Answer: I've had a similar experience with arborvitae and I traced the problem back to winter snowfall. The snow was late in the season and fairly wet, which weighed down some of the inner branches. By the time the snow melted, the damage was done and the branches never quite recovered. I pruned them out, the shrub recovered, and now you'd never know it suffered damage. If your arborvitae looks otherwise healthy and is putting out new growth, you can safely prune out the affected branches now. Over the summer the plant will grow new stems and branches, eventually filling in the bare area.
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