An arboretum is a botanical garden containing living collections of woody plants intended at least partly for scientific study. An arboretum specializing in growing conifers is known as a pinetum. Other specialist arboreta include salicetums (willows), populetums and quercetums (oaks). The term 'arboretum' was first used in an English publication by John Claudius Loudon in 1833.
So, the correct answer is 'Big trees are cultivated in the form of forests'