The mass of a tree is primarily carbon. The carbon comes from carbon dioxide used during photosynthesis. During photosynthesis, plants convert the sun's energy into chemical energy which is captured within the bonds of carbon molecules built from atmospheric carbon dioxide and water. Yes, the carbon from carbon dioxide in the air we breathe out ends up in "food" molecules (called glucose) each of which contains 6 carbon atoms (and 12 hydrogen atoms and 6 oxygen atoms).
<https://www.canr.msu.edu/news/where_do_trees_get_their_mass_from>
<https://www.canr.msu.edu/news/where_do_trees_get_their_mass_from>
sincerely,