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Name: John H Hersey
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Why do some trees in the winter lose their leaves and some are able to keep their leaves?


You can view the leaves on a tree as its food manufacturing factory. For some trees, especially those which live in areas which become cold in the winter, the 'factories' do their work during the normal growing season and are forced to 'shut-down' over the winter months. There are many evergreens which hold their foliage all year, simply dropping some of them periodically as they age and become less efficient to be replaced by new needles. Larches in the area where I live are conifers which lose their needles, which is quite rare. You can see that foliage on a tree presents a problem: water loss from a tree is greatest in its foliage. A tree has to 'decide' (and this has occurred over millennia by evolution) whether to hold its foliage or shed it. If it decides to hold the foliage, then it must provide a means of insuring conservation of water, especially in the winter months when dry cold winds remove water which is not easily replace due to liquid water in the environment being frozen. Many plant's adaptation has been the production of a waxy cuticle to 'seal-off' the leaf from the environment and reduce water loss. You can understand that in the northern latitude's winters, sunlight duration and intensity drops, and for some plants the best solution has involved shedding the leaves for the winter, and growing a new set of 'factories' in the spring.

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