Background and Lore

Since at least the 1940s, a population of trembling aspen (Populus tremuloides Michx.) with a crooked architecture was observed growing at the edge of an agricultural field near Hafford, Saskatchewan. For years, local residents have speculated on the cause of this growth form - everything from soil contamination to effects of a meteorite crashing into the area and altering the development of the trees.

In recent years, a group called the Friends of the Crooked Bush has emerged and they have posted a sign at the site stating that no one knows what causes the grove of trees to grow this way and go on to provide various possible causes.

While environment can have significant effects on tree architecture, for example the twisted ‘Krummholz’ vegetation typical of trees growing in windswept areas and in response to saltspray, there is no evidence that the phenomenon in the Hafford aspen is related to factors of the environment.

Evidence for genetic mutation

When the trees are vegetatively propagated and grown in sites in Manitoba, the crookedness is retained, and it is likely that the crooked trait stems from a genetic mutation.

In essence, because the trait can be propagated, it has little to do with location or soil factors.

Unusual stem form

There are many woody plants with unusual form that result from genetic modification to create pendulous branches. Many of these are horticultural varieties. Genetic modification has also been shown to result in a twisted stem from such as in the tortillard beeches of France.

Architecture Studies

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.

A study 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

Other studies

Because of the importance of the bending in creating the crooked form, we have initiated studies to try and determine what causes the bending.

There is some evidence that lack of strength of the shoots at critical times during the growing season may be involved.

The MSc Thesis of a graduate student, Ashley Linden looked at anatomical differences of the stems on the upper and lower sides of the bend. He has noticed that phloem fibres on the upper side do not develop fully compared to the lower side and compared to all locations around wild type trees. He has also conducted strength experiments using weights and preliminary results suggest that strength may be involved.