Answer: Unfortunately, based on your description I am not able to tell why the browning or yellowing is happening. It might be due to a rooting problem or to a disease of some sort. It would be a good idea to have a firm diagnosis though before you replace it, especially if you want to replace it with another arborvitae. You might want to take photos of the overall plant and some closeups of the yellowing and consult with a professionally trained nurseryman to try to obtain a more specific diagnosis. Since it sounds as though the majority of the plant is affected, you might also want to consult with the nursery where you purchased it about their warranty. I'm sorry you are having trouble with it!
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