Woodland of all sorts is good for wood cutting, and roads in forests and Prime timber increase lumber production.
Lumber is a raw material, produced by cutting trees. Colonists working on forest terrain tiles produce some Lumber each turn, depending on the specific forest type, and the presence of a river or road. An Expert Lumberjack increases Lumber production.
Lumber is also produced when terraforming a forest tile into a non-forest tile. A one-time shipment of Lumber is placed in the warehouse of the nearest colony.