To leave the best trees and take the worst, you need to know how to tell the difference. A healthy tree is straight, with a full crown, and produces plenty of seed to feed wildlife and germinate more trees like itself. Trees can be unhealthy due to disease or infestation, some signs of which are fungus growth, discolored or weeping bark, and holes made by bugs. Trees can also be stunted from overcrowding and losing the race for sunlight. These trees are small for their age with old bark, and they do not grow noticeably for years and even decades.
Left to their own devices, forests have mechanisms for relieving overcrowding, such as wind, fire, pests, and diseases, but too often they also get rid of all of the trees. These are natural rejuvenation processes, but despite their long-term benefits, none of us really want to have any of them occur in the woods around our homes. A better alternative for the people who live near these woods is called sustainable forestry, a set of principles and methods directed toward mimicking, in a much tidier manner, the natural processes that keep a forest healthy and growing. Though individual trees get old and die, the forest survives.