Why do Leaves change Colour in Autumn?

One of the great joys of walking during Autumn is the opportunity to enjoy the stunning, rich colours of our native trees, woodland and forests. Every year I find myself amazed to watch the transition from the multiple shades of green to the vibrant oranges, gold and deep reds. But why do deciduous trees change colour like this before their leaves finally drop off? Time to re-visit that 'O' level biology ....

Leaves contain a variety of pigments - chlorophyll plus a number of others known as carotenoids/xanthophylls and anthocyanins. The relative proportion of these pigments varies from species to species.

Chlorophyll gives leaves their green colour and is also the predominant pigment in Photosynthesis - perhaps the most important biological process there is. During summer, when temperatures are warm and there is a lot (!) of sunlight, the chlorophyll in leaves is continuously being broken down and resynthesised. As sunlight reduces and temperatures fall, the tree reduces the flow of nutrients to the leaf and the regeneration of Chlorophyll slows until eventually there is none left in the leaves.

Carotenoids give leaves their orange-yellow and orange colours (such as in the Beech). Xanthophylls produce the brighter yellow colours. Anthocyanins are present in the leaves of some trees (such as Maples) when sugar levels are high and produce a red or purple colour. These pigments are all more stable than Chlorophyll and so linger much longer in leaves as temperatures fall - so changing the predominant colour of the leaf. The actual leaf colour even amongst the same type of tree can vary depending on its age, how much sunlight it's had during the summer months and soil conditions.

And good news - there is a bonus from the rather awful summer this year in the UK as suggested by Simon Toomer, the Forestry Commission's Director at Westonbirt Arboretum.

"Because of the less than glorious summer we have experienced in the UK this year, we expect to see prolonged autumn colour well into November due to the mild, damp weather conditions and no shortage of water."

For a some Autumn walking ideas that include woodland visit here.

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