Little trees is what I call the quilting that is done in lines top to bottom and bottom to top. I don't think I can get my typing to show what I mean, so I will try and explain it. With your thread at the top of the area you want to quilt drop down lets say an inch, take your free motion and go up at a 45% about 1/2 inch. Cross over the stitching to the other side. Drop down at a 45% angle again to form a point like an arrow. Now you have made 1 unit. You can make these smaller or larger. Some of the Japanese quilts at festival had this kind of stipple. You can have these little 'trees' as close together as you wish.
For me I have found you are either a round shape free motion person, or an angular one. I am a round! Stippling the way you have it is round. You can do the same stipple and make it look angular. You can also make it look like pine needles, and little branches of trees. With any of these suggestions I would hope you would practice. Some freemotion does well with sectioning off areas, like blocks, with a chalk pencil, or even with your stippling.