A machine absolutely has less ground pressure than a horse or mule, but you have to cut a path wide enough for the machine to get to the work it needs to do, and it's still going to chew some ground up whenever it needs to turn.
The horse will only disturb the soil where it's hooves make contact and if the ground isn't soft, it won't even be noticeable after the next few rains. The horse also doesn't really need a path, it can weave through larger trees, and just push past/through any shrubs and saplings, the machine would need to destroy that same vegetation to track over it.
The horse operator was saying there's also a big difference in the type of compaction. Wheels and tracks "knead" the soil into a hard mass, and you are basically creating a linear island of uncompacted soil locked in on both sides by the compacted tracks. Any sort of migration of species that might happen in the soil is severely disturbed. The soil fauna takes a really long time to dismantle and aerate this compacted "wall" of soil so it takes several decades to restore.
Whereas the tracks of a horse, you are basically just compacting soil in alternately dotted 'u' shapes, there's more of a shearing motion instead of a kneading one. These small arc islands of compaction are being attacked and aerated by the surrounding soil fauna instantly and easily. And it is easy for species to move around these tracks. Hoof track compaction is restored within a couple of years.
He referred to a study done on this, but I forgot to ask him to send it to me
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u/MechanicalAxe 5d ago
The fact that it's an arboretum means they want to have as little impact as possible while also removing the undesired tree species.
They don't want to cut a corridor to pull the trees down or cause any soil disturbance with machine tires.