From the Super Mario Wiki, the Mario encyclopedia
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The Acorn Tree is a tree introduced in New Super Mario Bros. U and New Super Luigi U that is a landmark of Acorn Plains. The tree grows Super Acorns, the item that transforms Mario into Flying Squirrel Mario, and the tree's base itself resembles two acorns.
New Super Mario Bros. U / New Super Luigi U / New Super Mario Bros. U Deluxe
In the beginning of New Super Mario Bros. U, Bowser and the Koopalings invade Peach's Castle, and the former throws Mario, Luigi, Yellow Toad, and Blue Toad far away into Acorn Plains, where the four crash into the Acorn Tree, causing many Super Acorns to fly away to various parts of the land.
In the Acorn Plains map, there are smaller Acorn Trees. There are no differences between these and the large one except that the bases of the smaller trees do not look like acorns.
The Acorn Tree can be seen in the intro, the Acorn Plains map, Acorn Plains Way (or Waddlewing Warning! in New Super Luigi U), briefly in Tilted Tunnel/Crooked Cavern, and finally in Spine-Tingling Spine Coaster/Spine Coaster Connections.
Super Mario Maker
The Acorn Tree was going to appear in the background at the beginning of New Super Mario Bros. U-style ground levels in Super Mario Maker, as seen in the game's E3 2014 trailer. It was removed from the game prior to its official release, despite ground levels still taking place in Acorn Plains.
Super Mario Run
A small Acorn Tree appears in Super Mario Run as a kingdom decoration. It can be purchased for 300 coins in the shop or obtained at random in Remix 10 mode. The item's description suggests that multiple Acorn Trees grow in Acorn Plains, whereas New Super Mario Bros. U and New Super Luigi U only show one.
The Acorn Tree appears in the premade world for the Super Mario Mash-Up Pack in Minecraft.