What would you call “cut tree trunks” that are lying on the ground of a forest?

Talked to a girl who’s currently studying in school in Canada. She mentioned today for science class they went to a forest near school.

And there were lots of “tree trunks” lying on the “ground” (can you call the ground of a forest ground?). I was surprised so I asked:”Why are they lying horizontally on the ground?” She replied:”Because some trees grow crooked (not sure this expression is correct either), so people need to cut these trees down. Then they would cut the trunks into a few parts and leave them there. Because they aren’t allowed to take these “logs/trunks?” away with them.

So my question is what would you call these logs/trunks? Thank you so much in advance.