I missed December's Accretionary Wedge about plants and rocks - I really need to keep up on my geoblogosphere reading! I am not a botanist, and my knowledge of plants is generally restricted to the local ones which can cause me trouble, such as poison oak and poison ivy. I do have one piece of botanical/geological advice:
If you are trying to locate pockets of soil deep enough to get a good non-surface soil sample, and you are ass-deep in boulders and cobbles and outcroppings, you are often limited in the areas you can get your drill rig. It is tempting to try and drill in the center of an area that is relatively free of trees. However, there's probably a reason the trees aren't at that area. Far better to maneuver to one such clearing, and then aim the drill rods right at the edge, close to the trees. You're much likely to get something other than a rock ledge with 4 inches of dirt on top.