Saplings are baby trees that are sold in small pots. From the day that they are planted in your village, it takes three days to fully grown tree. Saplings can be bought from Tom Nook's Store. Saplings cost 80 Bells. There are two different kinds of saplings, an oak tree sapling and a cedar sapling. Cedar saplings look like fir trees and are decorated like Christmas trees around New Years and Christmas. Oak trees look much like fruit trees, just without fruit.
When you first start the game, one of your first jobs at Tom Nook's Store is to plant 3 oak tree saplings and 3 flowers around his store to make the shopping experience for costumers better. You can not plant saplings on bricks, or cement. Saplings will not grow if they are too crowed, or in a bad spot in your town.
Animal Crossing, any type of tree or sapling could be planted next to each other or even next to buildings and rivers. This was stopped in Animal Crossing: Wild World and had more rules where to plant. Saplings and trees died easier if placed in bad areas in the newer games after the old Animal Crossing.