There are also trends in the appearance of trees along rainfall gradients. Generally, individual leaf sizes of trees get smaller as you move from a very moist tropical climate (e.g., the central Amazon Basin) to a rather drier hotter climate (e.g., the southern Amazon Basin), even if it is still covered in forest. It has been suggested that this is because drier climates can get hotter (see Chapters 5 and 6 for the reasons for this) and a big leaf cannot loose heat as well as a small one when it is heated under the sun. And being hotter, it looses water by evaporation quicker so the tree is more likely to suffer drought, if water is in short supply. So, perhaps a plant that has small leaves looses heat faster so that it won't loose water so fast? Perhaps this is a good reason to have small leaves, but it begs the converse question of why it is any advantage for a tree in a moist climate to have big leaves. Are they in some sense cheaper to make and maintain, perhaps? So far, no clear answer has emerged on this.
Generally speaking, along a gradient of increasing rainfall in a forest zone there is an increase in the number of leaves per unit area of forest. In the very moistest climates, old growth forest can have on average seven or eight leaves over any particular point on the surface, all soaking up sunlight (the number of leaves stacked above any particular point is known as the leaf area index or LAI). The increase in leaf area is made possible by the abundant water supply. With more moisture around their roots, trees can afford to produce more leaves despite the extra evaporation that this entails. Since more leaves mean more photosynthesis and more seeds and young being produced by the tree, there is a selective advantage in growing as big as possible given the climatic conditions.
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