Man, ploughs a garden; and lets no one walk upon it.  For the broken up, permeable soil from ploughing allows gas and fluid movement in the soil biomass, as well as room for growth.  So that the soil sea should not be compressed.  Compressing the soil squeezes and crushes the life out of it; and also the life spaces that could allow replenishment of new life too.  Loosened, permeable soil gives food, oxygen, water to microscopic life, and eases micro-organism's growth/movement thru the loosened soil too.  The work and growth by the microscopic life by having already fractured the soil then offer less resistance to grow thru by the plants.  Also, from the byproducts of the microscopic life, so much more food, per inch of growth is given too in the now richer ground.  None of this is a single act, but part of an orchestration.  So trees are less stressed by having a richer food source and less effort to get said food source in permeable ground.  This is all a self perpetuating, yet very delicate fauna of life, enriching and loosening soil biomass below the ground where you can't see.  Trees protect this soil from trampling invasions with the lines of trunks around the soil sea garden and then breathable mulch matt layer on top of it.  The mulch then also serves as seed barrier, light starvation to competing seeds, refuse dump, recycling disposal and food source; as it protects the live soil sea from wind, temperature, humidity, light and body weight extremes etc.  Proper mulch beds provide a shared, protecting feeding area for several plants, that then allow them linking underground to help each other further.