Using the 1-km grid map of plant communities prepared by the Environment Agency of Japan, we recognized the five natural forest vegetation zones in Japan based on climatic data.
0. alpine : : Pinus pumila
and alpine grass-land (boundary WI= 23)
1. subalpine : : Evergreen coniferous trees (boundary WI= 45)
2. cool-temperate : : Deciduous broad-leaved trees (boundary WI= 74)
3. semi-temperate : : Evergreen coniferous replaced by deciduous broad-leaved trees : (boundary WI= 117)
4. warm-temperate : : Evergreen broad-leaved trees :
In Japan, which receives sufficient precipitation in summer, the thermal condition during the plant growing season is the overwhelming factor that sets the limits of plant distributions. Using the 1-km grid map of monthly air temperature prepared by the Meteorological Agency of Japan, we made a grid map of WI (warmth index of Kira's), defined as the annual sum of monthly mean temperature above 5°C excluding negative values. This value, an indicator of summer warmth, ranges from 0.8 at the top of Mt. Fuji (3, 776 m) to some 200 in the subtropical islands of southernmost Japan. WI is also an approximate indicator of annual potential evapotranspiration and net radiation, which are major factors of plant productivity.
W e calculated the boundary values of WI for the natural forest zones using the grid map of plant communities and the newly prepared map of WI. At a boundary value, the two adjacent zones have the same grid numbers, that is, occupy the same area. The boundary values are given above. These values were used for estimation of potential vegetation for the grid cells whose natural vegetation had been lost by human land use, and for simulation of probable vegetation zones under different climates cooler or warmer than the present. Before doing this we analyzed statistical relationships between plant occurrence and local topography such as the solpe and its direction, and the Laplacian value of height which indicates degree of convergence or divergence of water movements on the hill slope.
18.6 % of the total area of Japan is covered by natural vegetation, the rest by human-induced secondary forests (25.9 %), artificial afforestations (27.9 %), cultivated fields and grassland (21.4 %), rural and urban areas including industrial areas (6.2 %) etc. For these human-influenced areas we estimated potential vegetation on the basis of the boundary values of WI. Areal occupation ratios of the potential vegetation zones of Japan under present and different climates. are as follow :
Areal Occupation (%) of potential vegetation zones under different climates natural potential -7°C cooler -1.5°C cooler +2°C warmer
(remaining) (present) (full glacial) (little ice age) (CO2
alpine 0.3 0.8 26.9 1.4 0.6
subalpine 4.6 4.6 34.2 14.0 1.2
cool-temperate 12.0 31.8 37.8 34.1 22.1
semi-temperate 0.7 45.3 0.9 46.8 38.4
warm-temperate 1.0 17.5 0.2 3.7 37.7
Judging from the Plates on separate pages and the table above, it seemed that the shift of vegetation, zones from the glacial time to the present skipped over the adjacent zone. The horizontal distance was short in the case of the shift in mountanous districts but long in flatter lowlands. In addition to this, human land use interrupted the horizontal shift of vegetation, especially in lowlands, for the last several thousand years. These historical situations influence strongly the distribution and composition of the vegetation of Japan.