We carried out a phytolith analysis of soil sequences of three regions, Haboro, Sarobetsu, and Tonbetsu, in the northern part of Hokkaido to discuss the fluctuation of Sasa since the Last Interglacial period and evaluate Sasa as an index of past snowfall and temperature. The phytolith record suggests that Sasa has continuously existed after the Last Interglacial in Haboro. On the other hand, Sasa almost disappeared during the Last Glacial period in Sarobetsu and Tonbetsu. Previous research shows that Sasa drastically decreased in the northern and eastern parts of Hokkaido during the Last Glacial period. The present geographical distribution of Sasa shows that Sasa needs conditions of a warmth index of more than 17·°C month and a snow depth of less than 50 cm for its existence. Other paleobotanical information suggests that Hokkaido satisfied the condition of a warmth index of more than 17·°C month during the Last Glacial period. Thus, the decrease of Sasa must have been caused by the lack of enough snow. Considering the slow migration speed of Sasa by vegetative reproduction, Sasa seems to have existed sporadically in refugia such as Haboro with an enough snow fall during the Last Glacial period and spread over the whole Hokkaido in the Holocene period.
We reconstructed the vegetation history since the mid-Holocene in the subalpine area of the central Kitakami Mountains, northern Japan, by pollen analysis using soil samples taken from two Abies mariesii forest sites, the Aomatsuba (AMT) site located in a scattered subalpine conifer stand on Mt. Aomatsuba and the Odagoe (ODG) site located in the central part of the developed subalpine conifer zone on Mt. Hayachine. Pollen diagrams show that the vegetation of the montane zone around the study sites were continuously a deciduous broadleaved forest composed mainly of Fagus, Betula, and Quercus subgenus Lepidobalanus species after the fall of Towada-Chuseri tephra (To-Cu) erupted in 6000 yBP. Abies pollen increased abruptly after the fall of Towadaa tephra (To-a) erupted in AD 915 at both sites. A distinct difference, however, existed between the two sites. At ODG, Abies pollen contined to occur in small quantities in layers from To-Cu to To-a, but no Abies pollen occurred in these layers at AMT. These results revealed that A. mariesii was distributed around ODG until mid-Holocene, whereas A. mariesii appeared approximately 1000 years ago around AMT.
The material plant of a tinny basket excavated from the San’nai-maruyama site (early Jomon period) of Aomori, Aomori Prefecture, was identified as a stem of a monocotyledon in 1993. On the other hand, a further observation of the basket suggested that the studied material did not derive from the basket. Thus, a newly obtained material of the basket was reexamined anatomically and was shown to be the torn inner bark of a Cupressaceae tree. The original material formerly identified as a monocotyledonous stem was re-identified as a petiole of a fern akin to Arachniodes standishii (Dryopteridaceae). Furthermore, a rope excavated from the Sakuramachi site (middle Jomon period) of Oyabe, Toyama Prefecture, was also made of the same material. These results show that the original sample of the basket was a contamination from sedimentary deposits. This is the first report of a basket made of conifer bark and a rope made of fern petioles in the Jomon period of Japan.