AUTUMN LEAVES
AUTUMN is the season where we reflect on the passing of summer and the onset of the darker and colder days of winter ahead. While in autumn the days can be dull and dreary, some aspects of the natural world can more colourful than ever. Summer flowers have been lost but deciduous trees and shrubs treat us to a spectacular show of colour and brightening up the landscape. In temperate regions deciduous trees shed their leaves to conserve resources in autumn and in the dry season in tropical and sub-tropical deciduous forests. The shedding of leaves also makes them less vulnerable to being blown over during these windier times. The word deciduous is from the Latin word decidere, meaning “to fall off”.
During the spring and summer months, leaves are busy producing the nutrients necessary for growth. They do this through the process known as photosynthesis where the simple molecules of water and carbon dioxide react to form glucose and oxygen. The light energy required to drive this chemical reaction is absorbed by green pigment in the leaf, chlorophyll, and leaves are predominantly green throughout the growing season.
The changes in daylight and temperature triggered by the arrival of autumn are detected by the trees and the process of photosynthesis gradually shuts down. At the same time, valuable nutrients within the leaves are reabsorbed by the tree and stored in the roots. Chlorophyll is one of the first molecules to be broken down, resulting in the loss of the green colour. Any remaining sugars in the leaf are converted to compounds known as anthocyanins which produce red, purple and pink pigments. Varying weather and underlying chemical processes can have a dramatic impact on the depth and colour range in autumn leaves. Bright sunny days can still produce photosynthesis. This concentrates sugars in the leaves and results in the production of more red anthocyanin pigments. Dry weather can also increase the sugar concentration, further enhancing the red tones.
Cold nights destroy more chlorophyll – producing more yellow areas.
A combination of cold nights and dry sunny days can deliver an amazing range and intensity.
Shedding of leaves is promoted by auxin, a plant hormone. As light levels fall and ambient temperature drops auxin levels lessen while another hormone, ethane, increases, causing cells at the base of leaves weakening with other cells expanding and breaking the connections between the leaf base with a result like tearing perforated paper.
Fallen leaves can be a nuisance, especially on roads and pavements, as well as blocking drains and drainage channels – but the rotting of leaves is an important part of the carbon cycle.
Leaf litter can be composted and used as fertiliser and mulch.
Decomposing leaves replenish soil and can also foster the growth of fungi. While it can be unsightly in winter, lawns can benefit by leaving leaves in place to decompose and fertilise the lawn. If you are not able to compost the leaves from your garden, please ensure that you send them for composting via your local garden waste scheme.