We have a rocky back yard with two large arbor vitae trees growing hear the large rocky outcropping. We have been in the house 20 years. This summer the smaller ((35 ft tall) tree began to turn brown and within a couple of weeks is totally brown. The larger tree is still green, although there are some brown needles. The summer was not hot, but was quite dry, and I was not watering the trees. When I noticed the problem, 2 weeks ago, I called an arborist and had a treatment of liquid kelp applied to the roots of both trees. Is this likely due to stress or is it possible that someone has applied a poison to the smaller tree? (I am not usually paranoid but I have a neighbor who is angry with me about a border dispute.)

I would suspect that the loss of the smaller tree was natural given the growing conditions. Evergreens will look perfect until they brown and fail due to a lack of soil moisture. I cannot speculate if a pesticide was applied. Local authorizes like your state department of agriculture would need to be involved.