The plants that grow on a forest floor, including shrubs and small trees, are called undergrowth. If you leave a marked hiking trail in the woods, you might have a hard time getting through the undergrowth.
You won't find a lot of undergrowth in a North American forest in the winter — most ferns and other plants die back, and many shrubs and trees lose their leaves. In the summertime, however, undergrowth can be thick and dense under the taller, older trees. This noun dates from the 1600s, a combination of under, "beneath or below," and growth, "something that has grown."