I came across this fantastic video that asks the questions where tree get their mass from. It is fantastically made and really shows how much we think we know and yet have never questioned.
Spoiler – Please watch the video first before reading on, it’s really worth it.
An Unintuitive Answer
The answer to the question “Where do trees get their mass?” was really unexpected for me. As was said in the video, I had never given this much thought, and since trees have a texture a little bit like earth, I assumed that by association earth had to have something to do with it.
The reason we do not intuitively consider air as a source for mass is – I guess – that we are completely immersed in air, don’t know how it is to be without air, and do not attribute mass to it. For one thing because it’s gaseous and thus has low density and also because any mass we attribute to objects is relative mass to the displaced air.
Taking another Look
I asked my brother this question about tree mass and he made the good point that a tree’s biomass is largely carbohydrates which of course contain hydrogen which mainly comes from water. Therefore I was curious to find out how much of the mass of trees actually comes from the air, and how much from rain, air humidity and from water in the soil.
“About 400 years ago a Flemish guy named Jan Baptista van Helmont also wondered where a tree’s mass came from. So he planted a small tree in 200 pounds of soil. The tree gained 164 pounds, but the soil only lost two ounces!” 
The dry mass of a tree comes mainly from the air as CO2.
Most of the plant dry matter is carbohydrate, with the general composition ratio of CH2O. The C and O come from CO2. The H comes from H2O. [...]. 6.7 % of its mass comes from H2O and 93.3 % from CO2. About 5% of the tree dry mass consists of mineral nutrients from the soil although this can be higher in some species. Thus, about 88% of a tree dry mass comes from CO2.
For example [...] new pine needles contained 85.8% water by weight in July and about 58% in December.
(The amount of water in wood is difficult to find information on, also because it varies drastically. I have found values of 50% in different sources. )
Therefore, about 50% of a living tree’s mass come from water and another 7% come from H2O that is bound in carbohydrate molecules.
Thus, 57% of a tree’s mass come from rain, air humidity and water in the ground and about 38% come from CO2.
I couldn’t find a source to estimate how much of the water comes from the air versus rain/the ground, so the quest basically ends here…
Not as cool as saying that trees are made completely made from air, but in a way it only adds to the magic that are plants!
Richard Feynman explaining what trees are made of