EFFECTS OF BENDING STRESS ON TAPER AND GROWTH OF STEMS OF YOUNG EUCALYPTUS-REGNANS TREES

A glasshouse experiment investigated the effect of bending stress on s tem radial and height growth and stem taper of Eucalyptus regnans seed lings. Eighteen-week-old, potted seedlings were bent continuously for 8 weeks with a static bending stress. The bending treatment was then r emoved and the seedlings grown for another 12 weeks. Other seedlings w ere stayed vertically throughout the experiment whilst central seedlin gs were neither bent nor stayed. Seedlings were rotated every 2 days t o prevent reaction wood developing asymmetrically in the stems of bent trees. Bent trees had higher radial growth rates, developed more tape red stems and had higher safety factors (the ratio of stem radius to t he minimum radius required to prevent the tree toppling over) than unb ent seedlings. They produced a band of tension wood in their stems and ceased height growth whilst bent. When bending ceased, they resumed n ormal radial and height growth. Unbent trees developed more cylindrica l stems. There were no differences in growth behaviour between stayed and control trees. Bent and unbent trees all developed a butt swell, t he taper of which was not affected by treatment. It was concluded that bending stress has substantial effects on both the size and taper of tree stems. However, the development of butt swell is independent of t he bending stress applied. The results were considered in relation to biomechanical theories of tree stem development.