Answer: Could be lack of sunlight or it could be due to reflected heat from the fence. Another thought - spider mites love hot, dusty locations and what you describe sounds like a perfect environment for them. They also like living in arborvitaes because the centers of the plants are often dry and dusty. Check for spider mites by holding a clean piece of paper towards the center of the plant and sharply taping a branch. If spider mites are there, they will fall down onto the paper. Spider mite feeding can cause browning of the foliage which is why I'm curious about them. If yo find the pests you can discourage them by periodically hosing off the arborvitaes, directing the spray of water into the centers of the plants and the foliage that is nearest the fence. Hope this sheds some light onto the problem!
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