Autumn Colour
Introduction
The quality of the annual display of autumn colour is dependent upon soil type, soil pH, soil fertility, tree species, tree provenance and the weather.
Leaf Pigments There are three types of pigments present in the leaves of trees and shrubs namely; a) anthocyanins, b) carotenoids and c) chlorophylls. It is the production and retention of these pigments that determines the autumn colours of the leaves.
a) Anthocyanins are pigments that produce the red colours we see during autumn.
b) Carotenoids are pigments that produce the yellow colours we see during autumn.
c) Chlorophylls are the green coloured pigments used in photosynthesis.
The Process of Leaf Colour Change
Leaves change colour during the autumn due to a change in the levels of pigments as the leaves prepare to fall from the trees. All leaves gradually lose chlorophyll during the growing season and this loss accelerates before leaf fall. Under optimal conditions this process of chlorophyll loss is very orderly and allows the plants to reabsorb much of the nitrogen in the structure of the pigment molecule.
Although carotenoid pigments are also lost during at this time some of them are retained after the chlorophyll is removed and this produces autumn leaves with yellow colours.
Very occasionally quite a lot of chlorophyll is left in the leaves when they fall. Such leaves are a pale green or yellow-green in colour from the mixture of chlorophyll and carotenoids.
The active synthesis of anthocyanin pigments just before the leaves fall from the trees produce the red colours. This is the most common colour of autumn leaves. The actual shades of red are the consequences of the amounts of anthocyanin, the retention of carotenoids and even a little chlorophyll.
Anthocyanin and chlorophyll produce bronze colours.
Anthocyanins and carotenoids produce orange colours.
In some plants the colour production is uniform whereas in others the leaves vary between individual plants. Colour production can also vary dramatically within an individual plant and sometimes even within a single leaf.
Weather Conditions Influencing Autumn Colour
Summer Weather
Wet summer months promote good tree health, good leaf retention and therefore good autumn colour.
Dry summer months (i.e. drought conditions) cause trees physiological stress hence they may lose their leaves early or start colour production early. The end result is a poor autumn colour.
Autumn Weather
The right weather during the autumn can promote more intense colour production. The anthocyanins (the red pigments) require sunlight for production and are enhanced by cold and sunny days.
Wet and windy weather during the autumn can knock leaves off early therefore shorten the display of autumn colour.
What Causes the Colours?
Deciduous trees and shrubs drop leaves in autumn by a process known as senescence. Underlying pigments in the leaf are masked for most of the year by the green pigment chlorophyll. As this starts to break down, other pigments show through: carotenoids to give yellows and oranges, and anthocyanins for reds, blues and purples.
High light intensities cause sugars to accumulate in leaves, leading to more anthocyanins. Therefore relentlessly overcast weather and shady areas will not produce good autumn colour. Consistently higher temperatures than normal are also detrimental. The ideal conditions for good autumn colour are therefore sunny days followed by crisp, cold nights.
Soil Type and Soil pH
Trees growing on their preferred soil type and pH are typically healthier and more vigorous than trees not growing on the preferred soil type. The general belief is that acidic soils i.e. with a pH of less than 7.0, produce the best autumn colour. Whilst for many trees and shrubs this is the case there are substantial numbers of plants which are either indifferent to soil type or thrive on alkaline soils and still produce good autumn colour.
Soil Fertility
Research conducted by Emily M. Habinck, a graduate of the University of North Carolina, found that trees growing in places where the soil was relatively low in nitrogen and other essential elements produced more anthocyanins.
Parentage and Geographical Origin (Provenance)
Trees selected from areas recognised for good autumn colour continue to display good autumn colour when grown elsewhere.
Conclusion
The best autumn colour is achieved when trees of good provenance are growing on their preferred soil type, preferred soil pH and poor soil fertility.
The ideal weather conditions are mild wet summers followed by a cold dry autumn with high light intensity.
References:
http://www.rhs.org.uk/advice/profiles1006/autumncolour.asp
http://www.telegraph.co.uk/gardening/main.jhtml?xml=/gardening/2007/11/02/garden-autumn-leaves102.xml
http://www.independent.co.uk/travel/uk/the-complete-guide-to-autumn-gold-545726.html http://www.sciencedaily.com/releases/2007/10/071025112042.htm