Why do tropical evergreen forests not shed their leaves?
Some rainforest trees shed their leaves during the cooler months of winter, others shed their leaves during summer as a result of dry conditions, then as soon as it rains, they shoot new leaves. some drop their leaves because of shading from other branches.
Well, some shed to some extent but the answer is complicated. Normally old leaves are shed when they no longer are producing sugar (more or less) and one way this can occur is if there’s not enough water so their stomata are closed most of the time. This will vary by species and is more common in dry tropical forests.
Looking from other perspectives, that is the simplest definition of an evergreen. If you are wondering why they don’t lose all their leaves at once, like a deciduous tree, just think about what useful purpose it serves a deciduous tree to lose all its leaves every fall.
In the fall in temperate areas, the temperatures begin to drop. If the trees kept their leaves through the late fall and winter, it is nearly certain that they’d freeze. If the moisture in the leaves froze, it would cause the cell walls to burst from the expansion of water. This would cause the photosynthesis to come to a halt and the sugar it produced would be lost. The tree would suffer, if not starve to death.
Instead, everything is drawn in and the tree goes through a period of dormancy through winter. It is able to store the sugars in the roots and at that point, the leaves are no longer needed. They drop.
In a tropical forest, though, there is nothing to prevent the tree from continuing to produce sugars through photosynthesis, for the tree to live, all year long. So the only leaves that need to fall are those that have done their job and are no longer able to function.
This is similar to animals, including humans, shedding skin, scales, and hair when the skin, scales, and hair no longer serves a purpose. They usually don’t shed the skin, scales, and hair all at once because that isn’t necessary.
In a tropical forest, new leaves have already usually taken the place of old leaves before the old leaves are discarded by the plant.
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