October 30, 2011

Life and death

In the parks and public places around us, pyracanthas have caught sudden fire. These tough, easycare shrubs are much favoured by councils: they withstand pollution, grow slowly and can be persuaded to fan out and cover ugly walls and fitments.

Happily, firethorns are also great for birds, particularly our native blackbirds and thrushes, and migratory redwings and fieldfares, all of which use them to stock up on body fat before the depradations of winter.

Elsewhere, bright colours mean decay, not life. As the green chlorophyll in leaves breaks down, the brighter oranges and yellows of carotenoids are revealed, and, in some species, red or purple anthocyanins are also synthesised – at some cost to the tree. Finally, mould begins to consume the decaying leaf – although sometimes all three processes occur at once, as on this London plane leaf.

Despite all our sophistication, debate still rages about this most simple yearly phenomenon: the changing colours of autumn leaves. Are the red anthocyanins a warning to overwintering aphids, a method of protecting the tree's last stores of chlorophyll or even a way of stunting the growth of competing trees? We don't know. But we enjoy their red blaze nonetheless.