[hosh]

Bamboo is a grass, and very fast growing. Some of the running species are vigorous enough to be considered invasive outside its native ecosystem. (The running bamboos spread via rhizomes, a couple feet deep, making them extremely difficult to get rid of once established). They can grow, be harvested, and regrow again faster than many trees, even if those trees are coppiced. They also produce a woody like construction material that is more durable than some of the fast growing trees (such as balsa).

Point being, growing more bamboo is easier than growing more trees in order to supply something like cardboard.