Crown development of a clone of Populus tremuloides exhibiting "crooked" architecture and a comparison with wild-type trees

Abstract

Populus tremuloides Michx. (trembling aspen) is a tree species native to much of North America and is normally ascribed to the architectural model of Rauh, characterized by an excurrent crown structure with a central main stem and orthotropic branches. A mutant clone of trembling aspen is located near Hafford, Saskatchewan, exhibiting an architecture with crooked and twisted tree trunks. It was the objective of the present study to determine how the architectural development of the crooked clone differed from the wild type. In a study conducted over a 5-year period, four mutant trees were compared with four young wild-type aspen in the Winnipeg, Manitoba, area. Based on detailed quantitative data, it was determined that the architecture of the crooked clone of aspen differs greatly from the wild type. The trees are built by the continuous 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. The development of the crookedness starts with the bending of the relay shoots, mostly in relation to the gravitational direction, which is followed in the subsequent years by various gravimorphic responses. In particular, the longest lateral shoots on a parent relay shoot occur in the middle regions, and the tip of the parent relay shoot generally loses vigour over time. The parent shoot may die back to the junction with a daughter relay shoot, causing a sharp bend at that point. Moreover, the divergence angles of relay shoots with the parent shoots were shown to be greater than in wild-type aspen, and this appears to exacerbate the crookedness. The new relay shoot may actually grow back towards the centre of the crown, opposite to the direction of growth of the parent. The results of this study demonstrate how a quantitative change in one architectural character can set in motion a series of developmental processes that result in a vastly different crown structure from the wild type.