Forest- Tree,Stump,Branches
This is how you get them-
After the player has both received the National Pokédex from Professor
Oak and completed Baoba's second challenge, Baoba will eventually call
the player to tell him or her, that he has come up with a new idea for
the Safari Zone. After receiving this call, the player will then be
able to place one block at a time of any of 5 categories of blocks
inside of the Safari Zone, up to 30 blocks in one area at a time.
Baoba will continue to call the player another three times after this
giving the player 6 new blocks each time. There are 24 blocks in total
Source- http://bulbapedia.bulbagarden.net/wiki/Johto_Safari_Zone#Forest