Tree models have been developed for optimized use in Blenders particle system.  Layer 1 you will find a test scene with backdrop and trees applied to the backdrop with a particle system. Actual tree models are always located on Layer 10.  Trees are currently set to wire display.  I suggest switching that to Bounds if you intend to use a particle system with several thousand instances of these trees.  Particle system setting can be seen in the particle system setup for the test scene.  Simply free the particle edit and you may apply the particle system to any surface by tweaking the number of particles and possibly defining density information with weight paint mode.  Edits may be made to the model as well as the materials.

Leaf material shader is setup to transition between summer and fall colors. A 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 orange colors

 Update:

An additional node has been added to create a randomized mix leaf texture of orange and green colors.  Read the node titles for factor value settings to achieve the look you want.

All materials are packed into the zipped Blend file.  You may unpack the textures if desired although this may not be required.