Autumn Leaves

Autumn has now truly greeted us with the cold misty mornings and changes of the leaves. Emma Nord provides an insight into the chemical reactions happening behind the changing colours of leaves in autumn.

Walking around the University of York campus, there is the smell of leaves and the sight of the crown of the tree decreasing in size as the leaves are dancing to the ground. Everything seems to be transforming. What is actually happening in nature when autumn embraces it?

One apparent characteristic of autumn is the new colours of the trees, shifting from green to orange, red and brown. However there is much more to it than an aesthetic feature. During the summer months photosynthesis makes the trees grow. Energy from the sun is used to turn carbon dioxide, water and light into chemical energy (glucose) and oxygen. This chemical reaction is taking place inside the leaves starting with a green pigment called chlorophyll. Other photosynthetic pigments colouring the leaves are carotenes (giving a yellow colour) and anthocyanins (giving a red shade).

A pile of fallen leaves. Image Credit: Emma Nord.

When the autumn and winter months are coming, trees will lose their leaves in order to save valuable energy and water so as to not dry out. This is the driving force behind leaves changing colour and descending the trees. The intensity of the shifting of colours depends on temperature, dryness and if there are sunny days allowing photosynthesis to happen with the remaining chlorophyll. A very yellow forest in autumn mostly stems from cold nights where the chlorophyll is demolished, making the leaf turn yellow. Whereas a red-leaved forest means that temperature has been above zero and therefore allows production of red leaves.

A red colour also occurs when there is little rain, making the concentration of sugar in the leaves high, sequentially producing anthocyanin to turn the leaves red. The reason for the leaves to lose their grip on the branches is due to the decreasing production of auxin, a hormone in the tree that triggers extension in the leave-connection and splits within that will later cause the leaves to fall or be blown away.

Having fallen from the tree the leaves that are assembled on the ground after a while will start to combust. It is from this combustion the smell of leaves comes from. The decomposition process is a fascinating one. Bacterias and fungi are taking complex carbon chains apart and turning them into carbon compounds that are much simpler. This process is part of the everflowing carbon cycle where carbon is transformed through two interconnected cycles. The decomposers (fungi and bacteria) release carbon dioxide as part of their cellular respiration of the fallen leaves.

It is difficult not to be in awe of the beauty nature creates in autumn times. Hopefully you now can explore the outdoors with new eyes on what is actually happening. Not only that, the poem from Robert Frost now gains another meaning, with the yellow leaves signifying recent cold nights…

Golden trees. Image credit: Emma Nord.

Two roads diverged in a yellow wood,

And sorry I could not travel both

And be one traveler, long I stood

And looked down one as far as I could

To where it bent in the undergrowth

…

Two roads diverged in a wood, and I—

I took the one less traveled by,

And that has made all the difference.

- The Road Not Taken by Robert Frost

About the author: Emma Nord is from Sweden originally but is now a postgraduate student at the University of York. This is her first article for Wild. When she is not reading or writing she enjoys going for long hikes, ballroom dancing or wild swimming.