Tree cover will lock moisture in better than most things, and you do get a windbreak effect from anything suddenly in front of a stormfront. Plus windbreaks help keep moisture in since wind will leach of surface moisture quite quickly.

I'm hoping that's what he means, as opposed to implying a few thousand trees significantly impact rainfall patterns. Which is a bit of a cause and effect confusion, since rainfall patterns impact tree quantity. Lots of rain equal lots of vegetation, lots of vegetation indicates lots of rain, but does not, of course, cause it in any meaningful way. In fact I'd rather guess rainfall globally decreases slightly as vegetation increases, since they tend to prevent as much evaporation from occurring as would otherwise happen.