Survival strategy of hiba (Thujopsis dolabrata var. hondae) and two broad-leaved trees in northern Japan in response to creep and glide pressures of snow.

We compared the tolerance of hiba (Thujopsis dolabrata var. hondae Makino), beech (Fagus crenata Blume) and oak (Quercus serrata Thunb.) to creep and glide pressures of snow. The top/root (T/R) ratio of hiba trees ranged between 7.1 and 41.4 compared with 3.6-10.9 and 1.0-12.4 for beech and oak, respectively. However, if prostrate stems bearing adventitious roots were included in the root component, the T/R ratio of hiba trees was comparable to the range observed for the co-occurring broad-leaved species (1.2 to 3.9). The adventitious roots originating from the trunks of hiba trees extended horizontally during the early growth stage. However, when stem diameter at ground level reached 6-7 cm, some large-diameter adventitious roots developed vertically to form, so-called, sustentacular roots. Because all of the dead hiba trees that we examined lacked sustentacular roots, we conclude that sustentacular roots are necessary to enable young hiba trees to withstand the pressures of snow movement. All of the large beech trees formed sustentacular roots and no dead beech trees were observed. Not all the oak trees formed sustentacular roots, which may explain why this species was found only on gentle slopes.