6 Dec 2012

Autumn leaves and a strange phenomenon

I was recently in Lubljana, the capital of Slovenia (see article here on this beautiful town), and I noticed a curious phomenon on a tree while walking around the castle that dominates the town. In a long line of horse chestnut trees, all of which were bare after the late autumnal loss of their leaves, the branches on the side of just one of these trees had not only retained most of their leaves, but these were still bright green (see photo) in late November. This lead me to investigate the mecanisms behind the changing colour of leaves in the auturmn, prior to their being shed by deciduous trees.

The mysterious chestnut tree in Lubljana, still with green leaves on part of it in late November

Why are leaves green or other colours anyway?

First of all we need to understand what makes leaves green. This colour is due to the presence of a substance called chlorophyll. This is in constant production during the growing season, as sap rises from the roots up to the leafy part of the tree or bush. It is utilised by the plant to absorb solar rays and produce food. When the sap slows down, and then ceases to rise, production of chlorophyll also slows down and then ceases. The last part of a leaf to change is, logically the vein system as these are the parts most loaded with chlorophyll. Leaves apparently contain as much yellow pigment as green during the summer months, but, when the stronger green colour ceases to be produced, the yellow shows through more and more. This explains why leaves turn yellow in the autumn. Of course some leaves may turn orange, red or brown. How come?

Different kinds of trees, and sometimes the same trees in different places (climates, soil types, environments will make for these differences), will produce a varied range of colours in the green leaves, but also in their leaves as these change colour before falling. One evolutionary theory about red colours in autumn leaves, for instance, is that these have some kind of insect-repellent function, particularly against aphids. In any case these pigments are either present in latent form in the leaves or not. Arotenoids make leaves turn yellow, orange, or brown. Not all leaves contain anthocyanins, another type of pigment which cause colors like red and purple to appear. Anthocyanin is formed when sugar gets trapped in a leaf after the chlorophyll is gone. Then, when the leaf is exposed to sunlight, the anthocyanin turns leaves red or purple.

So how and why do leaves fall off trees ?

If  I have understood it right, it is part of a survival system. Leaves contain water and this would freeze in the winter. The tree uses sunlight energy to form a kind of seal between the branch and the leaf. This area, called the abscission zone, swells when the sap ceases to pass throught it, causing the connection to be severed and the leaf to drop to the ground, where it finds a final use as compost to fertilise the soil as it decays. A protective layer is immediately formed where the leaf previously met the bark, stopping the entry of mushrooms or predators and preventing water evaporation. The annual change of leaves also eliminates certain predactors like insects which attack the leaves and live in them. Growing new ones each year is a another form of response to these attacks.

And why were the leaves still green and still attached to this part of just one tree?

Unless some prankster glued them on, I have no idea! If anyone has a suggestion, I would be very grateful. It has been observed that global warming and higher levels of CO2 have slowed down the colour-change process, but, if this was the case here, then surely all the horse chestnuts in this row would have retained some leaves. No there was just one, and the leaves were green! 

And it looks like our friend Richter has the changing colours of leaves in his mind when he painted this one

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