Clearly this quote is literally meant to refer to the length of time it takes to grow trees to maturity. While no trees are mentioned specifically in the question, it would not be out of the question to assume that a great many trees don't slow growth until they are somewhere around 15 years of age. Contrast this with the amount of time it takes to cut down a tree, and you understand the quote. Consider then, the enormous amount of trees that can be cleared in a matter of hours. According to National Geographic, 80% or more of the worlds natural forests have been destroyed. Even if loggers planted one tree for every tree they clear, these trees would take years to regrow. Consequently, another effect is the loss of natural habitats that are destroyed with little hope of rebuilding them for years. It always takes longer to build something up than it does to tear it down.