I did a project similar to this with my 2nd graders last year on a more basic level and was inspired to step it up a little bit by a post I saw on the A Faithful Attempt blog.
My JH students did resist trees using painters tape or masking tape. Then they used watercolor paint to create a fall outdoor environment. After their paper was filled and dry, they peeled the tape off and added shadowing to create trees.
I was impressed with the results even from my usually lower end students. Although the results are good, here are a few things I would change:
-Use only painters tape...I used a combo of painters tape and masking tape to get different widths. The masking tape tore the paper in some areas which added a little texture but was a bit of a headache
-Have my students practice some before going head on into the project with watercolor. Some did not understand the use of water and creating some transparency through the color. I stressed it and all of my students have done watercolor before so I don't know why they didn't quite "get it". Some of the paintings look like they were done with acrylic, but they weren't.
-Perhaps use a different size of paper? I'm not sure if I would change this or not. I used 18X24 inch paper which for my JH students has been the largest project we have done. I think they enjoyed it, but for me it took a little longer than I had planned on which was ok.
Here are some results :)