Because trees grow underground with a nearby light source, and also 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 torch light to grow.

Why won't my trees grow underground in Minecraft?

Most trees in Minecraft will not grow when there is a block next to them. This can be something as small as a torch. You'll need to make your room larger.

Where can trees not grow in Minecraft?

A spruce or jungle sapling can't grow if it's part of a 2×2 square of saplings that doesn't meet the requirements for a giant tree.