Difference in monoculture vs. multi-species pasture!!

In the spring, my neglegted, natural multi-species pasture thawed and started to sprout while the neighbouring monoculture tilled field still had a foot of snow!

Possible explanations:
- Root activism creates heat?
- plant residue does not let the soil freeze so deep
- remaining messy sticking-out biomass does not let the snow to make a compacted thick carpet
- something else???

Wow, the difference was so drastic! You are right about why a polyculture field warms up faster than a bare one without cover. Microbial activity is still going on in cold weather slowly, both underground and on the surface, to generate heat and warm up the soil. Beside that, tall sticks poking through the snow absorb heat to accelerate snow melting too.

In the book " a soil owner's manual" by Jon Stika, he talked about choosing cover crops to achieve the desirable C:N ratio of resulting plant residues ( page 59-61). If doing it right, the nutrients are recycled with the right amount of mulch to protect the soil in spring time. This is a little book on restoring and maintaining soil health and maybe you will find it helpful. Gabe Brown's work was cited in there too.

Looks interesting! Those trees probably create a microclimate that would change the air flow above ground and also add more microbial activity underground.

Yes, it is warmer under the trees, even when they don't have any leaves.  I used to have a large tree next to a garden; I noticed that the part of the garden that was under the tree canopy was protected from early frosts even though the tree had already lost it's leaves.  The ground under your trees -- in addition to everything already mentioned -- probably didn't freeze as deeply as the ground out in the open.