If the trees are still standing, the shear or forestry mulcher are the most efficient way of accomplishing this kind of task.
The shear will take the entire tree down, and if you're deft with the controls it will leave a very short stump, maybe an inch to two inches in height, depending on the design of the shear. This requires resting the thick edge of the shear on the ground, so you have to watch for rock that may get between the cutting arm and the tree. You'll have the tree trunk and branches to deal with, either by limbing and bucking, or by dragging to a burn pile, tub grinder, or to be otherwise hauled off.
A mulcher grinds the entire tree down, and depending on the soil conditions can operate to a few inches below grade, leaving no stump once the surface is graded. The mulcher disturbs the soil extensively, and will turn the chipped/ground material into the top layer, mixing it with the existing soil material. The mulcher heads come in differing styles, and use tooth designs and materials that can operate in rocky soil in some cases (round-point carbide) to square, chisel teeth that are not intended to contact rock or hard debris.
Here where I work, we use a combination of methods depending on the starting conditions, and the desired end-point. We just finished clearing a 25-acre section on a base of loose soil, river rock, hard limestone and dolomite/granite using a Cat D3G dozer, a Bobcat with a shear, a Fecon mulcher, and hand felling/pruning. The mulcher and dozer were used in sections that ultimately will become road and trail, as we don't want stumps or other sub-surface debris that will "bubble up" over time. The remainder was cleared with the mulcher. There is still a substantial amount of handwork in selective clearing, as there are areas where the shear or mulcher can't be operated because of clearance or terrain.
