Trees are a Bio-Dome component; unlike pads, they do not produce resources and require no maintenance.
Creating a tree requires an Engineer or Constructor Bot.
They produce 2 oxygen during the day, but consume 1 at night (each tree reduces your nightly oxygen availability by the equivalent of one person).
They can also be used by any colonist to increase their happiness levels, making them an inexpensive way to boost your colony's morale, especially in the initial phases.