The population plays an important part in the growth of the forest,
the Pokémon residing within it, and items dropped. In order to expand
the forest and get more Pokémon and items, you'll need to use the
Entralink to visit another player's Black City and invite characters
over. At maximum population (ten characters), White Forest has lush
tall trees, abundant grassland, and gleaming water surfaces. When very
few people are around, the grassland is sparse, there are fewer,
smaller trees and fewer or no water surfaces.
You need 2 DS systems and one of the DS needs Pokemon White and the other one will have Pokemon Black.