Epic Games has just released 20+ free environment and vegetation products onto its Unreal Engine Marketplace.
Epic Games Unreal Engine
These products are permanently free on the Unreal Engine Marketplace and include hundreds of optimized plant models and materials, a shader-based wind system, an interaction system, a dynamic low-poly grass system, a dynamic ivy creation pack and more.
Some of the assets that are available include:
Below are just some examples of the available assets, but these you can view all assets on the Unreal Engine Marketplace here: Epic Games Project Nature Free Resources