You're a gardener. He, obviously, is not. I'd tell him that he may very well end up with dead trees. Perhaps you could print out something from a serious tree site (more impressive than comments here) and give it to him.

Yep, it's about as helpful as real volcano mulching . . .

Resin

Here is a link that might be useful: {{gwi:445552}}

LOL, it sounds like you already know the answer and are just trying to let off some steam here. Neighbors do the craziest things...

Getting serious, what would you really say? I really don't talk to the guy and he seems to himself.

I want to say something casual without insulting him, although I would like to.

I would go with the second approach - you just found out yourself and you want to make sure he knows the best way to do it also. Also might save some money on mulch as he is overdoing it.

Agreed, I'd definitely go for the second approach. The first one is only going to make for an angry neighbor.

Be sure to let us know how it goes!

Then just wait until he starts pruning .... (cringe).

Rain2Fall

The only thing worse than a volcano mulched tree is a volcano mulched tree using red mulch.

I'd let him kill the barberrys (assuming they're Japanese barberry and as invasive in your area as they are in mine). I know that lots of landscaping companies in my area charge for mulch by the applied cubic yard - so, in that case, it's in landscapers best immediate financial interest to apply LOTS of mulch.