Did you know why some leaves change their colours in autumn?  We didn’t, so we decided to find out.  It turns out it’s got a lot to do with cold weather and a thing called chlorophyll.

Most deciduous trees have broad leaves that can be damaged or frozen during cold, dry, and windy weather.  By dropping their leaves, trees conserve water and energy, and reduce themselves to their strongest parts – their trunks, branches and bark – ready to survive harsh weather.

During their growing season, trees create chlorophyll, which they use for photosynthesis.  It’s the pigment that makes leaves green.  In autumn, when days become shorter and nights become cool, plants save energy and reabsorb nutrients in their leaves and store them in their roots for later. Chlorophyll is one of the first molecules that gets broken down for nutrients.

As chlorophyll is broken down, other colours in the leaves that are usually eclipsed by the green appear.  Carotenoid pigments (also found in carrots and banana peel) produce yellow, brown and orange colours, while anthocyanin pigments (also found in blueberries, cherries and grapes) produce red and purple colours.

Trees can sense a change in day-length thanks to their chemical light receptors.  Environmental factors can influence the timing of leaf changes – summer drought can delay the changes, a warm autumn can tone down the colours.  Street lights can also disturb the cycle.  (Have you seen some greener leaves around street lights?  Have a look for them.)

Most Australian trees are evergreen, not deciduous, because our climate is much more variable than many other places in the world.  Good growing conditions can happen almost anytime, so the plants need to be ready to take up water whenever it rains.

Fun fact – The process of shedding leaves is called abscission.  It shares the same Latin root as scissors: scindere, which means ‘to cut’.

You can separate pigments in autumn leaves yourself, if you want.  Try this leaf chromatography activity at home.