Treelines of the Japanese Alps – altitudinal distribution and species composition under contrasting winter climates

Abstract

Summary In Japan, contrasting winter climate conditions between the Sea of Japan and the Pacific Ocean side have created conspicuous differences in species composition and altitudinal distribution of subalpine forests and treeline ecotones. In the Japanese Alps the altitudinal distribution of principal subalpine and treeline tree species was investigated along the slopes of four mountain ranges, two located in snowy regions facing the Sea of Japan (Mt. Hakusan, Mt. Naeba), and two with little snow cover located near the Pacific Ocean (Mt. Akaishi, Mt. Fuji). On Mt. Fuji many years of temperature records at different altitudes up to the treeline and at the summit provided the database for calculation of monthly lapse rates of air temperature, particularly the decrease of absolute and mean minima. High and prolonged snow cover causes the descent of the upper distribution limit of the subalpine coniferous forest by 800 to 1000 m altitude towards the Sea of Japan. A subalpine deciduous broad-leaved woodland of 500 m of vertical spread is formed only on snowy mountains, and does not exist on the Pacific Ocean side. With reference to Betula ermanii, the thermal upperdistribution limit of treelines in the Japanese Alps unimpaired by snow is determined by two periods of minimum temperature: (1) periods of subzero temperature which determine the occurrence of spring frost injury and affect the onset of the growing season, (2) minimum temperatures during the growing season which affect the thermal-time requirement (TTR) for fully differentiated tissues in overwintering plant organs. It is suggested to quantify TTR as thermal sum [(t–5.5), given in day-degrees], where t is the daily minimum air temperature above 5.5°C during the growing season. For B. ermanii the TTR is 210 day-degrees, which corresponds to 82 d of growing season length at the species’ upper distribution limit at 2800 m altitude. The descent of deciduous treelines on snowy mountains in humid northeastern Asia below the general thermal forest line of the Northern Hemisphere is discussed in terms of a hypoxia-induced carbohydrate limitation hypothesis. Tree survival will be im-paired by high and prolonged snow cover if oxygen deprivation causes hypoxia in wood parenchymas, which in turn will accelerate the depletion of nonstructural carbohydrate reserves. The impact of oxygen deprivation will increase towards spring, when most of the nonstructural reserve carbohydrates have been consumed so that enhanced ATP demand of live tissues at that time cannot be further compensated.