W would have no air and we wouldn't be able to breath.
What are the disadvantages we face if trees are destroyed?
Actually most of the oxygen in the air we breathe is waste gas from the unicellular organisms of the oceans.
Trees are contributory to oxygen in the air but their most important role, as far as humans and other animals are concerned, is the carbon sink they provide. Trees take up enormous amounts of carbon ( as CO2 ) that other wise would end up in the atmosphere greatly exacerbating the greenhouse effect.