A young tree with a slender trunk is known as a sapling. Just as a young duck is called a "duckling," a young tree is called a sapling.
The suffix "-ling" comes from Old English and often shows up today at the end of words to mean "young" or "youngster." Brothers and sisters are called siblings. A baby goose is called a gosling. Someone who works under you is called an underling. The way to remember the meaning of sapling is that trees make "sap" — the sweet, sugary liquid that, in sugar maples, gets turned into syrup. A young tree, then, is known as a sapling. One day, it'll make sap.