Why do trees lose their leaves in temperate climates?

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Eduardo Godoy

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Eduardo Godoy asked the Naked Scientists:

Why do trees lose their leaves in temperate climates? Seems wasteful.

Eduardo Godoy
San Pedro,CA

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Offline dentstudent

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Why do trees lose their leaves in temperate climates?
« Reply #1 on: 20/11/2008 15:41:18 »
Here is an updated post from a while back in answering a slightly different question, but should help...

So - there are evergreen (coniferous) and broadleaved (deciduous) trees:

Coniferous = cone bearing
Deciduous = leaf dropping

The leaves of a deciduous tree are generally broad and flat. The leaves of a conifer are generally needles, and are thin and pointy. The leaf area is the primary area of photosynthesis in any tree.

Deciduous trees tend to drop their leaves annually at the end of the growing season during Autumn, whilst conifers retain theirs all through the winter. Conifers keep their needles for several years, though this changes with species and with a trees position in the forest.

There is a strong habitat difference between the two tree types deciduous trees tend to be at lower altitudes or in latitudes of shorter day maximum day length, but where the sun reaches a higher elevation. Conifers are found more commonly in more harsh environments, where the growing season is somewhat shorter, where there is an acutely angled solar radiation and water is potentially scarce due to soil depth, freezing conditions and so on. This is perhaps the key driver of the differences in the leaf morphology.

Deciduous trees require large quantities of water in order to grow, which in winter is less dependable.  They have become highly adapted to making use of spring and summer water, and putting on their growth during this period. As they grow, waste products are stored back in the leaves, which are then removed during leaf fall. You can see these products (tannins and so on) in some of the oranges and reds in the leaves as the green pigmented chlorophyll dissipates - some of these colours are also accessory pigemnts that help the tree photosynthesis more efficiently through using other light wavelengths. Broadleaves also have a large storage system within the trunk for carbohydrates, ready for the following years new growth. So the leaves become filled with waste products, and also the pores (stomata) within the leaves become blocked by dust and so reduce the trees growing capacity through reducing its water and gas exchange processes. It is more efficient for the tree to grow new leaves and then have a strong growth capacity than it is to retain the leaves and have a resultant reduction in growth.

Conifers do not have the same level of carbohydrate storage capacity as boadleaved trees, and so need to make use of this "evergreen" system. Because the needles of coniferous trees are continuously available to the tree, the tree can take advantage of short growing periods without having to firstly grow its leaves. Therefore, the conifers can strongly out-compete broadleaves in the harsher terrains. Their needles also have a waxy coating, which helps their adaptability with reducing water-loss.

There are other reasons as to why there is a generalisation of broadleaves at lower altitudes and conifers at higher elevations.  For example, conifers can easily become water-stressed in hot temperatures due to their smaller root system. Deciduous trees can become frozen at higher elevations due to the internal cell structures of the wood. But, hopefully this answer sheds light (rather than leaves) as to why there are leaf differences between the two tree types.