Until recently Britain’s annual leaf turn has been colourful, but muted in comparison to the New England ‘fall’. Now climate change could be set to liven things up. The Woodland Trust’s Nick Collinson and Shaun Nixon demystify the underlying science.

For many people autumn is a time of beauty, when the natural world treats us to a last burst of colour before the onset of winter. The annual colour change and leaf shedding that is characteristic of broad-leaved trees is intertwined with photosynthesis. As most people know, this is the process by which plants make food, converting carbon dioxide and water into sugars using sunlight to power the reaction. The sun’s energy is captured by chlorophyll and converted to sugars which are either stored in the leaves or transported to other parts of the tree. The chlorophyll is a green pigment continually produced in leaf cells throughout the growing season, not least because it is destroyed by bright sunlight. Conversely, it is not produced when there is little or no sunlight. As summer turns into autumn, the shorter days and cooler nights trigger two major changes in the leaf, both of which have consequences for its colour.

Less well known is the presence of carotene. This yellow pigment is also constantly present in leaf cells throughout the growing season and plays a secondary role in photosynthesis. In spring and summer leaves it is masked by the green of the chlorophyll, but unlike the latter it is not affected by sunlight and temperature. Thus as temperature and day length reduce in autumn and chlorophyll production slows down and eventually stops, so the yellow carotene is revealed.

At the same time a layer of corky cells forms across the base of leaf stalk, in preparation for leaf shedding, which restricts the movement of sugars back to the main part of the tree. When weather is sunny and dry at this time, sugars can become concentrated in the leaf and can produce a further pigment known as anthocyanin.

Nick Collinson is head of conservation policy and Shaun Nixon is project development officer at the Woodland Trust. 

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