By impregnating fibers with a preservative, wood is eliminated as a food, therefore we can prevent biological decay.
All wood species are composed of two systems of interwoven cells – one from the roots to the top of the tree (fiber tracheids) and the other (ray tracheids) from the branches to the bark. When the tree is alive these cells transmit water and minerals throughout the tree. When seasoned, the natural passageways are useful when the softwood is vacuum/pressure treated with preservatives to transport chemicals deep into the wood.
American Wood Preservers Association (AWPA) sets standards for a wide range of end use treated lumber. These standards allow for several species of wood to be preserved.