The trees on the card on the left are simple triangles folded from a square piece of paper.
The tree on the card on the right was designed by me, and is folded from 4 pieces of paper: 3 of 3" (75 mm) for the branches and 1 of 4" (100 mm) for the trunk. No glue is require to assemble the model.
You can find the diagram here.
The trunk of the tree can also be folded at an angle to allow the model to stand up. I also created a bicolor version that resembles a snowy tree.