Autumn colours

Bill Meek celebrates nature’s changing seasons.

The magical colours of autumn are one of the delights of living at a temperate latitude, and provide some compensation for the dreary winter ahead.

This is the time of year when ornamental and native trees complement each other beautifully in a transformative explosion of colour.

All leaves contain a number of pigments. First amongst these, and most abundant, is chlorophyll – which makes photosynthesis possible. So dominant is this pigment that it makes leaves in summer simply appear green. Chlorophyll is degraded by light (just like ink fades on paper when left in the sun) and must be constantly replaced.

But leaves also contain carotenoids and flavonoids – yellow, and yellow, orange or red respectively – which are normally hidden by the abundance of chlorophyll.

As the days become shorter and cooler in autumn, chlorophyll is the first pigment to break down, ‘unmasking’ the colours of the others beneath.

But the very deepest reds and purples are caused by another set of compounds, which are not present throughout the year – these are the anthocyanins. These pigments are created afresh by the leaf in autumn, in response to a build-up of sugars in combination with bright sunlight. Interestingly, when tree sap is more acidic, redder colours are produced, with more alkaline sap creating purples.

Native trees
Many of our native trees show consistent colour changes in autumn. Beech will turn a brilliant translucent golden orange, with oak a more opaque russet.

Ash, our most common wild tree, flushes a smoky olive yellow before the leaves fall. Our roadside limes turn a fine yellow which contrasts with its dark twigs, while wild cherries may show simultaneous yellow and vivid red. Alder does not change at all, the leaves falling while they are still green.

The maples are famed for their autumn colours. We only have one native maple, the Field Maple, but another, sycamore, is common everywhere and abundantly naturalised – an ‘honorary’ native. Both turn rich yellow in autumn.

It is around the edges of towns – in parks, cemeteries and the older, leafier suburbs – where the variety of planted species makes autumn colours the most extravagant. Here, dark, unchanging evergreens and purple-leaved varieties of beech, cherry or maple contrast with native trees, street plantings and exotics celebrated for their autumn colours, such as ornamental maples and False Acacia.

In gardens, Virginia Creeper (though not a tree) and Stag’s horn Sumach provide some of the deepest and richest reds of all.

In terms of the display of colours in autumn, not every year is the same. Colour tends to be most intense when hot, bright autumn days are coupled with cold, clear, but not freezing nights.

During fine days sugar is produced in the leaves, but cold nights stop it from being re-absorbed so readily, facilitating the development of red anthocyanins.

If strong winds or frosts come too early, a disappointingly short display can be the result. If the autumn remains calm and bright throughout, a colourful, prolonged and ever-changing spectacle may last well into December.

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