If your trees are using trans-mapped leaves you can look at downsizing the transparency map size for the leaves if it won't impact your render. As the transparency maps tend to create a large memory footprint which may be slowing down your system. The grass would also be a suspect for slowing the scene down. Try to minimise the use of grass to where it is most effective in the scene and look for props that have been optimised as much as possible for fast rendering. Really 50 minutes isn't really a long render time for a complex scene.

You could also use a utility like the SimTenero - Iray Memory assistant available from DAZ 3D to assess your scene asset by asset if you are using Iray.

@Iwaves 50' would be 50 minutes using the long, lat degree system, It's not usually used for time though as far as I am aware.