…that wines from barrels made from American oak are characterized by stronger vanilla notes

than the ones from French oak barrels?

Why?

This is the result of different material (woodblock) acquisition methods used during the construction of the above-mentioned barrels.

American sawmills use tracks to saw wood with the use of sawing machines, while French coopers use wedges to split the log.

The former, ‘American way’, irreversibly destroys the coating of wood pores which allows vanillin contained in oak to relatively easily pass to the wine stored in the barrel.

The ‘French’ method on the other hand leaves the pore coating intact, in other words, vanillin will not pass to the wine so easily.