I wrote this to a client that was wondering about the health and browning of the Arborvitae pictured here.
All evergreens drop older foliage in the fall. The dropped needles become a mulch for the tree. The browning always occurs near the interior of the tree on the older needles. You can distinguish natural foliage drop from foliage drop caused by insects, diseases or environmental problems by a uniform appearance over the entire tree and by observing that neighboring trees of the same kind have the same appearance. http://homeguides.sfgate.com/cedar-trees-normally-turn-brown-fall-71076.html
Although the majority of the Arborvitae browning is normal there is some discoloration and die-back that “stems” from broken branches occurring during installation.
In the spring you should “shake the tree” down and get rid of the old loose stuff that will clog the interior from breathing.
Also…..if you want to keep the rabbits from chewing the bottom it would not hurt to wrap the bottom 3′ with burlap (found at Menards) for the first couple years. It will stop chewing teeth as well as give them a little buffer from the harsh wind.