We are going to reduce the number of trees being cut down 去掉

我们将减少正在被砍伐的树的数目.
being cut down 是正在被砍伐的,在句子中作定语；如果去掉being,意思成了“已经砍伐的树的数目”
We are going to reduce the number of trees which are being cut down (now).（完整的句子）
We are going to reduce the number of trees which were cut down (last year).