Umm... trees (complex objects w/ transparency?) and multiple lights will definitely slow down the render process. Depends on the number, and the settings. Also, if you have any volume materials at all, it's a sure bet that's adding quite a bit of time to your render. But 17% in 38 hours... unless you've got a TON of trees and lights, as well as a large/complex volumetric, that's crazy. Rosh