Autumn colours in November can be spectacular! But why do leaves change colour and fall?

The different shades of green we see in the leaves of deciduous trees (and most other plants, including the needles of conifers) are created by the presence of ‘chlorophyll’, a green substance that enables trees and plants to manufacture energy in the form of natural sugars – in other words, to make their own food. This process, known as ‘photosynthesis’, is powered by energy from the sun and uses the raw ingredients of water, carbon dioxide from the air and nutrients from the soil. 

These natural chemicals vary in quantity depending on the species of the tree, which is why trees turn different colours in the autumn - from the yellows and golds of beech (Fagus sylvatica) to the reds and purples of dogwood (Cornus sanguinea). It is thought that these chemicals might remain longer in the leaves to help the tree reabsorb vital nutrients before they drop.

But not every autumn is the same and cold nights are the key! If we have a period of particularly warm and sunny weather early on - often referred to as an ‘Indian summer’ – the trees will stay greener for longer. If this is then followed by very chilly nights, the cold speeds up the breakdown of leaf chemicals and trees will all turn at the same time, their leaves quickly changing through their spectrum of colours as one substance vanishes to reveal another. This is when we get a glorious autumn spectacle! As the process continues, hormones are triggered that seal off each leaf from the rest of the tree and they drop, spreading a carpet of colour on the ground.

Autumn is also associated with mushrooms and other forms of fungi. Fungi are not plants; in fact, they are more closely related to us and other animals because we all share a common ancestor – although that was 1.5 billion years ago! Instead, they are classified by science in a group of their own. 

The cell walls of fungi are made from chitin, the same stuff that makes butterfly wings and the exoskeleton (the hard, outer structure) of insects. The part of any fungi that we see is just the ‘fruiting body’, stimulated by the warm damp weather to emerge and release thousands of minute seeds or ‘spores’, which are dispersed on the wind or carried on the coats of passing animals brushing against them. 

The main body of a fungus is below ground. Unlike plants, fungi cannot use photosynthesis to make their own energy. Instead, they develop thread-like structures known as ‘hyphae’ that spread through the soil into vast networks called ‘mycelium’. As mycelium come into contact with dead organic matter like fallen leaves, wood or the bodies of insects and animals, the fungi use enzymes (natural chemicals) to break it down. They can then absorb the nutrients released back into the soil, which also feed the surrounding woodland.   

Mycelium will extend further into the root system of trees and plants, increasing the amount of nutrients and water that they can reach. In return, fungi receive energy packed sugars from their host, manufactured by photosynthesis. It is estimated that around 90% of plants in the world rely on fungi in this way! 

The way in which trees are connected by mycelium also allows them to communicate with each other – something we affectionately call ‘the wood wide web’! We are only just beginning to understand how it works. 

Fascinated by fungi? Then have a read of 'Hidden kingdom: a beginner's guide to fungi' written by a Wildlife Trust expert!

Stacks of logs and branch trimmings, old plant pots and patches of long grass will also make ideal shelters for solitary bees over winter, as well as other invertebrates like beetles and woodlice - all part of your garden’s mini ecosystem! 

For more advice on how to offer wildlife a helping hand this winter visit our ‘Help wildlife in the cold’ page!