Vegetation models have been developed for optimized use in Blenders particle system.  In the base Blend file Layers 1 through 9 and layers 11 through 19 are sample terrain surfaces with particle systems applied to them for your use.  Layer 20 has a sample scene with multiple particle systems used for learning guidance and examples of particle system use. The actual vegetation and rock models are located on Layer 10 and are currently set to wire or bounds display. I suggest switching them all to bounds if you intend to use a particle system with several thousand instances of these trees.Some of the vegetation material shaders are setup to transition between summer and fall.  The leaf color mix value node, colored in orange, may be set to the following values as well as everything in between for customized colors :) 0.00=Summer green color 1.00=Fall colors All materials are packed into the zipped Blend file.  You may unpack the textures if desired although this is not be required. Below are some simple instructions on how to utilize the model pack.  Enjoy:) 

1. To begin first open Blender and change the rendering mode to Cycles. 

 2. Now got to File and click Append. (Note that you may link models as well to save on memory.  I suggest doing this with the more complex models such as trees or large shrubs) 3. You will need to brows to the Grass Rock Shrub blend file you download with this pack.  From there you can append any of the Groups, Materials, World settings ect.  For now we will append the already setup World from the Grass Rock Shrub blend file.  Click the World folder. 4. Now click the tree.world file and then click Append button to the right side of the screen. 5. Now we just need to select the world settings. 6. The process is the same for the vegetation model Group appending.  Simply go to File then Append then select the Group folder. 7.  From there select the grass.green file then click the Append button to the right side of the screen. 8. The model group will come in on which ever layer you are on.  I suggest moving to layer 10 and keeping all the model groups there. 9. Now select the default cube or any terrain surface you have modeled then click the Particle Systems tab and then click the + symbol to add a particle system. 10. You will need to scroll down and select the grass.green model group and well as setting up some of the particle system settings.  Please review the settings displayed below to get a good general setup. 11. Again more of the particle system settings shown on the right side of the screen. 12. Note that you may use count to limit or increase the number of a specific model included in the Group. 13. Here you can see some of the camera settings I have modified.  I have also increased the Display Camera Alpha to 1.0 which blacks out the area beyond the camera view. 14. Some simple setup for the sun lamp.  The values I use on the right side of the screen work the best for exterior rendering with full sun light. 15. And finally your ready to render with the adjustments of a few render settings which you can follow on the right side of the screen in the image below to optimize the quality vs render time. I hope this guide has been helpful:)  Know that you should feel free to experiment and combine groups and particle systems to create some very diverse environments.  Remember nature is chaos, Happy Blending!