Tree models have been developed for optimized use in Blenders particle system.  Layer 1 you will find a test scene with backdrop and man profile.  On layer 2 you will find the display trees (not for use in particle system).  The actual tree 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.  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.  On layer 11 you will find a simple snow terrain mesh. Actual tree models for use in particle systems are always located on Layer 10.  

Edits may be made to the model as well as the materials. The pine needle material shader is setup to transition between summer and winter.  A Winter Frost Mix value node colored in blue may be set to the following values as well as everything in between for customized colors :) 

0.00=Winter Frost color 

1.00=Summer Green colors 

There is also a Frost Gradient value node which allows the frost to appear only on one side of the tree as it might in nature.  Note if used in a particle system the random rotation cannot be set very high because the frost should appear mostly on the same side of the trees like a shadow:) 

0.00=no gradient 

1.00=full gradient and frost only on one side of tree 

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