Otherwise we will be planting trees on the one hand, while not caring about cutting down the already mature trees we have on the other. In time there will not only be no big trees at all, but people will have no way of knowing what the significance of planting trees is anyway.
否則一方面種樹，一方面卻毫無顧慮的砍掉已經長大的樹，期待一棵大樹不僅不可能，人們也永遠無法得知種樹的意義。