These chemicals are sequestered :: Now Smell This

But terpenes can also be dangerous to a plant itself, Whitehill says, and so in firs, for example, these chemicals are sequestered away from the rest of the tree’s tissue in a protective liquid botanists call resin. The thick liquid is kept in specialized ducts in the outer bark and needles where it can be oozed out as the tree needs, such as to patch an injury and reduce the likelihood of infection. It hardens when exposed to air, and it’s what makes your hands sticky after handling a Christmas tree.

— Read more in What Gives Christmas Trees Their Crisp, Cozy Scent? at Scientific American.