[abakker]

Its interesting because last year I did an experiment where I put ~6" of wood chip mulch and slow release fertilizer over the root zones of several trees (climate 6a), and now that its spring, every tree that got the thick coat of mulch has leafed out or budded significantly more than those that just got fertilizer. (sample size of around 10 trees in bad dirt in boulder. YMMV)

[0xbadcafebee]

I mean, could have been the fertilizer, or could have just been an insulating layer?

[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!