How much seasonal, internal die-off of Arborvitae is natural and expected?

Question

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 am aware of the original arborvitae die-off questions, including on internal die-back and about new plantings, but I believe this is a different topic.

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.

Answer 1

I agree with Ecnerwal. Both white pines and cedars in the wild will drop needles or leaves in the fall. If you can find wild stands of these trees you will notice there is little else growing on the forest floor due to the "duff" or ground cover of old needles.

As to how much is normal this can only be answered by "it depends". Competition and availability of light, water and nutrients is the usual triangle of needs for plants.

• Hot, dry summer => more drop
• attacks by pests => more drop

This publication offers more detail and this has great pictures of cedar yellowing.

Answer 2

Internally shaded parts of pretty much all dense evergreens will die off due to lack of light, as the exterior parts grow and shade them out.

When light levels reaching a leaf become too low for the plant to get a net benefit from the leaf, it cuts its losses and shuts the leaf down.