The colour of leaves come from pigments, natural substances made by the cells in the leaf. Chlorophyll makes the green colour of leaves and during the spring and summer the chlorophyll is working hard catching sunlight to use as energy and converting the carbon dioxide in the air and water from the ground into sugars which the tree uses as food. During this time the chlorophyll is working so hard that the green pigments cover up the other pigments present in the leaf.
As the weather cools and days shorten, photosynthesis is reduced and the production of chlorophyll slows and eventually stops. At this point the woody cells at the base of the leaf become corky, stopping the leaf from absorbing water and preparing for the shedding of the leaf. The corky cells also restrict the movement of sugars back to the woody part of the tree. As water in the leaf is reduced the remaining sugars become trapped and concentrated in the leaf. The green pigment in the leaf starts to be broken down, leading to the fading of the green pigments and the yellow and orange pigments of xanthophyll and carotene becoming visible. They are always there, but hidden during the growing season and the hard working chlorophyll!
Once the tubes within leaves close off to prevent water getting in, sugars left within the leaf can become trapped and chemical changes in the leaf influenced by the weather, produce the pigment anthocyanin, making the reds and purples we sometimes see in later autumn.
As with the emergence of leaves in the spring, the colour changes that occur in leaves can be determined by a reduction in night time temperature and day length. If temperatures stay above freezing then production of the pigment anthocyanin is increased and leaves are redder. Drier weather during the summer and early autumn increases the production of anthocyanin, but to a greater levels. Once the leaves have used all their food and water they begin to die. As the pigment is reduced the leaves start to fall and become brown.
So for the best, or most vivid, autumn colours you need a dry summer followed by a dry and sunny autumn with cool nights. Looks like it could be a good one! Autumn’s a great time for planting a tree as you really get to see it at its best, and the soil is still warm enough to allow some rooting to take place, enabling a good start for the tree in the following spring.