I take as a basic law of nature that "trees do not grow to the sky". In fact, the tree metaphor is very useful as we try to impose a science on the vaguely defined term, "sustainability." Trees make up forests and forests grow and recede according to many factors. Trees develop from seeds to saplings to mature trees, using nutrients and water in the soil and C02, O2 and light from the atmosphere to grow. They do not grow to the sky, and they eventually dies. Forests do not cover the earth; but they grow, recede, and are potentially sustainable. Trees and forests are a function of their environment. Homo sapiens are a function of their environment too, although we sometimes forget.