Are the tree growers obligated to sell their trees back to Trees4Trees™?

No. The families that own the land that the trees are planted on are given the trees with no obligation to sell them to Trees4Trees™ or any other organization related to the planting. Trees4Trees™ does however offer a channel to the market for the trees which helps ensure that the grower will receive fair market prices for their timber when the trees reach their maturity. But it is the grower's sole discretion to decide who to sell their trees to.