Autumn Colours

Towards the end of the year in the northern temperate climates, like Britain, as it starts to get colder, we have the autumn, or fall. The deciduous or broad leafed trees detect a climate change, and realise that they will use more energy than they get, if they were to try to continue to grow, and with all their leaves on they would be likely to loose branches when the snow falls, so they go into a form of hibernation, they stop feeding the leaves, and they dry out, and fall off. However they are cleverer than this, in that they extract 'stuff' from them and store it in their roots until wanted again the next year and flower before the majority of leaves grow, so their flowers are more visible to insects and pollen can more easily be spread in the wind.

Understanding what's happening, and how colours are produced

The process of photosynthesis steadily degrades the supply of chlorophylls in the foliage, plants normally replenish chlorophylls during the summer months. It is the chlorophyll pigment that gives the leaves their green colour through the summer.

When days grow short and nights are cool, or when plants are drought stressed, deciduous trees decrease chlorophyll pigment production allowing other pigments present in the leaf to become apparent, resulting in autumn or fall colours. These other pigments include:

Carotenoids that are yellow, brown, and orange,
Anthocyanin pigments which produce reds and purple colours.

These are not always present in the leaves, but are produced in the foliage in late summer, when sugars are trapped in the leaves after the process of abscission begins.

Parts of the world that have showy displays of bright fall/Autumn colours are limited to locations where days become short and nights are cool. In other parts of the world the leaves of deciduous trees simply fall off without turning the bright colours produced from the accumulation of anthocyanin pigments.

Click on image
to see larger version

Leaf drop or abscission involves complex physiological signals and changes within plants. A number of deciduous plants remove nitrogen and carbon from the foliage before they are shed and store them in the form of proteins in the vacuoles of parenchyma cells in the roots and the inner bark. In the spring these proteins are used as a nitrogen source during the growth of new leaves or flowers.

The beginning of leaf drop starts when an abscission layer is formed and activated between the leaf petiole and the stem. This layer is formed in the spring during active new growth of the leaf, it consists of layers of cells that can separate from each other. The cells are sensitive to a plant hormone called auxin that is produced by the leaf and other parts of the plant. When the auxin coming from the leaf is produced at a rate consistent with that of the auxin from the body of the plant, the cells of the abscission layer remain connected, in the autumn/fall or when under stress, the auxin flow from the leaf decreases or stops triggering or activating it, which involves cellular elongation within the abscission layer. The elongation of these cells break the connection between the different cell layers, allowing the leaf to break away from the plant, it also forms a layer that seals the break so the plant does not loose sap.

Click on smaller images to see larger version

In tropical deciduous forests this same pattern does not occur, instead they relate to water shortages etc, and this happens at different times in different places even in close locality or between different plants.

Non deciduous plants like coniferous trees with needles, don't have the same needs and grow far faster, so can afford to loose some branches, they don't go through the same cycle.

The best conditions to create good displays

The best autumn displays come about when you have had a relatively wet and warm period, (series of low pressure areas) leading up to a sudden change to a dry spell with cloudless cold nights (high pressure). In this condition there are loads of the pigments in the leaves and the sudden triggering of the change means all trees change at the same time, producing a stronger and more consistent colour change.

Some others say you get good displays with a dry summer followed by a dry cool autumn, while some others that the brightest colorations usually develop when the days of autumn are bright and cool, and the nights are chilly but not freezing.

Click on smaller images
to see larger version

Geographic variations

If the weather does not change suddenly then as the nights get colder, the further north trees are affected before the ones in the south, and often north facing slopes or exposed trees before south facing or trees bunched closely together.

Knowing when they change

With today's climate depending on which area of the country you are in will depend on what and when you will get the colours at their best. In 2008 the Forestry Commission had an online map that showed the status of very many woods across Britain as the colours changed. However they do not appear to be doing a similar thing this year, but taking a look at their website and the Woodland Trusts website as well as keeping an eye open in your local area will help you identify the best time to catch this glorious time of year.

You may be able to find out when different places traditionally have their best colours, Westonbirt Arboretum in Gloucestershire for example with its special collections of maples are said to be at its best towards the end of October. This is later than some other places. Generally you should start looking out for them from the end of September.