Often carpenters will make quick work of cutting fiberboard sheathing by making their cut marks with a chalkline instead of a carpenter's pencil. Hook the end of the chalkline on one end, hold down the other end, and snap to make the line. The same idea can be used on other sheet material.
Make sure you wear proper safety gear and make three simple cuts. Make your first cut at a slight angle, no more than two-thirds into the tree. Make a second cut to form a wedge. Then make a final cut on the opposite side, cutting into the wedge from above. Cut carefully, watch for the wind, and the tree will fall safely away from you.