Proper LOD’s (level of detail) for starters can help tons. As can using instanced geometry. I don’t know what effects Ultra Dynamic Sky adds but that could be causing a performance impact also. What kind of performance do you get with UDS and without UDS, but with everything else the same? What about without the trees vs. with trees? Also use the built in tools to narrow down what is causing the performance hit.
Here is the documentation link for performance and profiling:
Overdraw is a major performance killer… lots of overlapping transparent geometry like leaves. Best to spread it out some. Also do make sure your actually using the foliage tool… and not just placing tree models… otherwise your draw calls will go through the roof.