With that hardware 60fps should be feasible for a scene like this.
The biggest bang for you buck will be to adjust the LOD screen size. You can adjust it for one tree type and then copy/paste to others.
You are using billboards with the trees, right? You must do this if not already. If you bought your trees, chances are most likely they should include some billboards that will be assigned to the highest LOD level.
Besides that, you can decrease texture resolution for some gain, and adjust how far you are drawing shadows.
In a scene like this you only need one directional light - that is where you adjust the shadow resolution values - search “shadows” in the directional lights details.
Another big performance boost would come from switching to forward rendering - that does change the look a bit so you have to decide if it is worth it, but that might net 10-30 more fps.
edit: actually I shouldnt say so much about the hardware. I was using a 1060 and had scenes liek this with 60fps, however that GPU had 16gb of RAM. I am not a big hardware guy so I guess I shouldn’t give such a certain answer. I think you may be under min specs for ue4, in which case getting 60fps on a foliage heavy scene may be unrealistic. But maybe somebody who knows more can say something about that.