Why do tree leaves turn darker in summer?

As the seasons progress from spring to summer, the light green colouration of leaves darkens. Naturalist, broadcaster and wildlife detective Ed Drewitt explains why this occurs.

Sunlight through beech leaves. © Richard Osbourne/Getty

Spring in temperate climates is a carnival of colour. As days lengthen and temperatures rise, the electric-green of fresh foliage transforms the dull landscapes of winter. Yet several months later, those bright leaves have turned dark green, giving woods a duller appearance.

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Spring foliage is a magnet for a multitude of caterpillars and other insect larvae, which can’t wait to get their jaws on the soft and supple, cellulose-rich leaves. So, to repel this army of leaf-munchers, plants defend themselves by producing dark tannins.

These bitter-tasting chemicals are a natural pest control. They become activated when the leaf is eaten, and are capable of damaging the mouth and stomach proteins of attacking insects. As summer wears on, the leaves get ever darker and more distasteful as tannins build up to fend off the onslaught.


Do you have a wildlife question you’d like answered? Email your question to wildquestions@immediate.co.uk or post it to Q&A, BBC Wildlife Magazine, Immediate Media Company, Eagle House, Bristol, BS1 4ST.

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Main image: Sunlight through beech leaves. © Richard Osbourne/Getty