Is it OK to cover gaps between exposed roots of Douglas-Fir trees with soil?

Question

My need is to establish more even ground levels on SLOPED areas along one-side-only of several Douglas-fir trees which are along the property line where my neighbor and I want (and need) to install a wood fence.

Answer 1

The advice given by arborists is not to put more than 2 inches of soil or mulch over tree roots. Roots require oxygen (leaves utilize CO2); covering half of a tree's roots with more than that may lead to problems.