I think the tutorial suggests you basically erase the jagged bits by hand until everything is smooth. Then you make a custom brush and put in the trunks on the leading edge toward the viewer. This gives you a slightly iso look but is in keeping with the style.
If you're using a layer to 'erase' the visible trees (white is erase black is reveal) you can save yourself a lot of work by revealing or erasing to suit.
(I think everyone runs into this
Sometimes you can blend two different patterns to put in some variety and also perhaps some smoothness.