Paving and other soil surfacing are generally considered harmful to trees because these activities reduce soil aeration and moisture, and because the cutting, grading, filling, and compacting required for some surfacing weakens or destroys tree roots in the area. The degree to which a particular surfacing alters the soil environment or damages tree roots depends on the type of surfacing. In general, load-bearing surfaces such as roads and footers must have organic matter removed, and the soil must be compacted.
At one end of the continuum, constructing driveways and sidewalks on top of the grade does a minimum amount of damage to trees. In such cases, the surface is prepared by simply smoothing and tamping. Some compaction and restriction of soil and air movement will still obviously occur, and it is certainly desirable to locate as much of this type of paved surface outside of a tree's protection zone as possible.
At the other end of the continuum, surfacing that involves cutting, grading, filling, and/or compacting to prepare a suitable base does substantial damage to trees. Such situations exist in constructing roads, parking lots, and other areas designed to bear heavy pedestrian or vehicular traffic. Residential driveways and sidewalks that are cut below grade also can cause substantial damage to trees. In these instances, many or all of the tree roots are destroyed during construction. Further, because of the soil compaction, few if any roots will grow into the area.
Obviously, planning is the primary method to reduce the impact of this type of surfacing. To the extent possible, locate areas to be surfaced outside the trees' protection zones. When this is not possible and where a grade reduction is not essential, the use of paving requiring a minimum of excavation, such as reinforced concrete, can reduce the amount of site disturbance. In areas where extensive paving is to be done and "islands" of trees maintained, some form of porous paving material, though relatively new and somewhat expensive, may offer an opportunity to maintain a suitable rooting environment for the tree roots. In relatively large pedestrian areas, such as courts or areas surrounding pavilions, discontinuous paving with bricks or interlocking pavers might be considered.
Certainly, when any excavating is done together with surfacing, it would be desirable to properly prune any tree roots larger than one inch in diameter that would be exposed during the excavation. This is preferable to leaving the torn, shredded ends. Also, trees around which substantial surfacing has been done will commonly benefit from judicious watering and fertilization. During times of drought, watering may be essential to the survival of trees that have lost substantial portions of their roots. This provides them time to regenerate an adequate root system.
Finally, it is important to note that herbicides are sometimes used when paving or surfacing to reduce the risk of vegetation damaging the structure. Contractors should be questioned regarding whether an herbicide is to be used, and if so, specifically which one. Some herbicides used for this purpose will damage or kill trees if they absorb the material.