FragileWarrior
Sorry, I should have expanded that answer.
Check out:
formatting link
formatting link
contains this bit: "Grade Changes Trees depend on the feeder roots in the upper 6 to 12 inches of soil for air, water and minerals. Most feeder roots are in an area under the tree crown in open-grown trees. If you extend perpendicular lines from the widest portions of the tree crown to the ground and connect these points, you?ll enclose an area that includes most of the root system.
Because most of the roots are in the upper 6 to 12 inches of soil, lowering the grade and consequently removing those roots under a tree crown can be harmful. Generally, protection is achieved by terracing the grade to keep soil disturbance as far away as possible from the trunk. If space is available, the tree may be unharmed if you let it remain on a gently sloping mound. Another way to protect trees from root damage caused by lowering the grade is to build a retaining wall between them and the lower grade. This is an effective way to save trees if the grade difference is less than 2 feet. Don?t forget to leave drain holes in the wall.
______________________________________________________________________ On the other hand, if you raise the grade by adding soil, the tree may slowly decline in vigor or die because of difficulty in obtaining essential air, water and nutrients. It?s often possible to add up to 2 inches of a light sandy-loamy soil without killing your trees. A tree well with perforated drainage tiles is necessary to allow drainage if you plan deeper fills. Contact a professional consulting arborist with help with lowering or raising the grade around trees during construction.
Trees with damaged or buried roots often do not die immediately; in fact, they may live several years before declining. Once damage becomes evident, it is probably too late to correct the problem." ______________________________________________________________________