Brown fir trees

Question

2 of my fir trees are going brown on the outside. They are 18 years old and about 14ft tall

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Answer 1

There can be several reasons for browning of Arborvitae.

These links will help you pinpoint the issue.
http://horticulture.oregonstate.edu/system/files/onn011001.pdf
https://www.gardeningknowhow.com/ornamental/trees/arborvitae/winter-damage-to-arborvitae.htm