Abstract
It is often said that the shallow rooted characteristic of coniferous tree species such as hinoki cypress and Japanese cedar leads to the collapse of relatively-young artificial forests when they meet with extraordinarily heavy rain. To examine this claim, we compared the vertical root distribution pattern between hinoki cypress (Chamaecyparis obtusa) and an oak (Quercus crispula). In a 48-year-old hinoki plantation, a hinoki individual (DBH : 14.2 cm, H : 11.9 m) and oak individual (DBH : 12.8 cm, H : 8.3 m) invading there were selected. Under each sample tree, a trench of 20 cm wide, 100 cm long, and 60 cm deep was dug, sited for the three directions from stem base, i. e. , up-slope, down-slope, and side-ward. The trench was further sub-divided into soil blocks of 10 cm deep and 20 cm wide. The horizontal distribution pattern of root weight on slope was different between the two sample trees. The oak sample tree developed much root in the up-slope direction but the hinoki in the down-slope direction. The vertical distribution of root weight density decreased exponentially with soil depth. The decreasing rate of root weight density was not different statistically between the two sample trees. The individual root weight was calculated by integration of the exponential equations for the arbitrary soil depth. The individual root weight from soil surface to 30 cm depth formed 89 and 94% of total root weight for the oak and hinoki, respectively. The two sample trees showed similarly the shallow rooted behaviour.
