a tree is a kind of plant & plants are a source of oxygen through photosynthesis ->  trees are a source of oxygen through photosynthesis  as the amount of a source of something decreases , the amount of that something will decrease & a source of something produces that something ->  as the amount of a source of something decreases, the production of that something will decrease  trees are a source of oxygen through photosynthesis & as the amount of a source of something decreases, the production of that something will decrease ->  as the amount of trees decreases the production of oxygen decreases  deforestation means the removal of trees in a forest & if something is removed then the amount of that something decreases ->  deforestation causes the amount of trees in a forest to decrease  as the amount of trees decreases the production of oxygen decreases & deforestation causes the amount of trees in a forest to decrease -> deforestation causes the production of oxygen to decrease