What are you using the 40 acres for?I am no expert and can only answer based on my experience. I have 40 acres in Oregon. That size cat wouldn't touch the doug fir trees on my property that size. However the soil here is very stable mixture of rocks, soil, and clay. Your conditions are likely different. The biggest issue is the weight of the machine. You won't get the traction needed. Assuming it would topple the tree it seems it would take some time to push them into a pile.
Growing timber on 32 acres currently 65 years old, thinning this year. 8 Acres of pasture that I cleared in the early 90's. Just realized you may have intended this question for the OP. I answered because it was a reply to my post...What are you using the 40 acres for?