THE BIOMECHANICS OF RAYS IN TREES

Storing and transportation functions of rays in living trees are well examined. The mechanical function of these radial orientated fibres we re mostly neglected. ALBRECHT proved that lateral strength is strictly correlated to lateral stresses using a fractometer to measure the dis tribution of lateral strength in the area of a root buttress. A close relation was found between ray size and the maximum lateral strength f ractometer values. Evidently, the rays raise the resistance of wood in this radial direction against splitting. The typical spindle-like sha pe of rays is optimized to deviate the force flow in longitudinal dire ction - i.e. along the stem - around the rays in a soft way. So danger ous shear stresses are minimized. For tensile stress in the tangential direction rays have to be considered as 'sleeping cracks'. In order n ot to fail, the tree has to push them laterally together by growth str esses. A decrease of water content in long periods of drought relaxes these growth stresses. So the rays can act as crack starters and the t ree can fail in worst case.