I think that your observation of water is probably spot-on. When it comes which RF frequencies get absorbed, with water the answer is pretty much "yes". Combined with what I'd imagine is some really fun scattering math tossed in, tree cover is a pretty good bet for reducing signal quality, especially if it's more than one.
The trees (forest) is line of sight between me and the only tower.
I presume it's all that water in the leaves.