How much "natural" (i.e., without damaging factors) die-back from the inside of an (otherwise healthy) arborvitae hedge should be expected each year? Stated another way, would an arborvitae in ideal conditions experience any natural, seasonal die-back, or would it remain entirely green? If so, how much internal die-off is normal or even "healthy"?
I'm mainly interested in the general case ("could it be normal?"), but I'll describe the specifics. One of the borders of my lot has a well-established (15+ year old) Arborvitae hedge -- specifically Thuja occidentalis / Northern whitecedar. Seasonally, in the fall, some amount of internal foliage turns brown and dies off. The plants otherwise appear vigorous. The plans seem to recover each spring to a reasonable degree, but this year's growing conditions, and the die-off symptoms, have both been worse than normal.
If it matters, I believe it to be the probably the Emerald Green / Smaragd cultivar. This is in the Northeast US. They are in full sun. The die-off is all internal, and doesn't look like whole branches or spotty (as for pests), or whole areas (as salt damage), or systemic or from the tips (as roots, water, or nutrients). However, if die-off always implies damage, I'll look harder!
There is no shortage of empirical evidence that Arborvitae don't live up to the "vitae" ("life") part of their name, and I have (separately) seen many of these symptoms. There are plenty of resources on arborvitae, but I see only passing references to "normal" die-back -- one such article.