1)
Here’s an obscure bit of first-hand knowledge for you. At age 20 I lived near New Orleans and my roommate worked at a company which provided blowout preventers to off-shore drilling platforms.
As the name says, blowout preventers close off an oil well drill hole if pressure is forcing fluid up to fast. Their are various kinds, depending on how quickly and thoroughly the brakes need to be applied.
The extreme is the blind shear ram. It slices through the drill pipe and any drilling hardware inside, instantly capping the well.
Stick your tree in one of those and voila!
2)
Here’s how logging companies do it (not in one slice). They have a crane/tractor with a complicated cutting head. It cuts logs in an instant with a chain saw, and then de-branches them. Fascinating. Youtube – 10 Largest and Powerful Forest Harvesters in the World