A 12-year old wound on a western hemlock, Tsuga beterophylla, caused by a black bear that chewed the bark. The dark boundary about the infected wood is called a reaction zone (straight arrows). It resist the spread of infections within the wood present at the time of wounding. The curved arrows show the size of the tree at the time of wounding. A strong boundary called a barrier zone formed around the tree the year after wounding (curved arrows). If decay would spread outward, it would stop at the barrier zone. A hollow would then develop in the tree.
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