Autumn Colours

The onset of autumn heralds remarkable changes in most broadleaved trees in Britain. The leaves mellow to colourful hues to drift off in the wind. But how and why does this occur? Read on to discover a bit more .......... Why do the leaves fall off?

• Shedding leaves is one adaptation deciduous plants have evolved to survive adverse conditions such as cold or sometimes drought.
• The changes in day length, light intensity and temperature in autumn cause a drop in the production of certain plant growth regulators or plant hormones such as auxin and gibberellin. When their levels, drop, the leaves begin to age.
• In the autumn, leaves undergo a process called senescence as the leaf tissue starts to die.
• A band of dead cells - the abcission layer - forms between the base of the leaf stalk or petiole and the twig it is attached to; when that layer is complete, the leaf will fall.
• The final breaking off of the leaves is aided by external physical forces particularly the wind.

Why do leaves change colour?

• Autumn leaf colours show a wide range of colours from yellows, oranges, reds, crimsons and purples; these colour changes mark autumn or fall in temperate climates.
• The colour changes result from the decomposition of chlorophyll, the substance which usually makes leaves look green.
• Vital in photosynthesis, chlorophyll is just one of several pigments found naturally in leaves. Others include red, blue and purple anthocyanins and yellow or orange carotenoids.
• These other plant pigments are there along with the chlorophyll but it is such a strong pigment in green leaves that it masks all other colours until senescence starts - as the leaf ages, chlorophyll breaks down, and the other hidden pigments are revealed.
• Like chlorophyll, carotenoids also begin to decompose rapidly in autumn but the anthocyanins - which cause bronze and red hues - accumulate in the leaves at this time of year.
• Anthocyanins build up as the leaves grow older; in autumn, although photosynthesis is still occurring, leaves contain more sugar than normal because the lower temperatures reduce its movement around the plant - the excess sugar in the leaves converts to anthocyanins, a process favoured by a high light intensity, so bright autumn days result in a good display of leaf colour.

Why does autumn leaf colour vary so much?

• Scientists argue about the fundamental mechanism responsible for autumn colour but its unpredictability is due to the interaction of many factors, including the weather.
• Both genetic and environmental factors contribute to the intensity of autumn colour - the balance of factors differs from plant to plant and site to site which helps explain why autumn colour is so variable both at the same places, between places and from one year to the next.
• Temperature, shade and leaf age affect the intensity of leaf colour - for example leaves grown in full sunlight tend to turn a brighter colour than those in the shade.
• Natural genetic variability accounts for some of the differences in intensity and duration of colour - artificial selection by man has enhanced that in garden and park stock.
• In a tree species with a wide distribution in latitude - say silver birch - leaf colouring and fall would be later in specimens from the south of its range compared with those in the north where winter comes earlier - this trait tends to persist if plants are moved around on a north-south gradient by man.

What tells trees that autumn is coming?

• A common environmental cue or trigger setting off senescence in trees in autumn is the change in photoperiod or the length of daylight; as autumn approaches, the days get shorter.
• As well as reacting to shortening daylight, lower light intensity and lower temperatures also play a role affecting the onset of leaf fall in some species.

.......... Some aspects of autumn leaf colouring and leaf fall are still not fully understood but we hope this brief introduction to the ageing process and the biochemistry occurring within leaves in autumn has been of interest - although the scientific explanations cannot replace the magic of a tree blazing with autumn colour.