Thermal Conditions of the Upper and Lower Limits of Vertical Distribution of Main Tree Species and the Seletion of Species Adequate for Palaeotemperature Estimation in the Tohoku Mountains, Japan

The present thermal conditions of the boundaries between vertical vegetation zones have been used as the criteria for estimation of palaeotemperature by means of the vegetation inferred from pollen stratigraphy. Not a few pollen stratigraphies, however, do not always show the normal assemblages usually indicated by typical communities, and vegetation types must be inferred based on dominant tree species. On the other hand, in some mountains the distribution of comtenporary vegetation zones is modified by the local conditions of climate and landforms; this is called the “top phenomenon.” This means that some of the thermal conditions calculated from the present status of vegetation distribution are not relevant to the palaeotemperature estimation which is done by assuming the “normal” vertical vegetaion zones. This paper calculates the present thermal conditions of the upper and lower limits of distribution of six main tree species in the Tohoku mountains, Japan- Pinus pumila, Abies mariesii, Tsuga diversifolia, Betula ermanii, Quercus mongolica var. undulatifolia, and Fagus crenata-, and discusses the species adequate for palaeotemperature estimation based on the historical changes in vegetation and the statistical characteristics of frequency distribution of thermal conditions.<br> The thermal conditions of upper and lower limits of the species examined can be represented by indices of warmth index, annual mean temperature, mean winter (January and February) temperature, and mean summer (July and August) temperature, with almost the same margin of error. But the index of mean summer temperature, with a low standard deviation of about 1°C for every species, is superior to the others. In the Tohoku mountains, the subalpine zones are normally occupied by coniferous forests dominated by Abies mariesii, but in some mountains Abies mariesii is not indigenous and the thermometrical subalpine zone is occupied mainly by Quercus mongolica var. undulatifolia; this zone is called the “pseudo-alpine zone”. The principal cause of the pseudo-alpine zone is explained by the “pushing-out effect, ” which <br>means that the habitat of Abies mariesii was pushed over the mountain peak due to the upward migration of the vegetation zone resulting from the rise of temperature during the Hypsithermal climax phase. After the Hypsithermal climax phase, the areas where Abies mariesii no longer grew were occupied mainly by Pin us pumila and Quercus mongolica var. undulatifolia. Through this process, the habitat of Pinus pumila <br>spread down to the altitude corresponding to the lower limit of the subalpine zone, with the possibility of substituting for the whole habitat of Abies mariesii. But Quercus mongolica var. undulatifolia can be replaced only as far as the lower part of the habitat of Abies mariesii. Thus, the thermal condition of Pinus pumila calculated here is not appropriate to a standard value for palaeotemperature estimation. Since Quercus mongolica var. undulatifolia, has its unique thermal conditions, however, the palaeotemperature can be estimated from the pollen stratigraphical data of the mountains with pseudo-alpine zones.<br> From the statistical point of view, the thermal conditions of the upper limits of Betula ermanii, Abies mariesii, and Tsuga diversifolia and both the upper and lower limits of Pinus pumila are closely related to the relative distance from mountain peaks, suggesting that their positions are controlled by phenomena at the peaks.