If I am not mistaken, most of the P. americana rootstock is grown via seed.
Unless the seed is collected within a stand of other P. americana trees, the
seed would have a pollen parent other than P. americana. This would make
the resulting seedlings highly variable in their characteristics and
performance. It is difficult to make a true assessment when everyone's
experience is stemming from the fact that they all have different
rootstocks.
Dean Kreutzer
Regina, Saskatchewan Canada
USDA zone 3