We report the morphology, anatomy, and biomechanics of Pachycereus pringlei root systems and explore the hypothesis that anchorage and nutrient absorption/storage are functionally specialized and spatially partitioned in response to selection in windy and arid habitats. The root systems of 18 plants were excavated, and root samples were anatomically and biomechanically examined. Our analyses indicate that anchorage is provided by a large bayonet-like root that typically emerges from the stem base in conjunction with the woody proximal elements of a horizontally extensive, shallow lateral root system that absorbs and stores water. Wood stiffness and strength correlated with the volume fraction of axial tissues, which dramatically increased toward the base of lateral roots. Calculations indicate that the ability to resist wind-throw decreases with increasing plant size, due to a disproportionate increase in stem size with respect to the depth of burial and size of the bayonet-like root. In contrast, the capacity of more distal lateral root elements to provide for nutrient absorption/storage increases. These and other lines of evidence support our hypothesis that the root system of this species is highly compartmentalized with respect to anchorage and nutrient absorption/storage.

Key words: biomechanics, Cactaceae, plant anatomy, roots, wind drag, wood