We "assume" that because we can demonstrate it to happen. Place a tree in an enclosure, increase the CO2 level but otherwise keep things the same as outside the

We "assume" that because we can demonstrate it to happen. Place a tree in an enclosure, increase the CO2 level but otherwise keep things the same as outside the enclosure. Trees within the enclosure grows faster than the ones outside the enclosure. This is not surprising, as we already know trees and plants consume CO2 to produce cellulose and other plant materials. This is heavily exploited in greenhouses which are CO2 enrichened to produce better yields - albeit used mostly for growing vegetables, not trees.