Forests or trees might be considered a non-renewable resource if they are cut down faster than they grow back. Many people think that the best wood is called "heartwood." This is the kind of wood that grows deep down in the middle of very old trees. If trees are lot allowed to grow for many, many years before they are cut down, we cannot get heartwood from them. If we cut them down too quickly, we'll end up with grasslands instead of forests, and there will be no trees left at all.

Another danger to cutting trees down too quickly is that the loss of forests could permanently change the environment there. For example, maybe an animal that used to pollinate the trees or help spread its seeds will disappear when the forest disappears, then even new trees will have trouble reproducing. Or maybe the trees will only sprout in the shade, and the shade all disappeared when the forest disappeared. There are many more examples of how harvesting trees too quickly can cause big, permanent problems.

The way to preserve forests as a renewable resource is to cut fewer trees down. For that to work, people would need to find other materials that could be used in place of new wood (like bamboo, fiberboard made from wood chips,or recycled lumber), or we would need to devote more land to growing trees.