The long, hot, dry summer pushed many trees and shrubs into early shows of autumnalcolours.
Indeed, for many deciduous trees a process called ‘abscission’ where the base of the leaf stem cuts off from the living plant and dies, is a good response to severe drought helping minimise catastrophic effects of serious desiccation. So readers will have observed trees and shrubs with their colours turning and leaves falling prematurely. However, as we move through September, October and November, stimulated by the first frosts, leaf-fall proper kicks in; often just touching the tops of trees in certain places and also licking the leaves of other sensitive plants like Lady Fern, Bracken, and the like. The result is the spectacular transformation of the landscape and in some cases wonderful arrays of reds, yellows, browns, and golds that suffuse the countryside. The impact varies with the weather and often the first bad storms can rip many of the leaves from the trees and they descend to Earth as if they were some magical, multi-coloured, rain. In some years, the mild weather allows the trees to hold their colours for longer periods.
The processes involved are part of the essential annual lifecycle of the deciduous trees and are a way of the plants ridding themselves of the waste products of metabolism from their summer growing season. Most tree-leaves appear green to us because of the pigments involved in photosynthesis, especially chlorophyll. However, as the first frosts set in, the leaves have stored waste products from the work of the summer and the photosynthetic pigments breaking-down. Important nutrients are salvaged and stored in the living trunk and roots for the winter. Come spring, these will provide a kick-start for the following year’s growth. However, as the green pigments breakdown and the waste metabolites accumulate, the leaves assume their typical autumn colours and of course these vary from species to species and as the season and weather progress. Some trees such as Beech, Maples, Sweet Chestnut, and Horse Chestnut have especially stunning shows, and others like Birch have softer, gentler displays of pale yellow and gold.
Professor Ian D. Rotherham, of Sheffield Hallam University, researcher, writer and broadcaster on wildlife and environmental issues.