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1 Frost resistance of Fagus crenata (Siebold’s beech) and Betula ermanii (Japanese mountain birch) was investigated with respect to the species’ altitudinal distribution on the Pacific slope of Mt. Fuji from 1996 to 1997. Flint’s Index of Injury, which is based on electrolyte leakage from freeze-injured tissue, was used to assess frost hardiness of shoots produced in the previous growing season.
2Fagus crenata is found on the lower slopes (700–1600 m a.s.l.). Mid- to late November hardening of shoots was enhanced, midwinter damage below −30 °C reduced and dehardening delayed nearer the upper limit. To here temperatures began to rise at least 3 weeks before dehardening began. Shade crown shoots were more susceptible to deep-freeze damage than light crown shoots. If the ultimate upper distribution limit was determined by frost hardiness, F. crenata would be expected to occur up to 1800 m altitude.
3Betula ermanii is found between 1600 m and 2800 m, and intensive hardening occurred at all altitudes during the second half of October. Frost hardiness increased considerably with altitude up to the forest limit, where frost acclimation preceded the temperature decline by 2 weeks. Once maximum frost resistance had been attained freezing to −47 °C failed to cause tissue injury. Dehardening began slightly later at the tree line, but the time–course was the same at all altitudes. Main and lateral shoots did not differ in frost hardiness.
4 Comparison of monthly air temperature minima over the past 66 years with the course of frost resistance showed that F. crenata and B. ermanii found on the Pacific slope of Mt. Fuji were unlikely to suffer damage by frost.
5 The observed uppermost distribution limit for B. ermanii at 2800 m altitude on Mt. Fuji is considered both with our observations and with previous hypotheses.
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In Japan, Siebold’s beech (Fagus crenata Blume) is the most dominant tree species of the cool-temperate deciduous broad-leaved forest zone. These Fagetea crenatae forests spread naturally over the northern half of Honshu and the southernmost part of Hokkaido (Ellenberg 1980; Miyawaki 1984, 1985; Sakai & Larcher 1987) between 700 m and 1600 m a.s.l. (Miyawaki 1984).
Japanese mountain birch (Betula ermanii Cham.) is a dominant deciduous tree in the mixed coniferous forest of subalpine regions, where it prevails at the tree line, commonly in co-existence with species of the genera Larix, Alnus, Salix and Rhododendron (Ohsawa 1984, 1990; Nakamura 1992). It can withstand severe cold (Sakai 1978; Sakai & Larcher 1987) and therefore also forms tree lines in Hokkaido, Sakhalin, the Kuril Islands and Kamchatka (Ohsawa 1990).
Mt. Fuji rises 3776 m from the Pacific coast of central Honshu and the altitudinal zonation encompasses patterns found in other Japanese forests. Deciduous broad-leaved forest is replaced above 1600 m by mixed subalpine coniferous forest, whose upper regions are characterized by Abies veitchii Lindl., Larix leptolepis Gord. and B. ermanii (Ohsawa 1984; Nakamura 1992). Subalpine deciduous forest, where B. ermanii is the dominant broad-leaved tree species, extends from 2300 m a.s.l. to the forest limit at 2500 m.
Clear skies in winter cause large daily temperature fluctuations and, except on the coastal plains, night frosts are severe here. Thus, frost hardiness determines the altitudinal distribution of F. crenata and B. ermanii in the montane and subalpine zones. Maximum frost hardiness of buds and twigs of B. ermanii was investigated by Sakai (1978), but its response to ambient low temperature and the seasonal variation of frost hardiness has not been investigated. Moreover, there have been no quantitative investigations of F. crenata for changes in its susceptibility to low temperature, a property that might determine the altitudinal distribution of this climax tree species.
The extent of and seasonal variation in frost hardiness were investigated for F. crenata and B. ermanii along an altitudinal transect on Mt. Fuji to enable a better understanding of the adaptive potential of a climax and a pioneer tree species to decreasing temperature. In this cold-determined Pacific winter climate, we assessed the upper distribution limits of F. crenata and B. ermanii in terms of the cold hardiness of the previous year’s shoots and disruption of the phenological cycle by spring and autumn frost.