Wood biomechanics and anatomy of Pachycereus pringlei

We report the longitudinal, biomechanical, and anatomical trends observed f or tissue samples drawn from the parallel aligned, prismatic woody vascular bundles running the length of a Pachycereus pringlei plant measuring 5.22 m in height. The main vertical stem of this plant was cut into five segment s (labeled A through E in the acropetal direction) measuring similar to 1.0 2 m in length. Four of the 14 vascular bundles in each segment were surgica lly removed to obtain 20 vascular bundle segments that were tested in bendi ng to determine their stiffness measured in the radial E-R and tangential E -T direction. We also determined the lignin content of representative sampl es of wood. A nonlinear trend in stiffness was observed: E-R and E-T were highest in se gments B or C (1.67 GN/m(2) and 1.09 GN/m(2), respectively), lower in segme nt A (E-R = 1.18 GN/m(2) and E-T = 0.35 GN/m(2)), and lowest in segment E ( E-R = 0.03 GN/m(2) and E-T = 0.20 GN/m(2)). Similar longitudinal trends wer e seen for axial tissue volume fraction and fiber wall thickness, which ach ieved their highest values in segment B (69.8% and 6.59 mu m, respectively) . Wood stiffness also correlated significantly with cell wall lignin conten t: with respect to segment B (which had the highest Lignin content, and was thus used as the standard reference for percent lignin content), lignin co ntent, was 15, 60, 85, and 43% in segments E, D, C, and A, respectively. Fi ber cell length, which increased toward the base of the stem and toward the vascular cambium in the most proximal vascular bundle segment, did not cor relate with E-R or E-T. Basic engineering principles were used to calculate stem stresses resulting from self-loading and any wind-induced bending moment (produced by drag fo rces). Calculations indicated that the less stiff wood produced in segment A eliminates a rapid and potentially dangerous increase in stresses that wo uld otherwise occur in segments B or C. The less stiff wood in segment A al so reduces the probability of shear failure at the cellular interface betwe en the wood and surrounding tissues in this portion of the stem. We conclude that P. pringlei wood stiffness is dependent on the volume frac tion and lignification of axial tissues, less so on fiber wall thickness, a nd that wood development in this species is adaptively responsive to self-l oading and differentially applied external mechanical forces.