"Unexpected?" Unbelievable. These guys are scientists?
Different species of trees vary, but a typical tree is about half carbon. It's what holds the tree together.
Take a typical 1 ton (small) tree. It would contain about 1,000 pounds of carbon. To get 1,000 pounds of carbon, it had to take 3,667 pounds of CO2 out of the air, and release 2,667 pounds of O2 (oxygen gas).
It takes about 1,400 cubic feet of air to get 1 ounce of CO2, so it took all of the CO2 from about 80,000,000 cubic feet of air. Keep in mind, this is a small tree (the roots count, too).
Not surprisingly, it takes a long time for the tree to make contact with, and grab CO2 from, this much air. Actually, it has to make contact with much more air, because it can't extract all of the CO2 from the air it contacts.
If the air contains much more CO2, it is hardly surprising that the tree can contact the CO2 it needs much sooner, and grow faster.
Farmers in Israel have taken advantage of this for many years, by growing crops near the Dead Sea, which is about 1,400 feet below sea level. There is much more CO2 in the air there, which more than makes up for the extra effort required to grow plants in the harsh environment.
Don’t forget that the released oxygen comes from the water the roots take up, not from the CO2.