Depth of ground cover to kill trees

[ArtB]

Is there a rule of thumb for the depth of ground cover (say chips or mulch) that will kill alder or Hemlock. One wild cherry also died.

Not trying to kill, trying to decide if 6" cover did in a bunch of alder/hemlock or if a combo of the 2001 earthquake follwed by ice storm is the main culprit. D.Fir seemingly not effected in areas with same ground coverage except obvious broken tops from the ice storm. Degradation of the trees started to be observed in 2002.
BTW, there are other nearby areas with lots of alders with dead tops (no recent moth infestation either) so am inclined to think EQ + the rare ice storm.

 

[texasnative]

You don't have the mulch piled up six inches around the trunk do you? If you do, this could be the culprit. You should keep the trunk clear at its base, then work your ground cover out from there.

 

[Kneejerk Bombas]

Depending on what the mulch is, yes 6" could do damage.
Tree roots need oxygen. Applying 6" of green mulch will cause a lot of CO2 to be formed from decomposition, and if the much is really green, say full of leaves, ammonia gases can be formed. When the soil gets water, the water pulls these toxic gases at ground level into the soil. Covering roots with mulch can be worse than covering it with soil in some ways, this soil gas exchange is one of those ways. Never mulch more than 3" deep.
Another way is if the mulch is toxic, like with Black Walnut, which has naturally occurring poisons in all its parts.
In certain rare cases, disease can also be spread by wood chips. I will defer any further comments on this subject to an arborist familiar with Washington's local diseases.
Some pictures might help generate more comments.