It's best to keep most settings at default, and only change detail and AA settings to start with. I see you have raised Ray detail multiplier to around 0.7, which is not useful at all, but might only slow down render time. RDM is mainly used if you have close-up transparent water and you want very detailed underwater details. In default 0.25 the detail is 25% of the amount of detail above a transparent 'sheet', to decrease the number of calculations needed, and thus keep render time low. So it also works for glass.
With a machine like yours it might not be too bad, but it;s good to keep in mind.
The 'trees', mmm. I would hunt for some (free) real trees; lots to be found on the web, maybe even in the file sharing section, XFrog has some, etc. For realistic placement set slope restrictions, and for high mountains perhaps an upper (altitude) restraint. Also use masks like power fractals, or simple shapes, but it seems you've already done that.
