Problem with the chainsaw idea, as any tree harvester operator will tell you, is the minute the chain touches the ground or a mucky stump it's blunt and you're out changing it. That's why so many leave high stumps and cause the forwarders problems.
Many North American feller bunchers use shears for the cut, especially favoured in frozen timber too. The blades are designed to put the shatter caused by the cutting force into the stump, not the log, which is pretty clever. It helps with the stumps rotting too.
Dave. S.