In cases where you're using pull-behind row planters (like this: https://www.kelcomaine.com/webpics/productpics/tp300/tp300-0...), catching on a stump could tear up the equipment or throw the rider off.

That is correct. The most costly operation in afforestation by planting is removing stumps. We did it with explosives.

BigDog would make an excellent tree planting robot.

It's way too slow. A drone has the advantage of being able to navigate extremely rugged terrain, largely avoiding obstacles, and can traverse across rivers without getting wet.

The sorts of areas that desperately need replanting are usually a wasteland to stumps, branches, brush, and lots of rocks.

One of the most efficient Soviet afforestation technology for pine forests is - wait until the good seed year comes in ripe forest. Cut it down the next year. Cones will land. Burn the place with controlled fire. Stumps will go, cones will open! Bummer, young forest in 3-4 years growing real fast.

IIRC pine forests often have microsaplings, lots and lots of tiny pine saplings that will start growing once the old trees goes away.

called podrost in Russia. Not in all types of forests. often they die when you remove canopy. bugs come.