So, it’s time to give some more informations 
On performances :
I have absolutely not optimize my scenes, the trees can have less poly, the lods can be tuned better, the quality settings are maxed … in short, there is lot of room for improvement.
On my i7-3770 / GTX970, in editor (1080p), I get around 30fps for the forest and mecha scenes and around 60fps for the snow scene.
What is killing the performance the most is the sun shadows. On “epic” quality, it divide my framerate by two ( is why the snow scene run at 60fps).
On the process :
(I use the 4.7.0 Preview 8)
I start by making some trees/plants in . I use trees from the store as a base, do some tweak untill I get something I like.
I activate the distance fields in UE4 (Project Settings - Rendering - Generate Mesh Distance Fields), and import the trees.
In the static mesh properties I check “Generate Distance Field as if TwoSided”.
For the leaves material, I use the new “Two Sided Foliage” shading model and plug the diffuse texture in the subsurface color slot.
For the lighting, I use a movable sky and sun (no special thing here, I just change the min occlusion in the sky if the AO is too dark).
Concerning the DFGI : It’s a great but it’s “the icing on the cake”. I don’t use it on the snow scene and it’s barely noticable on the mecha scene. The that give good result it’s the DFAO.
I totally forgot to say that the mecha scene was inspired by the artist Simon Stålenhag
https://farm9.staticflickr/8633/16434187458_2064167409_z.jpgUE4 - by kooooola, on Flickr
(I hope it’s understandable, my english is not very good.^^)