Although I may be too late, you could try this trick, I did it once once on my Winter Cemetery pic.

 

Make two copies of the tree in question, give one a snow texture, shrink it's dimensions by 1% or so and raise it up out of the other tree so it's just peeking out of the top.  It makes the needles look frosted (or get rid of the leaves and just do it on the branches if you can) to make the branches look coated in a little snow.

Just a thought