Lighting and its concequences...

I tried to create an extremely realistic lighting technique for my scenario.
However, once I had done this, I found that for a single frame to render, it would take 3 and a half minutes. This doesn't sound too bad, but when you have around 300 frames a scene, it will take a long time to render!
I think this is due to my environment for the animation. As i have included a large number of trees, and I enabled shadows, I think it is that that takes the time to render.
Maybe I should make my animation into a night scene...