A forest limit marks a distinct change in landscape, but it includes a variety of elements. In this study, the forest limit altitudes all over Japan were compiled, the relationship between the specific features of their distribution and thermal conditions were discussed.
The forest limit altitudes on 211 mountains ranged from below 1, 000 meters in Hokkaido to 2, 800 meters in Central Honshu, and difference in these altitudes extended about 2, 000 meters between 35° and 45° North Latitude. Horizontally, these altitudes are distributed concentrically, focusing on the Taisetsu Mountains and the Hidaka Mountains in Hokkaido and the Akaishi Mountains in Central Honshu. The zone ranging from the southern part of Northern Honshu to the northern part of Central Honshu is crowded with isopleths, forming a division between the sides of the Japan Sea and Pacific Ocean. Calculation the Warmth Indices (WI) of forest limit altitudes, we find that their frequency distribution ranges from 16.1°C·M to 50.4°CEM, and their modes are 25 to 35, 34 to 40, and 25 to 30°C·M in Hokkaido, Northern Honshu, and Central Honshu, respectively. Reviewing these data individually, we see that forest limits are seldom equivalent to WI 15°C·M, which has been set as a boundary between the alpine and subalpine zones. In reality, it becomes increasingly difficult for the forest limit to converge on a particular WI value due to a variety of causes. It is worth noting, however, that at least there are some mountains on which the forest limit altitude is extremely close to WI 15. In such a situation, the WI 15 is of great importance, because it indicates that forests are capable of growing at least to that extent in terms of thermal conditions. This is the reason why WI 15°C·M is worth notice, and therefore the relationship between its altitude distribution and the forest limit altitude was determined. Additionally, the relationship between mountaintop altitude and forest limit altitude was also investigated. The correlation coefficient of the latter is more closely related than the former. But we must emphasize that the latter is only a seeming relation. Why? It is true that the higher the mountain, the higher the limit altitude, but the fact is that the limit altitude will not rise endlessly in step with the height of a mountain.
Then, another relationship was examined, concerning differences not only between WI 15 altitudes and forest limit altitudes but also between mountaintop altitudes and forest limit altitudes, using the variation coefficient to verify the dispersion in these differences. As a result, we found out that dispersions are smaller in the former (WI 15 and forest limit) th an in the latter and that forest limit altitudes are more closely related to thermal conditions. On the other hand, the differences between WI 15 altitudes and the forest limit altitudes are defined by mountaintop altitudes. Additionally, the depth of snow cover strongly affects the difference in forest limit altitudes, if mountaintop altitudes are the same.
The role of thermal condition, mountaintop altitude and snow depth condition for determining the forest limit altitude should be easy to determine as Fig. 10. Namely, altitudes of forest limit are primarily decided by thermal conditions depended on their geographical situation. They are secondarily modified by altitude of mountain. When the mountain altitudes are equivalent, snow depth conditions affect the forest limit.
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