Science Questions

Why do leaves turn red in autumn?

Tue, 21st Oct 2014

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Question

Amelia asked:

Why do leaves turn red in autumn, and how do they know when it's time to drop?

Answer

Now, weíre entering autumn, you may have all noticed the beautiful red and orange colours of the leaves on the trees, but also, the substantial piles of them gathering on the ground.  But what exactly causes this to occur?  We spoke to Professor Beverly Glover, the director of Cambridge Universityís botanic gardens who got to the root of this problem...

Beverly -   So, weíre sitting here in the garden looking at some of the trees changing colour and there's lots of nice oranges and reds.  The leaves are changing colour because at this time of year, deciduous trees withdraw the nutrients from their leaves because they're going to drop those leaves off so that they don't waste particularly the proteins, the nitrogen.  The leaves is very difficult for trees to get a hold of and they don't want to waste any.  Leaves on trees and other plants are usually green because they contain a pigment called chlorophyll and it does the photosynthesis.  It gives the plant its energy from the sunlight.  So, when they breakdown the green chlorophyll, that reveals the yellow and orange pigments called carotenoids that were there all along anyway.  but they're not so easy to breakdown, so they last longer.  The reds we see on some of the trees are produced by  new pigments that are being made by the trees.  These are pigments called anthocyanins.  We don't really know why trees make red.  Some people think itís to protect the leaves from UV damage.  Other people think itís actually a signal to insects to say that these leaves arenít good to eat anymore, and so, don't waste your energy and time coming here.  So, the leaves fall off deciduous trees because they can't really do anything useful over the winter.  There's not going to be enough light or warmth to photosynthesise.  Actually, having a lot of leaves can be quite dangerous if itís very windy.  It adds to the surface area of the tree and makes it more vulnerable to losing branches or even being blown over entirely in strong winds.  So, this whole process starts off because day length starts to decrease.  That sends signals around the plant to start this process of breaking down the chlorophyll and getting the leaves ready to shed.  So, people often ask if climate change is likely to affect this process.  It wonít affect the onset because that's regulated by day length and of course, the reason trees use day lengths to decide when itís winter is that itís the most reliable cue.  It doesnít matter what the weather is doing.  If the days are getting shorter then winter is coming.  So climate change wonít affect that, but the degree to which itís warm will affect how quickly the process occurs and how long those lovely colours are around for, and how quickly the leaves are shed.

 

 

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