Q: Difference between "spruce" and "fir" when used in "Christmas tree" context What is the difference between the words "spruce" and "fir" (or even "fir tree")?
Could they be used interchangeably, for example, when referring to a kind of evergreen trees people decorate on Christmas and New Year holidays?
EDIT (more details about the roots of question):
I've had a conversation with my North American friend about species of trees that are symbols of the holidays in different cultures and was confused about discovering that in some dictionaries both "spruce" and "fir" could be translated similarly into Russian.
Thanks to @TimRomano and @deadrat, I see now that those are clearly distinguished species from the botany point of view. Therefore, let me narrow the question down a little.
When you want to say "Christmas tree", but more specifically in terms of a kind of the tree, could you say, for instance:

Christmas fir (spruce) is the symbol of Christmas in this country.

Or it sounds unnatural as the phrase and better to say something like:

They use fir trees as Christmas trees in their country.


A: Spruce and fir are different species and are distinguished by different physical characteristics as noted here.

To tell spruce and fir trees apart, it helps to know that spruce
  needles are sharply pointed, square and easy to roll between your
  fingers. Fir needles, on the other hand, are softer, flat and cannot
  be rolled between your fingers. Spruce needles are attached to small,
  stalk-like woody projections. When needles are shed, these projections
  remain. As a result, the branches of spruce trees feel rough. Fir
  branches lack these projections, and thus have smooth bark.

Spruce and fir are distinguished from pine species in that the latter have their needles attached in clusters and the former don't.  The term conifer (cone bearing) covers all three.
All of this is likely of interest only to arborolgists.  It's hard to imagine that if you announce that you're decorating your Christmas tree that anyone would ask, "Is it an Ables grandis or a Picea orientalis? (That is, fir or spruce?)

A: At least in the US, Christmas trees have traditionally been any evergreen conifer (meaning that the leaves are not lost seasonally and cones are the fruiting bodies): pine, fir, spruce, or any of the many others.
The term has even sometimes included other evergreen trees and shrubs besides conifers, as well as artificial trees. It is not the case in the US that a "Christmas tree" is necessarily, or even typically, a fir tree or a spruce tree. 

A: A rare question on EL&U where a picture is worth a thousand words:

Source: http://www.slideshare.net/PSURNR3xt/tree-identification-42010160
As for usage:

Country X uses the fir/spruce tree for Christmas

In my opinion, you don't this you need Christmas in there twice, it is implied by use.
