World Forests and Japanese Forests (Part 7) by Yoshishige Tachibana

Forests in Arid Zones
When we say arid zones, we do not mean dry areas where only grass grows, such as deserts, steppes, or savannas, but rather arid climate zones (B climates) where forests can at least grow. Japan is a humid zone (A climate) nationwide, so there are no B climate zones here, but there are many B climate zones in the world. One could say that most of the continents of Eurasia, North and South America, Africa, and Australia are B climates. Our neighbor, China, has an A climate only in its eastern coastal region, while most of the west is a B climate. Moreover, it has vast, extremely dry desert areas.
Everything has a middle ground, and climate zones often have intermediate or transition zones as well. Shanxi Province in China, where we are helping with afforestation, is located in such a transition zone. However, due to the influence of mountain ranges, there are cases where there is no transition zone and the climate changes abruptly. For example, the eastern part of Mexico drops sharply from the central plateau, and from that line, it changes suddenly into a humid zone. It is an abrupt change from B to A.

Warm-Temperate Deciduous Forests
Japan also has warm-temperate deciduous forests, but their boundaries are not clear, and they are often mixed with evergreen trees. These include forests of sawtooth oak, daimyo oak, and konara oak, but they are on a small scale, changing even just by crossing a single ridge.
The warm-temperate zone is the southern part of the temperate zone that borders the subtropical zone. In terms of the warmth index, it naturally exceeds 120. Therefore, if there is sufficient rainfall, evergreen forests would form, but because water is scarce, they become deciduous forests. This can be due to a year-round average lack of rain, or it can be seasonal, with distinct dry and rainy seasons.
There is a city called Changsha in the southern part of the middle reaches of the Yangtze River, almost in the center of China, and the warm-temperate deciduous forest in a nature reserve on the outskirts of that city was impressive. In terms of latitude, it is close to the subtropical zone, but because it is on a continent, the temperature difference is significant, and it seemed that tree species sensitive to cold do not grow there. Chinese tallow, hackberry, Chinese toon, paulownia, and trident maple were all familiar species as street trees in Japan.

Eucalyptus Forests
Eucalyptus trees are a specialty of Australia, and they are technically evergreen trees. I say "technically" because, although they are green, they have a brownish-silver hue that is hard to call green, yet they are not deciduous. They are resistant to drought and develop in climate zones that would otherwise be warm-temperate deciduous forests. When grown in Japan, they show quite a bit of green, but in their native land, they were truly silver. It is a large genus with 300 species, distributed from the tropics to the subtropics, with some species found on mountain ridges and others in the wetlands of valley floors. Australia is a dry continent, and its vegetation zones are clearly divided by the duration of rainfall (Figure).

They have been planted in various parts of the world with similar climate zones, and they are widely planted in California in North America, Madagascar, and Uruguay in South America, where they have spread as if they were wild. The old trees in Japan are of the globulus species, which is common in Tasmania and suited for cold regions; it is said that they were brought by missionaries at the end of the Edo period or the beginning of the Meiji period. Many species were introduced, but they were not suited to humid Japan and eventually disappeared, leaving only this globulus species.
(Published in Green Earth No. 53 (January 1997))