Brush Wood model pack by Martin Teichmann
69 models including all lod stages (36 unique models), plus very low poly collision meshes for rocks, boles, stumps and trees. Additional textures for normal, height and spec maps are included. For transparent areas especially on foliage you need an 2-sided alpha-test material.
The alpha channel of the foliage textures contains black and white color to represent opaque and transparent pixels. In Unity a special set of nature shaders does this work. In Modo's LXO files a RGBA rendering effect has been applied.
The Unity package contains additional prefabs with re-scaled capsule colliders applied. So you can paint trees on terrain and the player will automatically collide with the meshes.