbs sunlight and enables plants to produce sugars out of water and carbon dioxide. In its structure, chlorophyll resembles the red pigment in our blood.

Science

When fall arrives

When the days get shorter and darker, a tree knows that it's time to prepare for winter. It starts slowly breaking down its precious chlorophyll into small molecules and stores them in its trunk and roots - they are much too valuable to throw away.

Science

Red shines through

As well as the chlorophyll, a leaf also contains red and yellow pigments. In spring and summer, though, we aren't able to see them because the green color of chlorophyll hides all other colors. When the chlorophyll vanishes, the vivid red and orange colors finally get their turn to shine.

Science

Color palette

Carotenoids give leaves their golden or orange colors. Anthocyanins make them red or purple. But you can only see those vivid colors when the deep green stops cloaking all of the others.

Science

Get rid of it!

Once the tree has broken down all of the chlorophyll, it no longer has any use for its leaves - so they have to go. A layer between the branch and leaf forms and that cuts the leaf off from its water and nutrient supply. It dies off and falls to the ground.

Science

Why all this waste?

A leaf's cells contain a lot of water. If the water froze over in winter and formed ice crystals, the leaves would break anyway. So there is really no use in holding onto them.

Science

On a winter's break

It's much too cold for trees in winter, anyway. As all water freezes, trees don't get enough liquid water to perform photosynthesis. So, again, why keep the leaves? Naked and bleak, they hibernate, waiting for the days to get longer and warmer, so they can start growing leaves again.

Science

Some have summer all year round

Evergreen trees like those in the Amazon behave differently. As it never freezes over in those parts of the world, the trees don't need to hibernate. They keep their leaves and chlorophyll throughout the year. So the forests never turn a vivid red and yellow in fall in those regions, and it's a pity, isn't it?

Red, yellow, purple, pink. Each fall the forests and parks become more colorful than ever. By the end, the trees and shrubs have shed their leaves until they're naked. Why do it?