Stories from a Plant Shop (Sequel): Plants of Arid Regions Part 22 by Yoshishige Tachibana
Plants of Arid Regions
Since Japan is a country with plenty of rain, I think there are few people who have experienced life in arid regions, and thus few who think deeply about the impact of water on vegetation. I have touched upon the world's arid regions in this magazine several times in the past, but this time I will write about the vegetation of the arid continent of Australia.
I have only visited the southeastern part, but that is Australia's only forest zone. When looking at the Australian continent from an airplane, unlike the green land of Japan, reddish-brown land continues endlessly. Eventually, you approach the southeastern forest zone, but there is no green. As the altitude drops, it becomes a silvery land. Just before landing, I realized that it was a eucalyptus forest. The eucalyptus trees planted in Japan shine silver, but they are generally green. Here in Australia, they are completely silver. Once I landed and drove around, I realized there are a truly vast number of eucalyptus species. However, most species are silver. Very rarely, there are some that are about as green as the species planted in Japan.
Eucalyptus Forest
The eucalyptus tree is a specialty of Australia, and it is technically an evergreen. I say "technically" only because I am being particular about the word "evergreen"; it is not a deciduous tree. It is resistant to drought and is a type that would normally develop in a warm-temperate deciduous forest zone. It is a large genus with 300 species, distributed from the tropics to the temperate zones, with species adapted to mountain ridges and valley bottoms. The one introduced to Japan long ago that has grown into a large tree is E. globulus, which comes from Tasmania, a rainy and cold place south of Australia, and it was green even in its native land. Also, the E. camaldulensis species found in the riparian forests of the northern subtropical region has a relatively good green color. This species was used for greening in Thailand and achieved good results.
Rainfall Duration and Distribution
Australia is an arid land, but 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 in places like California in North America, Madagascar, and Uruguay in South America, they grow into large, lush trees, so much so that one might wonder if those places are the original habitat. Many species were introduced to Japan, but due to the heavy rain and typhoons, they were all wiped out, and there is not a single place where planting has been successful. I introduced over 30 species to the botanical garden where I worked and established an Australian section for planting, but the only one that remains is the aforementioned E. globulus species.
Surprisingly, the wild trees I saw in southeastern Australia were only large trees with a diameter at breast height of about 50 cm, but in Madagascar, there were many that exceeded 1 meter in diameter. I cannot help but think of this as the relationship between the UK and the US, if I were to use countries as an analogy. It may be the same as how the ginkgo and camphor trees, native to China, grow more vigorously and larger in Japan than in China.
(Published in "Green Earth" Issue 124, November 2008)