Why do leaves turn brown in autumn?
One of the delights of living at the top of a hill is watching the trees in the valley below come into leaf each spring. This wave of green sweeps up the hillside until it eventually includes the trees in our garden and surrounding fields.
Every autumn, we are then treated to our very own “firework display”, as the woods below turn a wonderful array of gold, orange, red and brown.
But why do deciduous trees change colour as the days become shorter
and the temperature drops?
The first question has to be: “Why are plants green?” This is due to chlorophyll, allowing plants to absorb energy from sunlight, like a natural solar panel. This is photosynthesis, memorised by generations of school children, in the celebrated equation:
Carbon Dioxide + Water (+ Light Energy) = Glucose + Oxygen The word ‘chlorophyll’ was coined in 1810 from two Greek words ‘chlorous’ meaning pale green, and ‘phyllous’ meaning leaf. Just over a century later in 1915 Richard Willstätter was awarded the Nobel Prize (the first ever for botany) for explaining how it works and what it does.
Curiously, this greenness we perceive as so important, is visible to us only because the colour green is of no importance to plants. Chlorophyll pigment is choosy when it comes to light, absorbing energy from the red and blue ends of the spectrum, but not from the middle, green region. As green light is not collected or absorbed by the plant, it is reflected back, which is why plants appear green.
Chlorophyll is the dominant pigment in leaves, but there are others that reflect different colours, yellow (xanthophylls), orange (carotenoids) and purple (anthocyanins). As leaves of deciduous trees cease to function in the darker days of autumn, the chlorophyll breaks down and the colours of these other pigments are then revealed, heralding the end of summer and the start of autumn.
One reason for the chlorophyll breaking-down first is connected with recycling and good housekeeping of limited resources. Plants take a lot of trouble to take up and transport nutrients, in particular iron and magnesium used in photosynthesis. The ability to reclaim these back before the leaves fall therefore conserves these valuable minerals.
Everyone should take at least two woodland walks each year. In spring for the flowers, and in autumn for those glorious colours. Why not take a look yourself this weekend?
Ian Brand
