Question: Why do leaves change colour in autumn?

Andrew Jackson answered on 20 Nov 2012:

awesome.. i was hoping someone would ask this! I have been working with Dr Sam Brown in Edinburgh recently on other questions, and in the past he has worked on the evolution of leaf colour change some years ago. Its fascinating, and not as easy an answer as my colleagues put forward below.

The ideas my colleagues put forward is that leaves age in someway with winter approaching, and as a consequence of this ageing process, they happen to turn orange, red or brown and then fall off the tree. There are two problems with this simplistic explanation.

First of all, the dropping of leaves by trees does not always occur along with a change in colour of the leaf. Many species of trees do not produce red and orange colours when then drop their leaves (and instead drop green leaves). Even more interestingly, trees of the same species show huge variation, with some individual trees dropping green leaves, and other turning red and orange before dropping. All of this implies that the traditional autumn colors of leaves do not necessarily go hand in hand with leaf drop.

Secondly, leaf ageing and loss of leaf health is not a good enough explanation for the change in colour. The trees actively produce new coloured compounds within the leaves during autumn. It would seem crazy for a tree to actively spend energy making its leaves turn bright colours if they are going to drop in a few days or weeks. Certainly, evolution would not put up with such a waste.

One of the theories for why trees might actively turn their leaves bright colours was put forward by a real science hero of mine, the late William D Hamilton – an amazing evolutionary biologist. Dr Sam Brown had the pleasure of working with him in his latter years, and helped to advance and publish his theory on autumn leaf colours.

The theory mirrors other bright colours in nature. A lot of times, animals and plants use bright colours to warn off predators that might attack them – bees, ladybirds, poisonous snakes and frogs are all toxic or venemous and often brightly coloured to warn others. The basic idea for the bright autumn colours of leaves is that it tells insect pests that would attack the trees, that the tree is in really good health and that its not worth wasting effort trying to attack the tree. Instead, the insect should go and find a less healthy tree that is not able to produce the bright autumn red colors. This theory works for both trees of the same species who compete with each other every year to survive and reproduce, and it also works for explaining competition between species.

While more work needs to be done to categorically prove this theory, it has solid backing from mathematical models and evidence from several experiments.

So… look more closely at trees and dont get fooled into thinking that every tree of every species is turning red and orange. This variation is key for evolution to select between the winners and the losers in the game of life. I think its fascinating to think that trees are trying to talk to insects on some very basic level and tell them: “get your grubby little 6 legs off me and go chew on a green-top stick down the road!”