In autumn, the changes of light levels and temperature give the signal to deciduous shrubs and trees to respond to the season. The leaves of some trees turn bright or dark yellow, orange, brown, red and pink. The leaves of others stay green until they drop, and I was surprised to learn that according to a study of deciduous shrubs and trees in various temperate regions, that's most of them. Only 12.1% turn red, 15.8% turn yellow, the rest remain green until they fall.
The yellow/orange pigments, carotenoids, become visible as in the Liriodendron tulipifera in the walled garden. The pigments have been there all along, but were hidden by the dominant chlorophyll. The trees will loose the nutritional value of carotenoid pigments, but keep the more essential nutrients. Other trees like Alnus glutinosa in the lower garden- this genus lives in symbiosis with nitrogen fixing bacteria - loose their leaves at the moment they are still green.
The pigment anthocyanin, giving the leaves of most trees red to purple colouration, is actively produced. It is generally not present when the leaves are still green, however in the early process of anthocyanin production, before chlorophyll is completely broken down, it may mix with anthocyanin to produce a brownish colour. One currently supported theory documents the red colour as a signal to insects like winged aphids to stay off the tree, as a protective mechanism to prevent aphids laying their eggs on the tree, thus making it also less vulnerable to viral or bacterial pathogens, which can be transmitted by aphids.
|Prunus x subhirtella 'Autumnalis'|
There are other pigments giving leaves a bright yellow colouration, such as 6-hydroxykynuric acid in Ginkgo biloba.
That's only a small insight into the mechanisms deciduous, woody plants have adapted as a response to their interactions with insects, organisms and weather conditions in temperate climates.
Tonia Friedrich, Trainee Botanical Horticulturist.