Pinus pinaster Ait. has a tendency to exhibit stem flexuosity that negatively affects the quality of its wood and its productivity. There is a wide geographical variability in this trait, and there is evidence of genetic control. We hypothesized that root structure and biomass allocation adjustments in response to a given mechanical stress might differ among populations of P. pinaster and might be related to the typical straightness of the stems of a given population. We analyzed root structure and biomass allocation in a provenance test in which plants were artificially tilted at 45° and naturally exposed to wind. Ten provenances were tested: five with typically straight-stemmed plants and five with twisted-stemmed plants. The wind affected the taper and the development of thickenings in the windward second-order roots, although the winds experienced were generally light. The straight-stemmed populations exhibited greater variability in the studied traits than the twisted-stemmed populations. This variability may reflect higher root responsiveness as well as various strategies to address mechanical stresses. Three possible additional distinguishing characteristics of various straight-stemmed populations are proposed: (a) greater allocation of biomass to the stem compared with the branches, (b) development of a thick, cylindrical taproot and tapered lateral roots and (c) strengthening of second-order roots with local thickening in the sectors of the root under tension.