Questionable Past Tense of “Tread”

The use of the adjective “well-tread” in the November/December 2016 issue of Writer’s Digest had me puzzled. The context was an article on the topic of writing about subjects that have already been frequently written about:

The title of the article is “The Road Already Taken.” This tag appears under it:

With the right approach, even well-tread subjects can lead to compelling new books or articles.

My first thought, albeit far-fetched, was that “well-tread” was possibly a typo for “well-read” or “well-tried,” but then it appeared again:

When an opportunity to correct erroneous information or offer a new perspective presents itself, a well-tread subject might just warrant a fresh approach.

When I saw it the second time, I realized that “tread” in “well-tread” was being used as a past participle form for the verb “to tread.” My confusion stemmed from the fact that I was familiar only with the past forms trod (simple past) and trodden (past participle) for the verb that refers to stepping or walking: trod and trodden.

Familiar only with these forms, I would have written “well-trodden subjects,” although I would not have stumbled at “well-trod subjects.”

Because the WD article’s title seems to be an allusion to Robert Frost’s poem “The Road Not Taken,” I can only wonder why the author chose tread and didn’t mirror Frost’s more conventional use of the past participle:

Two roads diverged in a yellow wood…
And both that morning equally lay
In leaves no step had trodden black.

Bottom line: When the context is retreading tires or treading water, the past and past participle forms in US English are treaded and (has) treaded.

The usual forms for the verb tread in the sense of stepping, walking, or well worn, are tread (present), trod (simple past), trodden or trod (past participle).

POSTSCRIPT: After writing this article about what I perceive to be a misuse of tread as a past participle, I heard a paleontologist giving a TED talk use the past form trod in a context calling for tread.  Apparently interchanging the forms of this verb is fairly common.