This page is regularly updated, please select here to view the latest version.
At this time of year as colour is starting to fade from the garden, it is time for the vibrant leaves of the trees at Portswood Rec to take the stage in the autumn sunlight. As the days get shorter, the trees respond by starting the process of abscission, or leaf drop, in order to conserve water and prevent damage over the winter months. The first stage of this process is to extract key nutrients from the leaves. Chlorophyll is broken down and resorbed into the tree, to be stored in its branches, trunk and roots ready to produce new leaves in the spring. With the green chlorophyll removed, the other pigments in the leaves start to show through. The yellows and oranges are the colours of the carotenoids that are present in the leaves all year round, and are the same pigments that give carrots, pumpkins and tomatoes their colours. Reds and purples, meanwhile, are produced by anthocyanins, also responsible for the colours of blackberries, plums and cranberries. These pigments are produced in the leaf in late summer, as the chlorophyll starts to be broken down.
Once the majority of the nutrients from the leaves have been resorbed, a layer of protective, waterproof cork cells is formed, and the leaf is detached at a point known as the abscission zone. Here, there are two layers of specialised cells: one layer with weak cell walls, and a second layer that expands in autumn to break the two layers apart and cause the leaf to fall.
A single tree can have tens of thousands of leaves, all falling to the ground in the space of a few weeks in autumn. In a deciduous forest, this can mean that every 10 square metres of forest floor is newly covered by 3kg of leaves. In this same area, there can be 1kg of earthworms, 2.7kg of fungi, and 1.7kg of bacteria, all feeding on the leaves, breaking them down, and providing nutrients back to the trees.
First, creatures like worms, ants and woodlice draw the leaf litter down into the soil. As they do so they also create structure in the soil, allowing water and air into it. The oxygen in this air is needed by many of the bacteria in the soil, like the actinobacteria that break down cellulose and lignin in the fallen leaves. These produce the pleasant, earthy smell of good soil. Without sufficient oxygen, anaerobic bacteria take over the decomposition and produce an unpleasant rotting smell instead. Alongside the bacteria, a succession of fungi also breaks down different parts of the leaves: first the sugars and starches, and then the cellulose and lignin in the cell walls. Once the leaves have started to decompose, they form the diet of many creatures in the soil, including nematodes, soil mites and insect larvae, as well as worms, millipedes, slugs and snails. These all break down the leaves further and release their nutrients in a form that can be absorbed again by the tree and used to produce fresh new growth.
Not only does all the life in the soil perform this constant recycling process, there are also huge branching networks of mycorrhizal fungi and actinobacteria that live symbiotically with the trees. They take carbohydrates from the roots of trees, and in turn provide the plant with accessible nitrogen. Filaments of these fungi can extend well beyond the roots of the tree, increasing its feeding range and connecting it to other trees nearby, even providing saplings with nutrients from surrounding trees.
For us, the autumn colours of leaves can seem like a final great flourish before winter. But this is just the beginning of the story happening in the soil, where this complex cycle connects the tree to countless other organisms, and provides nourishment for the bright, fresh growth of the spring.