I have been a Beta tester on ArtRage since about October last year.
All that time I have been mentioning Bob Ross, and doing Conifers with the edge of a big brush. Ambient are probably sick of me mentioning it!
But they are the best design guys in the world and will eventually solve this problem.
Although there are lots of other ways to get the effect they are all time consuming. You always end up putting in one little tree mark at a time.
If you want to do Bob Ross stuff there are a couple of downloads in Tips and Tricks. They are as close to Liquid White and Black as I can get. Just import one of them into ArtRage when you start. Try using the Palette Knife set to 0% for "smooshing" paint about, it's very subtle.
Making a black canvas, or just importing a black .jpg doesn't work properly.
The Palette Knife is wonderful, but you can't load it with a "little roll of paint". The Airbrush does things just as good, especially "little misty areas".
Have fun, ArtRage is cheaper then a real Bob Ross Palette Knife!
Aldrik's way of doing round leaf trees is the other method I use.
Luck is infatuated with the efficient.