Complex sentence: He started growing stools as a more practical way of saving energy, with trees being trained into shape as they grow, they don't need as much energy to produce as the whole process of growing trees to then be cut down and turn into furniture.
Split into simpler sentences: He started growing stools as a more practical way of saving energy. With trees being trained into shape as they grow, they don't need as much energy to produce as the whole process of growing trees to then be cut down and turn into furniture.