The answer is very simple. Trees in the forest are not competing with grass for water and nutrients. Heavy equipment and other land uses are not available to contribute to soil compaction. The natural cycle of falling leaves and other natural waste is decomposed into fertilizer for plant and tree life. The forest floor is soft, allowing easy penetration of life supporting elements. In today's world, trees are being removed from their natural habitat and planted in newly developed residential or commercial sites. With soil compaction and grass competing, countless hours are spent trying to preserve nature in unnatural settings. Irrigation, aeration, soil modification are all necessary to help maintain a healthy root system. Strong healthy root systems make it easier for a tree to cope with pest, disease, and temporary drought. This is why you should be sure to call a qualified experienced arborist or consultant to diagnose and help you with your urban tree problems.