[zdragnar]

Since OP mentioned both with and without the mulch got fertilizer, it's definitely insulation.

They also mentioned "bad dirt" as well, so there's a very good chance the mulch also helped with moisture retention.

[abakker]

yeah, we have alkaline expansive clay, so moisture retention was an objective. I added the nitrogen to offset the fact that fresh wood chips tend to be a nitrogen sink rather than a source.

I theorize that thermal protection and moisture retention together have kept those plants happier.

[x______________]

Nutritional "thermal protection and moisture retention" given that you've also added fertilizer.

As a side topic to the original post, do you have any comparison pictures to support the topic and theory? I'd be interesting to see your progress!