My city looks most beautiful in autumn. Credit goes to the trees lining the streets – Maples, Ginkgo, Claret Ash, Red Oak, Japanese Pagoda, American Sweet Gum, Chinese Pistachio, English Elm, and many more – and the shades of yellow, orange, brown and red their leaves transform into.
These temperate deciduous trees shed their leaves after changing colour from the normal green. This annual fanfare rekindles in me a line from Tagore’s song “jharaa paata go”:
“jharaa paata go basanti rang diye
shesher beshe sejechho tumi ki e”
[Fallen leaves, what colourful parting dress you have adorned yourselves with!]
Science has figured out the what and how of autumn colours and is still trying to make sense of why.
Autumn colouration doesn’t happen to all deciduous trees everywhere. It happens in temperate regions only. In other regions deciduous tree leaves usually just turn brown and fall off. Brown merely indicates dead cells.
Yellow and red and their various shades make autumn a visual feast. And they do represent two different mechanisms of colouration. Interestingly, their geographical distribution is also different. Yellow dominates the European autumn whereas North American deciduous trees usually go red.
Colours in plants are associated with pigments. The universal chlorophyll imparts green colour to normal foliage by absorbing other wavelengths. Shades of yellow and orange come from another type of pigment – carotenoids. These pigments are present all the time but the dominating chlorophyll masks their colour. Chlorophyll decays faster during autumn. This process unmasks the yellow-orange of the carotenoids.
The red colour comes from an entirely different mechanism. The pigment anthocyanin, responsible for the red colour, is produced during autumn as chlorophyll levels are falling. Anthocyanins also give red colour to fruits, but they are produced in the leaves of only certain species.
The reason for yellow colour is pretty obvious from the above. Winter is a period where temperate life switches to maintenance mode. Metabolism is at an all time low. Animals prefer to stay quiet. Invertebrates find relatively warm areas under the ground surface or beneath the fallen leaves. Most mammals hibernate. Life practically comes to a standstill. Plants in such regions also go through their own hibernation though scientists use the term dormancy for them. The idea is to somehow just survive the winter. Autumn is the time to prepare for that.
If you have enough food stored for survival, why spend energy making food? The fuel supply (sunlight) is also not encouraging. Let’s shut down photosynthesis factories and eventually dismantle them. But the leaves possess many useful nutrients. Though eventually everything goes into the soil and enriches the earth’s life system, the tree wants to retain some of the nutrients for itself – notably the precious nitrogen. Chlorophyll content of the leaves decrease for this reason in the autumn. The shades of yellow and orange of carotenoids – masked hitherto – show up.
The red colour is puzzling because the pigment responsible – anthocyanin – is actually produced in autumn. Why would the tree spend energy producing something when there is actually a need to conserve energy?
There are many theories. Some say that their antioxidant behaviour and/or absorption of harmful high energy rays prevent destruction of photosynthetic plant tissues. But why would plants want to protect photosynthetic structures in leaves that are about to fall? Carotenoids attract aphids that suck sap from leaves. Red colours in evolved species could help repel such insects, says one theory. Another reason given is that anthocyanin in leaves helps trees resorb more nitrogen in the trunks and less in the decaying leaves thus conserving the valuable resource.
They may be serving to warn the animals to prepare for the winter. The red colour could dissuade herbivore predators from eating the leaves, but this ecological reason appears to be more valid for young red leaves especially in the tropics than for the senescing autumn leaves of the temperate regions.
It could be a signal for animals to start preparing for the winter. Birds migrate to warmer parts of the globe. Animals such as squirrels store acorns and nuts for the winter. Bats breed during winter hibernation. Early autumn is mating time for them. The males show off their singing skills. Many other animals too have a last hurrah in the autumn before going quiet. The appearance of red, as per one theory, could simply be a warning for animals to go look somewhere else for survival. ‘Beware! We are about to fall.’
Whatever the motive, as the life’s sun goes down, o falling leaves, you brighten up my remaining soul just as Tagore wishes:
“ostorobi laagak poroshmoni
praaner momo shesher sombole