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The stem or trunk of a tree has three parts: bark, wood and pith. The pith is central part and around it is wood. Between wood and bark is cambium, a thin layer that produces new wood and bark. When cambium ring is severed, as by a wire cable, tree is killed, and since cambium protects against insects and disease, anything driven into it can wound tree severely.
Outside of man himself, trees have countless enemies.
There are 200,000 known kinds of insects that attack trees, in addition to diseases such as blight, rust and rot, storms and droughts. Luckily, birds help to keep caterpillars, borers, beetles and other insects in check.
Paul Curran is CEO of Cuzcom Internet Publishing Group and webmaster at Trees-and-Bushes.com, providing access to their nursery supplier for a range of quality plants, trees, bushes, shrubs, seeds and garden products.Visit their trees section to find a great selection of trees for your garden