Yes, trees can do wonders for the environment. But there's a catch, you need them in sizeable numbers in order for them to effectively work their magic. I think that it's important to mention that water retention in soil is affected directly by the amount of overhead canopy. Ferns and mosses only grow where the moisture in the air is high. As for the trees taking water out of the ground and releasing it into the air, I think you meant to say that it's really just the moisture from precipitation that is trapped by the vegetation, not plants releasing H2O. All this that I'm talking about refers to temperate rain forests. I understand that you're more interested in the conditions specifically relating to the Snohomish/Stillaguamish watersheds. I'm not sure I really answered your question, but hope this is informative at the least.