Effect of stem bending on hydraulic conductivity and wood strength of loblolly pine

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Abstract
Stems of 30-year-old loblolly pine (Pinustaeda L.) trees were bent with a winch to determine the effect of bending, which might occur from severe wind stress, on the hydraulic conductivity and wood strength of tree stems. Stems were bent to a point of imminent stem failure and then released. After release, stems leaned an average of 6.25° from their original vertical position. Measurements of sapwood permeability and viable conducting area were made on stem sections cut from bent trees and were compared with sections taken from unstressed (control) trees. Dye was used to determine the percentage of functional conducting elements. An average loss of approximately 30% in conducting sapwood area was attributed to the bending treatment. Loss of functional conducting area was greater along the axis of bending than on the lateral sides of the stem perpendicular to the direction of bending. Damage was greatest on the compression side of the stem. Despite this damage, no difference was observed in sapwood permeability due to treatment. Modulus of elasticity and modulus of rupture were reduced by 19% and 34%, respectively, on the compression side of bent stems compared with control stems. Loblolly pine appears to be capable of sustaining significant damage to its water conducting system without compromising its hydraulic conductivity. However, reduced wood strength due to bending may result in increased susceptibility to wind breakage and (or) decreased commercial value.

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