The function of buttress roots: a comparative study of the anchorage systems of buttressed (Aglaia and Nephelium ramboutan species) and non-buttressed (Mallotus wrayi) tropical trees

The anchorage mechanics of mature buttressed trees of Aglaia and Nephelium, and of non-buttressed Mallotus wrayi have been investigated by combining a study of the morphology of their root systems with a series of anchorage tests. Both types possessed tap roots, but only buttressed trees possessed sinker roots, which branched from the ends of the buttresses, The anchorage strength of the buttressed trees was almost double (10.6 kNm) that of the unbuttressed ones (4.9 kNm), and the maximum moment was generated at lower angles. In buttressed trees, the leeward buttresses were pushed into the soil before bending and eventually breaking towards their tip, whilst the windward buttresses pulled out of the soil or delaminated if they possessed sinker roots, The tap root rotated in the soil to windward. In contrast, during failure of unbuttressed trees the tap root both moved and bent towards the leeward, the windward roots were pulled out of the soil, and the leeward laterals simply buckled, Strains along buttresses were much higher than along the laterals of unbuttressed trees. These results suggest that buttresses act in both tension and compression and make a much larger contribution to anchorage than the thin laterals of non-buttressed trees. The relative contribution of the buttresses was determined by carrying out a further series of anchorage tests in which both buttressed and unbuttressed trees were pulled over after all their laterals had been cut away. These trees were therefore only anchored by their taproot. Failure of both types was similar to intact unbuttressed trees, and they had similar anchorage strengths to each other, 4 kNm, around 80% of the value for intact non-buttressed trees, but only 40% of the strength of intact buttressed trees. Buttresses therefore contribute around 60% of the anchorage of buttressed trees, producing around six times more anchorage than the thin laterals of unbuttressed trees.