This may be from lack of sunlight or proper nutrients. When this happens, pruning the plant down may provide the proper light to the branch, rejuvenating it. Pruning will also help to stimulate new growth.
Pruning arborvitae may also be strictly ornamental.
A row of arborvitae trees may be pruned down to shrubs, for a more aesthetically pleasing screen.
Pruning arborvitae may also entail removing branches and growth that shoots out of the border of the plant. Removing these will make the plant seem fuller and healthier.
Cutting it down to just a few inches off the ground may rejuvenate an arborvitae that is overgrown or weak.
This is the fastest and most reliable way to renew a shrub. More information about pruning arborvitaes can be found online or in libraries.
Garden centers and nurseries will also be able to provide help and tips for pruning arborvitaes.