58. BARRIER ZONE
The barrier zone, or wall 4, is a separating boundary between wood present at the time of wounding [W] and wood that continues to form after wounding. Many factors, still poorly understood, regulate the size of the barrier zone. It may be small [S], moderate [M] or large  both in the longitudinal and circumferential direction. The severity of the wound, time of wounding, and the type of pioneer microorganisms that infect playa major role in the process. Some tree species form small barrier zones but very strong walls 2 and 3. Other species form strong walls 4 but weak walls 1,2,and 3. Eventually, the wood behind wall 4 will no longer store energy reserves.
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