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How do trees change colour in autumn? 

 

Trees in Autumn Andy Goldsworthy

“Autumn is a second spring when every leaf is a flower.”  ― Albert Camus 

Trees display a vast array of impressive colours throughout autumn. These various hues of yellows, oranges, pinks, reds and browns are what provide most of us the first real indication that the seasons are changing. They provide us with a final colourful spectacle before winter sets in – the year’s last, loveliest smile. 

The changing leaves on the trees are the main association most of us have with the autumn season, and we can get instantly nostalgic from walking or kicking our way through the fallen leaves. But how and why do tree leaves change colour? AWA arboriculturist James Brown explains… 

Autumn changes in leaf colour are regulated by three factors; day and night length, leaf colour pigments and weather changes. The timing of the change in colour of leaves is primarily regulated by the decreasing day length and increasing night length. 

The shorter days and the longer nights experienced at the beginning of autumn trigger trees to stop producing chlorophyll and begin to break it down and reabsorb it back from the leaf. Chlorophyll is the pigment used by trees for photosynthesis; the process through which plants use sunlight to manufacture sugars for energy. It is this chlorophyll pigment that gives leaves their green colour. Other substances such as proteins that can be reused by the tree are also absorbed back into the tree at this stage.  

This absence of chlorophyll in the leaves allows other pigments such as carotenoids and anthocyanins to be unmasked and show through in the leaves. Carotenoids are the yellow, orange and brown pigments in leaves and are what gives vegetables such as corn and carrots their colour. Anthocyanins are the red pigments and give colour to fruits like apples, cherries and strawberries. It is these pigments mixing in different proportions that give the leaves the array of yellows, oranges, reds, browns and sometimes purples fondly associated with autumn. Anthocyanins are produced from sugars that remain in the leaf. This production requires warmth and bright light during the day for the remaining chlorophyll in the leaf to produce the sugars. Because of this, the dull and overcast autumn days in Britain are the reason it usually experiences more brown shades of autumn colour and less red shades as less anthocyanins can be produced. 

 The temperature and the amount of moisture in the soil also affect autumn colours. The countless combinations of these two highly variable factors assure that no two autumns can be exactly alike. A late spring, or a severe summer drought, can delay the onset of autumn colour by a few weeks. A warm period during this season will also lower the intensity of autumn colours. As a rule, the most brilliant autumn colours are generally associated with a warm, wet spring, favourable summer weather, and warm sunny autumn days with cool autumn nights. 

Genera such as Prunus, Acer and Sorbus all have tree species with excellent autumn colour and certain colours are characteristic of particular species. Many Quercus species turn red or brown; many Cornus species turn purplish red; and Fagus species generally turn a light tan colour in autumn. Acer species have perhaps the most attractive autumn colours of any genus. Within the Acer genus there are many species that display glowing yellow, orange and brilliant red throughout autumn, with many Acer species displaying the whole range of autumn colours on the same tree. Leaves of some trees such as many Ulmus species have no real autumn colour and the leaves simply shrivel up and fall, exhibiting little colour other than drab brown.  

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