I think that one reason why they must leave a certain amount of trees standing is so that they do not harm the environment. They must leave two trees standing and two down. They also have to leave a certain number of trees when there is a stream of water. The bigger the stream the more trees that have to leave. They are also forces to replant the area within two years and make sure that the trees thrive within 6 years. In the video they said that most of the time they go above and beyond these regulation to make sure the are is a healthy forest environmet.