I recently stumbled over this and wondered how it is done:
It appears to me that the tree details are textured onto the low-poly mesh. I am aware that low-poly forests can be done like that with billboards (rectangles which are rotated toward camera), but it seems that is not what is happening here.
Anyone an idea what the trick is, how the result can look right with a mesh like this, independent of camera angle?
Those low-poly trees are just “proxies” and automatically being replaced with high poly versions when rendered.
As far as I know there is no native Blender functionality to do this. But Scatter addon does it.
Hi, i’m the creator of this trick/addon/gif (where did you saw this by the way ? twitter ?)
See more info below or on the official Scatter thread
https://www.youtube.com/watch?v=rmUdyP7rSkc&list=PLmXeQcLPRwQD1GwSuOoVJU3RpxLSeLXwG&index=3
How to make a proxy (low poly object just for display) in blender 2.8?
Basically it’s a particle trick, in the “Use Count” option, i automatically switch the total render particle to 0 and proxy to 1, and vice versa via a script.
Those are called proxies in every other software and are in fact really basic and important viewport display features, that blender, unfortunately, doesn’t have 
Thanks for the clarification. Was really baffled by just looking at the gif 
I found it when reading this thread:
