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.  Layer 11 contains a simple snow terrain. 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.  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 as well as summer to Dead.  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 :)  I've added a Frost Gradient on Tree mix node as well that when the Winter Frost Mix is set to 1.00 the Gradient can make the frost appear on only one side like wind blown snow. 

 

Winter Frost Mix Node

1.00=Frost all over tree

0.00=Summer Green colors

 

Frost Gradient on Tree

1.00=Frost on one side of tree

0.00=Frost all over tree

 

NOTE that there is also a Blue Spruce material shader as well as a dead tree material shader saved within the blend file for your use.  This can be used to create some variation in coloring between the green and blue and orange spruce colors.