Trees' being cut down and farmlands' being occupied have polluted the air, which is the reason why we need to plant a increaing number of trees.
或:
When we face the fact that a number of trees have been cut down and a quantity of farmlands have been occupied by factores,resulting in the air pollution, we are obliged to plant more trees.
还有很多种译法,就不一一列出了.自认不是很地道,但语法没问题,你将就一下吧.