Tree farming is the process of planting a large number of saplings and waiting for them to grow into trees. These trees are then harvested for wood and more saplings, which can be used to grow another generation of trees. This can be repeated indefinitely, yielding a regular supply of logs without the hassle of covering large areas of terrain.
Since trees will grow quite happily underground with a nearby light source, and will grow when in direct or diagonal contact with other trees, quite compact arrangements can be used for efficient use of space.
Underground saplings rely on a torch light to grow.
Trees can grow and block the torch light to other saplings. Underground tree farms should stay clear of magma because after all... ho wants to die because of a tree?
-Well happy planting