The increase in anchorage with tree size of the tropical tap rooted tree Mallotus wrayi, King (Euphorbiaceae)

The mechanical development of the anchorage system of the tap rooted tropical species Mallotus wrayi King (Euphorbiaceae) was investigated by pulling over and examining trees with a diameter at breast height (d(bh)) of 4.2 cm to 14.3 cm. The mode of mechanical failure depended upon the size of the tree: thicker trees (d(bh) approx: 9 cm) failed in the ground with their tap roots pushing into the soil on the winchward side; in smaller trees (d(bh) approx. 7 cm) the trunk snapped before anchorage failure; and in very small trees (of d(bh) < 6 cm) neither type of failure occurred and the trees returned to their original upright position undamaged after the test. The anchorage strength of the trees was correlated with the second power of trunk diameter rather than with the third power that theory suggests is optimal because tap roots did not show an isometric increase in length or diameter. Therefore as trees grow larger the 'factor of safety' against anchorage failure falls, making them prone to fail in their roots. These results suggest that only relatively small tree species can rely solely on the tap root to prevent uprooting. It may be for this reason that most larger trees develop thick lateral roots. (C) 1998 Annals of Botany Company.