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Shedding their leaves is an adaptation that enables plants to survive cold, wintery conditions. Plants defoliate at other times as well, e.g. when placed under extreme stress such as in drought conditions. This state is known as dormancy and it enables plants to slow processes to a minimum for survival.
Why Leaves Fall
Leaf fall is affected by environment and climate. It has been shown to be a 'photoperiodic phenomenon', or is one related to day length.
Controlled environment experiments have shown that shorter daylight hours induce autumn colour, and that plants with a continued longer day length do not lose their leaves at all. Lower temperatures also induce leaf fall, and in some species (e.g. Liquidamber, Robinia pseudoacicia, Quercus alba) this is a more significant factor than day length.
In autumn leaves undergo a process called senescence where the leaf tissue begins to die. Dead cells form in an abscission layer at the base of the stalk (or petiole) and the twig to which it is attached. The leaf will fall off when the abscission layer completely covers this point. Beech (Fagus sp. ) and hornbeam (Carpinus betulus) do not form this layer and their dead leaves remain on the tree until the following spring.
The final severing results from external factors such as wind. | |
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Autumn colour at its finest, Nyssa sylvatica |
A 'photoperiodic phenomenon', i.e. related to day length
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Fagus sylvatica, Beech, doesn't lose its leaves in winter | |
Colour Change
The range of autumn colour extends from yellow to bright red and purple. The colour change occurs because the leaf stops making food or chlorophyll; it is the chlorophyll that makes tree leaves look green. Chlorophyll is such a strong substance it masks the other pigments present in the leaf, and then, as the level of chlorophyll in the leaf drops, the other pigments are revealed.
The intensity, colour range and duration of the autumn colour is affected by the rate of chlorophyll destruction, sugar levels present in the leaf, the concentration of other pigments and the rapidity of the leaf fall. And these are all dependent on climatic factors.
Sugar levels in the leaves rise in autumn, for, while photosynthesis has slowed, sugars are still being produced and lower temperatures mean that these are moved around the plant more slowly. The sugar in the leaf is converted into a substance called anthrocyanins, and these give us the bronze and red colours. More light increases anthrocyanin production so clear bright autumn days means more vivid autumn leaf colour. Cold but frost-free nights delay leaf fall and reduce the rate of sugar removal from the leaf, this intensifies foliage colour.
Genetics and Environmental Factors
Genetics and environmental factors also influence the intensity of leaf colour, with the degree of influence varying from plant to plant. Seed grown plants can have quite different degrees of autumn colour, and some will disappoint with none at all. We have all seen the vast range of leaf colour produced by cultivars of Acer palmatum, the Japanese maple, the result of different genetic makeup in seed-grown plants. |
Temperature, leaf age and shade also affect autumn leaf colour- plants grown in higher temperatures exhibit less colour than those grown in lower temperatures. Thus autumn colour is less spectacular in the subtropical climates than temperate and areas experiencing cooler temperatures have the most spectacular displays of all. Night temperatures of 14-18oC produce much brighter autumn displays than those of 25+oC. Cornus and Fothergillia grown in shade have been shown to develop much less colour than those grown in full sun.
A low nitrogen level encourages earlier displays of colour. We don't know exactly why but it could be the result of the sugars (which produce the colours) binding with nitrogen instead and forming proteins. If nitrogen levels were low this occur on a reduced scale. Other nutrients may also influence autumn leaf colour.
Amazing Autumn Colour
So, autumn colour is spectacular and unpredictable. It relies on a complex and variable combination of climatic, environmental, genetic and physiological factors. It is all the more amazing to see a brilliant and vibrant maple in full autumn dress when you consider all the factors at work to produce those vivid colours.
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Claret Ash, Fraxinus augustifolia 'Raywood' from this.. |
... to this |
 More Garden Botany
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