Question
We used a backhoe to clear salal from an area in our Washington State garden. We damaged a fairly large root of a young Douglas fir (about 30 feet tall). How can we tell whether we have fatally damaged the tree? And can we raise the grade around the tree at all with topsoil?

Answer
Normally, twenty percent of the tree's roots can be cut before any signs of stress will appear, however, keep in mind that tree roots do extend outside of the protected root zone. Think of the surface under a tree as a pie and slice through the "pie" to determine the amount of the roots that were cut. The damage will show up either this growing season or the next by the foliage starting to die back.
healthy root systems below ground are vital for tree vigor and longevity. Roots are responsible for water and mineral nutrient uptake, energy storage, and anchorage. If for any reason tree roots are damaged, tree health will be jeopardized.
Because roots work quietly out of sight underground, most people have a poor understanding of this important subterranean network. In general, roots grow where the resources of life (water, oxygen, and mineral nutrients) are available. They usually will not grow where there is no oxygen or where the soil is compacted and hard to penetrate. This need for oxygen explains why a majority of tree roots are located in the top 12 to 18 inches of soil. Root systems are also extensive. They often extend outward from the tree trunk to occupy an irregularly shaped area four to seven times larger than the crown (branch) spread. It is easy to see why any type of soil disturbance near trees can, and usually does cause damage. As trees mature in the landscape they attain a rather delicate balance with their surrounding environment. In fact, trees grow best in an environment of minimal change. Unfortunately, our urban, suburban, and even rural landscapes are places where drastic changes like driveway and sidewalk installation, grade changes, road widening, and utility trenching occur frequently. Such construction activities near trees can cause substantial root injury which may be fatal to established trees.

Of all the soil disturbances previously mentioned, grade changes and their impact on tree roots may be the least understood. Since roots are near the surface and depend on oxygen from the atmosphere, raising or lowering the soil level around an established tree can have serious impact. Scraping the soil away from a tree removes or injures important absorbing and transport roots, eliminates nutrient-rich topsoil, and exposes other roots to desiccating (drying) conditions. And if heavy equipment is used during the grading process, additional tree injury occurs because of soil compaction. Instead of lowering the grade, valuable trees might be protected by raising the grade elsewhere. If soil removal becomes absolutely necessary, grade changes should be limited to areas outside the branch spread of trees.

Soil fills which raise the grade around trees are equally harmful. Soil additions reduce the oxygen supply to roots, compact the soil, and often raise the water table. Soil additions six inches or less will probably not harm "fill-tolerant" trees (Table 1) especially if the fill material is good topsoil, high in organic matter and loamy in texture. But, irreparable damage will result if as little as two inches of clay soils are used as fill, particularly around "fill-intolerant" trees (Doug fir is a fill-intolerant tree). If fills deeper than six inches will occur, it is still best to limit those grade changes to areas outside the branch spread of the tree. Never pile the fill up on the trunk of the tree.
So in summary--Is the tree damaged Yes but you will have to wait to see how much. And No raising the grade near the tree is not a good idea.