Some deciduous trees are still hanging on to their leaves, such as Oaks and Hornbeams, but most are bare, with a dense carpet of coloured leaves beneath them. Deciduous trees like Oak go through a period called senescence or biological aging. For them, senescence is their dormant period, where they are not actively growing and photosynthesising; unlike annual or biannual plants, trees have multiple life cycles, meaning they have many years maturing and growing until they die. During senescence, the hormone Ethylene is produced in large quantities, causing plants and trees to age and fruit to ripen. In annual plants, this leads to death, but in trees, only the leaves are affected; this is because of a clever layer of cells called the abscission layer that the tree sends to the base of all the leaves to prevent the spread of Ethylene to the rest of the tree. This means that there are large, concentrated amounts of Ethylene within the leaves, causing them to breakdown and eventually drop from the tree.
Now, this whole process occurs within the tree, hidden beneath the surface; the only evidence we see of this is the colourful change and falling of leaves. Within a leaf cell, chloroplasts are organelles (structures) that contain different coloured pigments that make up the leaf colour; the leading pigment is chlorophyll, which makes the leaf green. During senescence, these pigments start to break down, starting with the dominant chlorophyll. As chlorophyll drains away, other pigment colours are visible, leading to the amazing reds, golds and yellows you see across the countryside.
Activity suggestion: Take a walk through your local woods and see how many colours you can find. For younger (and older!) minds, collect as many different coloured leaves you can find and create some winter art: Click this link to see what you can create with leaves, as suggested by The Woodland Trust.