I was taught trapunto another way. Using a layer of cheap flannel (holes to pull needle thru) or good cheesecloth, put this under say the large trees only. Sew around the tree. Buy the Hallmark yarn, the fuzzy stuff to tye gift packages with, and with a very large needle, about 5" long with a big eye, take the yarn strands apart, using only one strand.
Pull the yarn in from the back of the tree thru the flannel or cheesecloth, leaving little tails, cut off each time, and then when you get the tree full,, take the other end of the needle and stuff in your ends. Because the trees are alike, I would count how many strands you pulled thru so all of them will be stuffed alike. If you put the flannel backing on and sew around all the large trees before you start the stuffing, you can then sit down and draw the yarn thru, even watching TV
or talking on the phone. Hope this helps. I would quilt one by hand and stuff it so you can see if it meets what you are trying to accomplish. This way if you do not like it, you can take the hand stitches out easier. You could use white, offwhite, or even green. You can also use a hoop when you put the yarn thru.