Air Temperature and Snowcover as Factors Controlling Distribution of Forest Zone in the Central High Mountains, Japan

Abstract

It is the purpose of this paper to clarify relationship between forest distribution and its controlling factors such as cumulative air-temperature (degree-day), landform characteristics and snow-cover duration using remote sensing images, DEM (Digital Elevation Model) and climatological data. The outline of procedure of investigation will be described below.
The spring, summer and autumn LANDSAT TM/MSS images were used to analyze seasonal change of vegetation and snow-cover. At first all images were converted into images resampled on a 56/58 m grid of DEM by using the nearest neighborhood method. Then the image data were processed in order to eliminate shadows caused only by azimuth and dip of each grid of DEM and direction of sunbeam. This processing is useful to improve the accuracy of landcover classification for a high relief mountainous region like this study area, the northernmost part of the Hida Range, Central Japan. Finally five categories ; dwarf pine, evergreen coniferous forests, deciduous broad-leaved forests, alpine meadow, bare grounds were interpreted, and a digital map of the estimated date of snow disappearance was obtained from the multi-temporal images.
Air-temperature for each pixel of the DEM was calculated using daily surface air-temperature at a local AMeDAS station (Automated Meteorological Data Acquisition System) and daily lapse rate in free air at the nearest aeorological observatory. The estimated values were tested comparing the values observed at temporal observatories of mountain huts during summer season. The cumulative air-temperature is defined as an integrated excess of 5°C of daily air temperature, and it is widely admitted that it controls primarily the distribution of forest plants in rainy Japanese Islands. The other controlling factors such as snow-cover duration and slope azimuth and dip were discussed statistically.
In conclusion ;
(1) Dwarf pine (Pinus pumila) is distributed in gentle and westward slopes of ridge zones with the maximum occupation ratio at ca. 2, 500 m. This is replaced by alpine meadows or bare grounds in and around snow patches durable to late spring or summer.
(2) Evergreen coniferous (Abies mariestii, A. veitchii, Tsuga diversifolia and Picea jesoensis var. hondoensis) forests are distributed with the maximum occupation ratio at ca. 2, 000 m. The coniferous forests will prefer a habitat which is West- to North-facing slope with inclination of less than 25. In the higher zone than 2, 000 m, these forests compete with dwarf pine or birch (Betula ermanii) forsts for occupation according to controlling factors. The coniferous plants are seemed to be snow-intolerant and easily damaged by a loss of the cumulative air-temperature due to snow-cover duration, and by mechanical and physiological injury by snow-cover itself.
(3) Deciduous broad-leaved (Fagus crenata, Quercus mongolica var. crispula, Betula ermanii etc.) forests are widely distributed in this area. The Occupation ratio curve has bi-modal peaks at 2, 300 m and 1, 500 m. The lower one indicates the normal altitudinal distribution of forest zone according to thermal condition. The higher one is introduced by occupation of Betula ermanii in alpine-subalpine transitional zone. Betula ermanii is snow-tolerant and distributed in mountain slopes of all directions with the maximum frequency in South to East. The east-facing slope is leeward of the winter snow-bearing winds, but receives much snow accumulation by the blowing effect. Betula ermanii will occupay a habitat unfavorable for the coniferous mainly because of much snow accumulation.