I also tried "photocopy" which is a good filter to use if you're wanting to make it into a silkscreen; it's necessary to have sharp edges.
This next filter which was "stylize - find edges". I thought this was the best one for making into a screen - some interesting texture lines.
After making it into a screen, I transferred the image onto some "sky" fabric which I microwave dyed for a mottled effect. I like the results and also the fact that it doesn't jump out and say "I'm a tree" - just some interesting texture.