I'm pretty well set on the trees. I have a very tall maple off the southeast corner of the house. There is also a tree line that wraps around from the south to the west. It definitely keeps the temperature down in summer time. Its one of the reasons I liked the property. I get the same reaction when people come to my house in the summer. "Wow, you don't have air conditioning?"
A new shroud is definitely in order. However, I'm really thinking that when I put the next incarnation of this thing together I'll use a squirrel cage fan instead of an axial fan. I believe it'll move much more air through the heat exhanger. Even on high that box fan wasn't moving a lot of air through the radiator. I'm sure that was in part due to the poor quality shroud, but also just do to the design of an axial fan which simply can't hold pressure like a squirrel cage can. Its pretty evident that I need more airflow just by looking at the low flow rate and lack of temperature differential over the radiator input/output.