There are many strategies to dealing with large models (with lots of geometry) such as 3D trees.


Over on the Land8Lounge SketchUp Discussion area, I had posted these two links that have many helpful hints to getting SketchUp to run faster/better.

Making SketchUp run faster

How do I make SketchUp run faster 


Faking Massive Amounts of Trees


Jim Leggitt has a tutorial on large models over at 


John Pacyga