Read an Excerpt
Chapter 1- A Time of Transition
In the United States, autumn is referred to as the fall, describing the process of leaves being shed by deciduous climbers, trees and shrubs. Before the leaves are actually discarded, they undergo a number of colour changes. In the north United States, this colour change travels south at a rate of about 60-70km (35-45 miles) a day, and is so pronounced that it can be detected from space. The progress of seasonal change is mirrored in Northern Europe as well, with severe conditions and rapidly dropping light levels being most prevalent the further one travels north. In some plants, the colour changes of the autumn leaves are slow and subtle, whilst in others, the changes are much more pronounced and vivid to the extent that, for some species, this can be the most colourful and attractive season of the year. Most leaves are shaded green through the spring and summer, but the chlorophyll masks the many different types and quantities of pigment that are responsible for the colorus of these leaves as they die.
As autumn progresses, a major recycling process begins as the plants take useful nutrients from the leaves back into the stem and branches. These are stored over winter for use during the major surge of growth in springtime. Chlorophyll is the first pigment to be withdrawn from the leaf, which means that other pigments are visible, such as red and orange carotene, yellow xanthophylls, and/or reddish purple anthocyanin pigments, which are the result of sugars building up in the leaves, rather than being transported into the woody tissues of the stems and branches.
The richness and variety of the colour display will vary according to the weather conditions being experienced during the period when the leaves are slowly dying. The ideal weather conditions for a good show of leaf colour are a cool, damp autumn, with very little wind or frost, as these conditions give sow colour change, with the leaves hanging on the plants for the longest possible time.
Once the plant has drawn as much from the leaf as it possibly can, the connecting veins linking the leaf to the stem are closed sealed by the plant. A layer forms across the cells, effectively isolating the leaf, and acting as protection against weather and harmful organisms entering the plant through these veins. The leaves will then fall from the plant within a few days of this abscission layer forming. This process, whereby plants "toughen up" in order to withstand low water temperatures, is know as acclimation, but it is worth bearing in mind that this will not make frost-tender plants hardy.
Factors such as temperature variations, or changes in the availability of water, vary greatly from year to year. They are less consistent than changes in day length over a one-year cycle, so for any plant a change in day length is a more reliable warning of the approach of seasonal change to come