Cambial Dieback and Partial Shoot Mortality in Cliff-Face Thuja occidentalis: Evidence for Sectored Radial Architecture

Cliffs of the Niagara Escarpment, southern Ontario, Canada support a presettlement forest of stunted, long-lived, and grossly deformed Thuja occidentalis showing pronounced strip-bark growth. Previous studies had suggested that longevity and axis asymmetry in these trees were both influenced by root-mediated shoot mortality, but no quantitative data to support this hypothesis were available. To understand whether such a process could explain cliff-face tree morphology, whole plants were collected, serial sectioned, and morphologically analyzed. Dates of initiation and death of each root or branch were recorded, and the spatial and chronological relations between root and shoot mortality events were examined. The results showed that particular roots that had died were spatially connected to parts of the shoot system that died on average within 2.2 yr. Shoot mortality never preceded root mortality for roots and shoots that were spatially related to each other. A quantitative relationship was found between the size of the roots and shoots that were connected. These observations could be explained either by a disruption of the upward movement of water in hydraulically separate sections of xylem or by an interruption of the transport of photoassimilate to the roots. In either case, radial architectural segregation may permit the plant to survive in hostile and resource-patchy environments.