Trees have higher longitudinal growth strains in their stems than in their roots

Longitudinal growth strains develop in woody tissues during cell-wall formation. This study compares stems, which have a mechanical role and experience longitudinal stresses, and nonstructural roots, which have little mechanical role and experience few or no longitudinal stresses, to test the hypothesis that growth strains are produced in stems of straight trees as an adaptive feature for mechanical loads. I measured growth strains in one stem (at breast height) and one nonstructural root (beyond the zone of rapid taper and/or beyond a major change in root direction) for 13-15 individuals of each of four tree species, Pseudotsuga menziesii, Thuja heterophylla, Acer macrophyllum, and Alnus rubra. Forty-seven of the 54 individuals had higher strains in their stems than in their roots (4.3 +- 0.3 X 10-4 and 1.5 +- 0.3 X 10-4, respectively). Growth strain was two to five times higher for stems than roots. The modulus of elasticity (MOE) in bending was also higher in stem tissues (from literature values) than in root tissue (from this study). Calculated growth stresses, the product of growth strain and MOE, averaged 6-11 times higher in stems than roots for the four species. The higher strain and stress in stems than roots indicate that the strains and stresses are adaptive features that are produced in response to, or in "anticipation," of mechanical loads. The existence of nonzero strain in these roots indicates that production of some level of longitudinal strain is a consequence of wood development, even in situations where it does not appear to be adaptive