Question
Hi Jim, I live in WA State and I am currently building a large retaining wall several feet from 3-4 very large Fir trees-approx 40-50 ft high.  I am currently digging a trench for the first layer of brick when I unfortunately came across a very large root that is blocking the path of my trench.  The root is large-prob about 12 inches in circumference, about the size of a human thigh.  I see no way out but to severe the root and remove it from the path of my trench.  However I an concerned that I may kill the tree, especially if I run across one or two more before its all said and done.  Should I be concerned?

Answer
Normally you can cut up to 25% of a trees roots without seriously damaging the tree. The roots zone extends out beyond the foliage area about  1/2 again the size of the branches. If you think of this area as a pie and cut across the pie area this would be the part of the roots that were cut. The closer you cut to the tree trunk the more roots are cut. If you cut within "several feet" it sounds like a large amount of the roots were cut off. The roots supply the foliage with water and food for them to grow as well as holding the tree up. The cutting of the roots will show up in the foliage dying back. I think you will see die back in the foliage to what extent will depend on how much moisture the remaining roots can supply to the foliage.  Sounds like you also have changed the drainage when you put in the retaining wall and if you filled in behind it. This change will also effect the amount of water the tree is getting. During drier weather you will want to water the remaining root area. At this point you will have to wait and see.