OK heres some info from the University of Nebraska
Cooprerative Extention
Most trees are adapted to stable conditions in forests, where soil disturbance rarely occurs. In a garden, repeated disturbance of soil under trees can damage roots and reduce overall tree vigor. Activities which cause extensive damage to tree roots include: 1) rototilling under the tree canopy; 2) tearing away tree roots to make planting room for annuals; 3) adding soil to create planting spots or raised flower beds; and 4) cutting trenches 6 or more inches deep to install edging materials or to create a natural edge. These practices cut a tree's fine feeder roots which are responsible for water and nutrient uptake. Planting annuals like impatiens or begonias every year damages tree roots. In their place, use perennials or groundcovers which do not have to be planted each year. You can minimize root damage by using smaller seedlings. Smaller plants will take longer to fill in, but will preserve tree health. Not growing any plants beneath trees and simply mulching the area is another alternative. These techniques will cause less disruption to tree roots.
The Problem of Above-Ground Roots :blob5:
Like a tree's trunk or branches, tree roots grow in girth over time. A root that was once well below the soil surface can increase in circumference and appear above the soil line. Also, tree roots that are growing over a dense, compacted soil layer may eventually emerge above the soil. If turfgrass is in this area, care must be taken not to scalp tree roots with the lawnmower. Removing even one large root can cause the loss of 5 to 20 percent of the tree root system. Cutting tree roots reduces the ability of the tree to take up water and nutrients and affects the stability of the tree during high winds and ice/snow storms. Adding soil to rebury the root is not a long-term solution. Shallow roots will eventually resurface and deeper roots can be deprived of oxygen. A better alternative is to plant perennials or groundcovers, or mulch with a 2 to 4 inch layer of wood chips (or other organic mulch). This eliminates the need to mow, reduces tree root and trunk damage and allows roots to obtain needed oxygen, while hiding the shallow roots.
Dealing With Grade Changes
Adding or removing even a few inches of soil within the PRZ can kill a mature tree. Raising the elevation puts surface feeder roots further away from moisture and oxygen sources. Lowering the elevation removes vital roots and nutrient-rich topsoil. Decay organisms can move into the severed ends of remaining roots. Dead and dying roots lead to branch dieback, a greater susceptibility to insect and disease problems, and decline and possible death to the tree. Building retaining walls outside the PRZ can create desirable grade changes around trees, yet keep soil depth at existing levels (Figure 3).
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Figure 3.
tive Extention # G021452A
