Why green leaves are sometimes red

Breath-taking autumn colours help to compensate for the passing of summer. I love watching how leaves change over the year: they start with bright, fresh promise in spring, then they mature, fade and leave the stage in dramatic style. But why do fading deciduous leaves produce such a dazzling display?

Even if the September gales have put paid to so many leaves, their spell hasn’t yet been broken. From my window, I’m captivated by a fine old elm at the bottom of the garden. A yellow pencil line sur-rounds some faded leaves; in others, the yellow has suffused all but the very centre; yet more leaves are purest yellow.

While deciduous trees are preparing for dormancy and photosynthesis is slowing down, they no longer need the pigment, chlorophyll.

Chlorophyll plays a key role in photosynthesis and its green colour masks those of other pigments in a leaf, so once its dominance fades, the other pigments start to become visible. As a general rule of thumb, a predominance of the pigment carotene produces yellow leaves, as we can see with Witch Ha-zel, Hamamelis vernalis. The fiery reds in Viburnum carlesii have a high proportion of anthocyanins, and the orange in Acer palmatum ‘Cascade’ occurs when the two pigments are roughly balanced.

Have a closer look at your shrubs and you’ll see how all their subtle shades are evolving. During a break while writing this, I caught anthocyanins at work on my Syringa meyeri. The exposed outer leaves were already much redder than the others. The overall impact is a feast to the eyes.

Plants produce anthocyanins that create antioxidants to reduce the risk of cells being damaged by strong sunlight. But why should trees waste any energy protecting leaves against UV light when photo-synthesis has stopped? Why not just let them drop?

This is a very complex problem which scientists haven’t cracked yet, but recent research outlined by David Lee in 2007, gives us a pointer. In 2003, a major study found that the red leaves in trees that produced large amounts of anthocyanins contained less nitrogen than those with yellow leaves. The scientists believe that these trees are able to reabsorb nitrogen which they then store in the trunk. They would find this invaluable the following spring.

When I look at the ancient alder woodland bordering our smallholding, I wonder if these nitrogen-fixing trees look so dull and drab in the autumn because they’ve got plenty nitrogen already.

Although we think of red leaves in autumn, this colour often occurs in young spring foliage as well. I’s thought that anthocyanins could be working to protect leaf cells here as well as in the autumn. It could be preventing damage from strong sunlight before photosynthesis gets properly underway and tougher leaf surface cells have grown.

This protection applies to many trees, especially those from the tropics where the sun is always strong. But it could also be a strategy used against herbivores. After a severe caterpillar attack, oaks produce a fresh flush of reddish leaves, possibly to confuse attackers.

Red can also be a harbinger of death, as our stressed, drought-stricken plants have shown all too clear-ly during the summer. This also happens with mosses. Earlier in the year Spaghnum species are fresh and green, but by the end of summer they can turn bright red through lack of water.

You may have seen recent reports that the moss fields of Eastern Antarctica have turned red. As a re-sult of climate change, the whole region is much drier than usual. Presumably stressed plants are pro-ducing anthocyanins to limit cell damage.

1. David Lee ‘Nature’s Palate’ 2007

Plant of the week

Geranium ‘Rozanne’ flowers for months and triumphantly finishes the year with a blaze of autumn leaf colour. Shades of red and purple infuse the green leaves; voted plant of the RHS’s centenary.