I never think too much about trees, but I saw a tree today and thought to myself, "Where does it get all that mass?" Obviously all the mass of a tree must come from the root intake or from the leafy respiration, and my layman's understanding is that the latter is not a significant contributor to wooden bulk.
Which leaves (hah) us with the roots. Now roots can grow, but they certainly cannot move, which means that they are limited to taking in whatever is directly next to them. If we assume that A) wooden roots are typically denser than the soil surrounding them and B) roots do not absorb 100% of the material around them then we have a bit of a problem.
If we assume a 100% matter to tree conversion, then we can easily visualize a root absorbing the material around it and then growing into that space with no wasted matter. But, with the two assumptions above, it is hard to see how there could be any additional matter left over for the above ground parts of a tree. There is some churn as the course of events brings new material to die under the tree and get absorbed, but there isn't enough turnover to grow a gigantic tree without some unexplained factor.
I will not be investigating in any rigorous fashion what that unexplained factor is; I will just sit here and live my ordinary banal life occasionally pondering the notion.
This, incidentally, is why the ancient Greeks never discovered anything true about the natural world.