The answer is quite simple. Trees in the forest do not compete with grass for water and nutrients. Heavy equipment and other land uses are not present to contribute to soil compaction. The natural cycle of falling leaves and other natural waste decompose into fertilizer for plant and tree life. The forest floor is soft which allows easy penetration of life supporting elements.
In today’s world, trees are being removed from their natural setting and planted in newly developed residential or commercial sites. With soil compaction and grass in competition, countless hours are spent in an effort to maintain nature in unnatural settings. Irrigation, aeration, soil amendments are all necessary to help maintain a healthy root system. Strong healthy root systems allow a tree to better withstand pest, disease, and temporary droughts.
This is the reason to be sure to call a qualified experienced arborist or consulting arborist to diagnose and help you with your urban tree problems.