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In page Douglas fir:

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The bark on young trees is thin, smooth, gray, and contains numerous resin blisters. On mature trees, usually exceeding 80 years,[citation needed] it is very thick and corky, growing up to 36 cm (14 in) thick with distinctive, deep vertical fissures caused by the gradual expansion of the growing tree. Some of the mature bark is brown, while other parts are lighter colored with a cork-like texture; these develop in multiple layers.[11] This thick bark makes the Douglas-fir one of the most fire-resistant tree native to the Pacific Northwest.[12]