Trees are renewable because they can be replenished in a reasonable time frame. It takes too long to 'produce' fossil fuels, so they're not renewable. Thing is, the way we're using trees in most parts of the world is not sustainable. Forests are NOT renewable when they're clearcut or severely reduced. Forests are an ecosystem. When it collapses, species go extinct and don't come back. While we can plant more trees, we can't restore the land to the way it was. Sustainable forestry practices result in a consistent ecosystem which keeps the forest/trees renewable.