It seemed to me as well as others that an oak farm revolving around a canopy diminisher with aura field creators in the ground, planting trees on top of them, using repeaters to detect tree growth, and then activating the field creator and replacing stuff, ought to be aura positive. But it is very slightly negative; the cost of creating the fields when a tree exists are heavier than the canopy diminisher can replenish. It seems to me that this ought to work; perhaps the cost of starting up aura fields should be slightly cheaper? I doubt you want to make the canopy diminisher more powerful, as that will further enable some-other-tree-farming-magic-block + canopy diminisher too much.

This is intentional.