Using 3D scan and photo data as reference, with some mix of hand paint and image compositing technique. Fine tuning the material to work well in PBR environment using specular glossiness workflow. (The leaves still a little non-physically lit to simulate light scattering)
This is 7 difference kind of trees with 4 variation for each one labled in S,M,L size plus another alternate one. Total in 28 different trees. Created by Unity's build-in tree creator. So every trees could be edit to match your preference. And it is capeable of many useful effects such as;

Paintable on terrain.
Growth your own forest. Distance trees could also fade smoothly into billboard. So it's not impact your game's performance.
Or place it manually for fully control your delicate scene.

Either way. You're possible to use windzone. Breath life into your scene.

*The terrain texture and rocks used in the preview are not included. Both of them are my previous work that already available in the asset store.