A mosaic distribution pattern of Black soil and Brown forest soil is commonly observed on the lower slopes of volcanoes in Japan. These two soils are often distributed on similar topography, and they are derived from the same parent material. One of the hypotheses concerning the genesis of these soils is that the Black soil may have been formed under grassland vegetation and the Brown forest soil under the forests in the past. Little research has, however, been carried out until the present. Plant opal and pollen analyses of soils were carried out to distinguish differences in past vegetation between the two soil types on Mt. Kurohime in Nagano Prefecture. The results obtained are as follows. (1) Plant opals from Panicoideae, Festuceae and Sasa spp. are dominant in the Black soils, whereas in the Brown forest soils those from Sasa spp. are dominant. (2) Non-arboreal pollens such as Gramineae and Compositae are dominant in the Black soils, although pollen of Fagus is a major component in the Brown forest soils. (3) From the results mentioned above, it is concluded that grassland vegetation covered the Black soils but the Beech-Sasa community grew on the Brown forest soils in the past.
Plant opals (PO) contained in the buried humic soils of volcanic ash profile are known to be mainly derived from grass. Thus, if we assume that PO in the soils is supplied at a constant rate from the grass and estimate the rate, then we can determine the ages of the soils from PO contents divided by the yearly supply or the accumulating rate of PO. The age here used does not mean the time passed since the soil formation but the period needed just for the formation of the soil itself. 1. Two methods for determining PO content in the soil were examined: (1) Direct isolation of PO from the silt and fine sand fractions of the soil by floatation on a heavy liquid (2.2-2.3 sp. gr.). (2) Counting PO particles in the fine sand fraction under the microscope and changing the count % into the weight % by a previously determined exchange rate. 2. Three methods for determining the accumulation rate of PO were compared: (1) Direct isolation of PO from plant tissues by wet or dry ashing followed by separation with a sedimentation method. (2) Counting PO particles in a weighed ash under microscope and changing the count % to the weight % of the ash by a preliminarily determined exchange rate. Methods (1) and (2) should be connected with the estimation of the annual production of grass species which can be obtained from ecological studies. (3) Determining the PO content in soil of known age by method 1-(1). The PO content divided by the age gives a mean annual PO accumulation rate. The values of the accumulation rates deduced by the three methods were concentrated in a narrow range of 0.36-0.47mg/cm2/yr when a few anomalous values were excluded. 3. In two profiles, Yubunebara and Tanukiko at the foot of Mt. Fuji, both being composed of alternation of tephras and buried humic soils, the ages of the soils were determined using the method 1-(1) and accumulation rates of PO, 0.36 and 0.47mg/cm2/yr. If we assume that the ages thus obtained could approximately show the periods during the formation of the soils in the past and that the period of deposition of tephras was extremely shorter than the time needed for the formation of the whole profile, then the sum of the ages of the soils should roughly be equal to the age of the whole profile. Comparison of the former with the latter obtained by 14C dating showed that the coincidence was good in the period of less than 5, 000-6, 000y.B.P.
Plant opals (PO) begin to suffer from chemical dissolution immediately after they come into contact with soil material. The dissolution effect appears as pinholes corroded on the surface of PO; these increase with time in size and number, suggesting that they could be useful as an age indicator of the buried soil.“Age”here used means period passed since the formation of the soil. The degree of corrosion was conveniently classified into one of seven grades (1-7), which could be judged by visual observation of the number and size of the pinholes on the surface of PO in a specimen mounted on a glass slide under the microscope and was designated as weight mean value of the grade numbers. To get a reliable result, the measurement of the degree of corrosion must answer the following basic questions: (1) How many PO particles should be observed to give a stable mean value? (2) Are the variances small enough among the values obtained from: (a) plural slide specimens of the identical soil sample, (b) repeated measurements of the identical slide specimens, (c) slide specimens of plural samples taken from similar soil, (d) slide specimens of samples from soil at a similar horizon but at different sites, and (e) measurements by several persons of the identical slide specimen? As the results of the examinations showed few problems with variances in the measurements, a standard procedure could be established: taking one sample from any soil at any site, preparing one slide specimen from the sample and measuring the degree of corrosion for 200 particles of PO. Soil samples from several sites at the foot of Mt. Fuji were subjected to measurement according to the standard procedure. The degrees of corrosion of PO obtained had a high positive correlation to the 14C or indirectly estimated ages of the soils at each site. But the regression coefficients varied from site to site, indicating that it is so far difficult to derive absolute ages exclusively from the measurement of the degree of corrosion of PO.
Two upland bogs in the Daisetsuzan Mountains, the Yutomuraushi bog and the bog lying on the eastern slope of Ponchubetsudake, have been investigated for the physical requisites for bog formation and vegetational succession. The Yutomuraushi bog lies on a gentle slope facing a landslide area, although the bog seems little affected by this landslide. 14C dating of a peat obtained from 175cm in depth of the bog suggests that the accumulation of peat began ca. 5000y.B.P. The bog lying on the eastern slope of Ponchubetsudake is developed on a plateau, which consists of Quaternary andesitic lava. 14C dating of a peat collected from the bottom of the bog indicates deposition ca. 7500y.B.P. Both of these bogs have blanketed the original slopes, filling up hollows and masking the original topography, and are considered to be affected by solifluction during periods of cool climatic conditions. The vegetational succession revealed by pollen analysis during the last ca. 7500 years is as follows: 1) ca. 7500-5000y.B.P. (Zone Po-III) In the upper part of the Daisetsuzan Mountains, Pinus, most probably P. pumila, was the dominant tree, whereas in the lower part such trees as Picea jezoensis or Picea glehnii were well represented, although an increase in Quercus occurred with climatic amelioration. In the bog lying on the eastern slope of Ponchubetsudake, there were ponds with Menyanthes during the early stage of this period. 2) ca. 5000-2000y.B.P. (Zone Po-II, Zone Yu-II) In the upper part, Pinus pumila decreased its area. In the lower part, Quercus dominated at the beginning of this period, while Picea, Abies and Betula were fairly well represented later, between ca. 3500 and 3000y.B.P. The vegetation of the bogs usually comprised Cyperaceae, Umbelliferae, Liliaceae and Artemisia. In the Yutomuraushi bog, ponds with Menyanthes were scattered during the early stage of this period. 3) ca. 2000y.B.P.- present (Zone Po-I, Zone Yu-I) An increase in Picea and a decrease in Quercus were noted. This would imply that the climate was cooler and wetter during this period.