I'm rather fond of these two tutorials: http://www.cartographersguild.com/showthread.php?t=5478 http://www.cartographersguild.com/showthread.php?t=5664 and of using Inkscape to make up for my own lack of ability and patience.

Putting that all together has gotten me this:
Name: treetest.png Views: 699 Size: 9.0 KB

It's not a proper tutorial (I may write one up later), but the basic process is to draw the rough tree shape, copy it a few times, jitter the nodes to get some major variations, copy , add nodes, jitter again but less so to get a large number of tree forms. Copy, offset, intersect to get the lit portion. Use the tweak shift tool to adjust the boundary between light and dark. Create a bar, add nodes, tile in a column, intersect with the lit portion, use the tweak contract tool to have it only cover the mid tone area. Create a basic trunk, copy, tweak shift to add some variety, repeat the shading trick for the trunks. Make each tree a group, distribute them randomly, do the shading trick one more time for the ground.

Obviously, use of tile clones can cover large areas easily. This method easily provides large numbers of source forms for the clones.

It's not entirely automatic like Gilgamec's work (http://www.cartographersguild.com/showthread.php?t=6639) but I can put together a lot of trees fairly quickly.