Tree bark is a defence against herbivores, insects and parasitic plants. The smooth bark of a beech tree makes it hard for insects and ivy to gain a foothold, but to keep the surface smooth, the tree must grow its bark quite slowly. This makes it slow to seal injury sites, where branches have snapped off and also limits the overall growth rate of the tree.
Oak trees have bark that grows four times faster than beech, which allows speedy repair and also helps to retain moisture, so oaks can survive in dry Mediterranean environments. But the rapid growth causes the bark to wrinkle and crack and this harbours insects. To counter this, the oak must spend a greater proportion of its metabolic resources producing tannins to make the bark unpalatable.
Trees in very damp environments, such as birch, often have very thin bark that they shed regularly. This is because they’re prone to lichen and moss infestation and exfoliating like this lets them get rid of these parasites.
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