We cleared our property of most trees in 2003 leaving 10 western red cedars in clumps. Over the ensuing time most of those ten have started to die from top down. The original forest cover in this area was mostly Cottonwood tres all of which we removed. Are the cedars that are over one third dead from top down likelyto recover? Or for that matter are any likely to survive? This property is in Langley B.C. and irrigation in the summer is not possible.
Thuja plicata is known to "compartmentalize" (I think it's called) when subjected to drought. It is possible only the upper parts now dead will be all that is lost at this time. How do the rest of their crowns look? If the foliage below the dead parts look normal and healthy then this compartmentalization phenomenon may be what you are seeing. Instead of the whole top dying the tree dies back, and survives. If the surrounding trees were bulldozed away direct physical damage to the root zones of the cedars could be a problem, in addition to any warming and drying that may have occurred since. The roots of trees often extend well beyond the spread of their branches.
