Question
Because we like the looks, we built tree rings around our large douglas firs 3 years ago.  The rings are 4 ft out from the trees, 18 inches high, filled with dirt and planted with rhodys and azalias.  I have since been told this is not good for the trees.  Is this true?  I dug into the rings and found a vast network of new tree roots.  Is this good or bad or...?  What are your suggestions?

Answer
Fill dirt is frequently added around existing mature trees so that a level or more visually desirable lawn can be established. Fill dirt changes the ratio of oxygen to carbon dioxide around tree roots and the roots may die. Only a couple of inches of soil will cause damage but 18 inches of soil usually will kill most of the root system. The roots extend out about 1  1/2 times the spread of the foliage and any fill over this area can kill roots or at least cause some branch die back.  

Doug fir like most conifers have a tap root system and can with stand more fill than hardwood trees. The damage should show up in the first couple of years after adding the soil. Since it has been 3 years I would do nothing --removing the soil will cause problem with the trees roots that have adapted to the new growing conditions. The majority of the root system was out side this ring so the tree was able to with stand the damage.