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Trees that lose their leaves

Trees that lose all their leaves once a year are called deciduous whereas trees which lose their leaves continuously and not all at once are called evergreen.
In temperate regions that experience cold winters, many tree species  lose their leaves with the onset of the cooler weather in autumn (fall). These deciduous forests once covered large areas of  temperate Asia, Europe and North America. Superb autumnal leaf colours can be seen in the deciduous forests of Canada and New England. They are not the only regions where deciduous trees are common however, and tree species in many sub-tropical and tropical regions which experience a strongly seasonal rainfall, also lose their leaves once a year with the onset of the dry season e.g. the African Acacia trees.  Such trees are common in the savannas and woodlands of Africa, South America and Asia.

Why do trees lose their leaves ?

Leaves are expensive organs for a tree to build and maintain. During winter (in cold climates) or the dry season (in warmer climates) it becomes difficult for the tree to maintain its water balance as there is less free water available in the soil. It is thus difficult for the tree to keep its leaves turgid and the cells of the tissues in the leaves would become damaged by the cold in temperate areas, or the heat in warmer areas. Instead of remaining actively growing during this time of the year the tree enters a dormant period.

Trees are adapted to the climate of the area where they grow. They do not wait for their leaves to be damaged by the harsh conditions of the winter or dry season before losing them. They prepare in advance for the onset of the unfavourable season by getting ready to lose their leaves. The enzymes which control the functioning chemical pathways in the leaves e.g. photosynthesis, contain nutrients which are valuable to the tree because they are in short supply in the soil from where they are absorbed. Before the tree abscises or separates off  its leaves, it breaks down many of the organic compounds and reabsorbs the valuable nutrients from its leaves. It will reuse these in the next growing season. Nutrients which are reabsorbed from leaves include nitrogen (N) and phosphorus (P).

How does a tree lose its leaves ?

Near the base of the petiole an abscission zone is formed. A layer of cells in the abscission zone called the separation layer becomes physiologically active and starts secreting special enzymes call pectinases and cellulases. These are able to break down the cellulose of the primary cell wall and the pectin of the middle lamella. The cells separate and the leaf will eventually fall off the tree. On the side of the branch a layer of cells actively begins to divide and form a protective layer of cork cells which protect the stem.

Why do leaves change colour before they fall ?
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When trees start preparing the to loose their leaves one of the compounds they start breaking down first is chlorophyll which gives the leaves their green appearance. Chlorophyll is an expensive molecule for the tree to make as it contains many molecules of nitrogen which the tree removes form the leaf and stores for the next growing season. The chlorophyll masks the colour of the other compounds in the leaf i.e. caroteins which are orange and yellow and phycoerythrins which are red. Thus as the chlorophyll in the tree's leaves is broken down in autumn the other pigments become visible and the leaves change colour to that of the other dominant pigments in the leaves, red, yellow and orange.











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