Influence of wind and slope on buttress development in temperate tree species

A buttress is a prominent lateral-vertical structures of surface roots of a canopy or emergent trees frequently observed in tropical forests. Buttresses are probably formed to enhance mechanical stability of tree trunks and/or promote nutrient acquisition. However, the morphological diversity and control of buttress development remain unclear. Therefore, we aimed to clarify variations in buttress development related to prevailing wind and declination of tree trunks in warm temperate forests of Japan. We chose two pairs of forest sites with similar precipitation and geographical locations, but with contrasting wind regimes. Buttresses of two Castanopsis species, C. sieboldii and C. cuspidata, were assessed, and the size and direction of the most developed buttress (MDB) were measured for each individual. The average MDB height at the stem of trees at the strong wind site, Sumoto, was less than half of that at the control site, Himeji. However, the result was the opposite at the other strong wind site, Muroto, and control site, Kochi. The average MDB length did not differ between the strong wind and control sites. MDBs were formed corresponding to the direction of the most frequent wind at strong wind sites but not at control sites. The direction of growth of MDBs was almost the same as that at slopes at all the sites. Our results suggest that wind loading likely influences the development of buttresses; however, its effect could also be site-dependent. These results suggest the potential roles of buttress formation on mechanical stability of trees, which have not been appreciated in temperate forests.