Casual observation of the crooked aspens reveals that a major factor
contributing to the crooked form of the mutant aspens is the bending
of shoots towards a horizontal or pendulous orientation followed
by subsequent development that leads to the crookedness.
was conducted in the 1990s to determine the architectural basis
for the crooked form of the Hafford aspens.
The development of crooked form was compared to the architecture
of ‘normal’ wild-type aspen. Unlike the wild type trees
which have a dominant vertical leader with side branches, the crooked
trees are built by the repeated superposition of vigorous relay
shoots with a mixed orientation, that is shoots that take over the
main growth of the tree, and have a more or less upright basal part
and a horizontal to pendulous distal part. These relay shoots are
more vigorous than those near the distal end of the parent shoot.
The development of the crookedness starts with the bending of the
relay shoots, mostly in relation to the gravitational direction.
Over time, the growth of the terminal portion of the shoot declines
to the point where it often dies back to the vigorous relay shoot.
Because the divergence angle of the relay shoot is quite large,
this often results in a sharp bend in the developing woody frame
of the plant. Hence the crookedness results. See a Simulation
of Crooked Aspen Development