The base is square and the folds spiral around, tucking in at the top.
The finished size of the tree depends of course, on the size square you start with. I used a 9" square of atlas page and ended up with a 3" tall tree.
The origami tree fold was designed by Jannie van Schuylenburg.
Link to folding directions by Leyla Torres.
I've made a bunch of other projects from one discarded atlas--the Dutch apple sliceform, and this rolled rose wreath. (THAT took forever to assemble, but I love it.)