Photosynthesis provides the carbon used to make plant cells and tissues. All organic life is made from a backbone of carbon molecules. The plants/trees fix the carbon from the air, and we eat plants (or we eat other things that have eaten plants).
Yes it is photosynthesis This article explains what happens. https://en.wikipedia.org/wiki/Photosynthesis
Dances with Weed
Tell me...what do you think the trees roots are for? The roots of any plans for that matter? Then tell me how you are this stupid?