December 2025 Ramsbury Nature Notes by Peter Marren

Honey fungus

The autumn colours have been particularly intense this year. Passing though Savernake on the way to Marlborough the effect on the trees in full sun was like sunlight streaming through stained glass, all vivid yellows, golds, copper-browns, and even red and purple tints, as well as the leaves that still remained green. Why are some years better than others? I suspect that our sunny, warm summer had something to do with it, but only something. The process of how deciduous trees prepare for the winter by shedding their leaves is quite complex. Trees react mainly to falling daylength, which is why the process begins at roughly the same time each year. This year the process has been prolonged by mild temperatures and the lack of strong winds. But what of the colours? Before leaf-fall, the tree withdraws the more nutritious bits, notably the green chlorophyll, the pigment that does most of the photosynthesis and so effectively feeds the tree. What is left are the hidden yellow colours which are carotenoids, the same pigments that are found in carrots and pumpkins. Some trees, like birch and field maple, simply go yellow. But another group of pigments are produced by trees like beech and oak, which provide the rich brown and reddish colours of their autumn leaves. These are anthocyanins and are produced towards the end of the season. Unlike carotenoids, they do not produce food by photosynthesis, so what is their biological purpose? One theory is that the red colours are insect repellents. Insects like aphids cannot see red, and so for them the leaves look dark, in other words dead, and so not useful as food. Finally, the leaves of some trees, like ash, stay green until they fall. Why? Maybe because ash grows on rich soils rich in minerals. It doesn’t need to withdraw its chlorophyll because it has enough nutrients already. As for Christmas holly, it may be evergreen, but it nonetheless sheds its oldest leaves. But it does so in the spring without changing colour, and so no one notices!

There’s space only to mention fungi, which have had a good year, perhaps because the trees produced so much sugar this year that they have it to spare, at least to fungi growing on their roots. Honey fungus has been particularly prolific, and I was interested to see that it is a key ingredient of dried fungi sold in supermarkets. Does it taste of honey? I didn’t go that far!