Japan has six native species of palm tree. Four of these are endemic to Japan. There are a further three species, which have been cultivated in Japan for centuries. These are:
Rhapis excelsa (Thunb.) Henry, J. Arnold Arbor. 11: 153 (1930).
Rhapis humilis Blume, Rumphia 2: 54 (1839).
Trachycarpus fortunei var. wagnerianus Becc., Webbia 5: 70 (1921).
Two native species are found only in the Japanese Ogasawara-shoto or Bonin Islands (typical oceanic islands, located 1,000 km south of Tokyo, Japan). These are:
Clinostigma savoryanum (Rehder & E.H.Wilson) H.E.Moore & Fosberg, Gentes Herb. 8: 465 (1956). Endemic.
Livistona boninensis (Becc.) Nakai, J. Jap. Bot. 11: 222 (1935). Endemic.
The remaining four native species are the subject of this article, and these species are found in the Ryukyu archipelago of Japan or Nansei-shoto. These are:
Arenga ryukyuensis A.J.Hend., Taiwania 51: 298 (2006). Endemic.
Livistona chinensis var. subglobosa (Hassk.) Becc., Webbia 5: 16 (1920).
Nypa fruticans Wurmb, Verh. Batav. Genootsch. Kunsten 1: 349 (1779).
Satakentia liukiuensis (Hatus.) H.E.Moore, Principes 13: 5 (1969). Endemic.
The Ryukyu Islands (Figure 1) are a chain of Japanese islands that stretch south-west from Kyushu to Taiwan: the Ōsumi, Tokara, Amami, Okinawa, and Sakishima (Miyako and Yaeyama) islands, with Yonaguni the southernmost. The largest of the islands is Okinawa. These islands have a subtropical climate with mild winters and hot summers.
This palm is very similar to Arenga engleri from Taiwan, but differs in the pinnae being strongly ribbed adaxially, and the stems are only to about 2 m tall, whereas, Arenga engleri stems grow to over 4 m. tall. Arenga ryukyuensis seeds are also very globose, short and fat, whereas, Arenga engleri seeds are more elongate, also generally larger. Arenga engleri in Taiwan occurs at 200 - 1050 m. elevation, whereas, Arenga ryukyuensis is a lowland species, and occurs from sea-level up to about 300 m. Indeed, the more common localities to see A. ryukyuensis is on the coral limestone coastal rocks, often in the spray zone.
A. ryukyuensis is usually seen growning together with Cycas revoluta, which is also very common, and can also be seen throughout the Ryukyu Islands growing on coastal rocks and cliffs.
Both the cycads and the palms are more frequent at lower levels, becoming more scarce at elevation, nevertheless, both are found at the highest elevations in Okinawa.
Another difference between Arenga ryukyuensis and Arenga engleri is the infructescence. A. ryukyuensis fruits are somewhat exposed and visible from above, growing out of the top of the plant. Whereas, A. engleri fruits are often hidden in amongst the leaves, and almost never visible from above. Fruit of A. engleri ripens from green, through an orange/yellow to dark purplish red. A. ryukyuensis ripens from green through yellow to orange then dark red.
Taiwan is a new island that started being pushed up from the sea-bed around 6.5 Ma, by the north eastward movement of the Philippine plate crashing into the Chinese continental margin at 8 cm. per. year. This would have crashed through the Ryukyu archipelago / Luzon volcanic arc, pushing any pre-existing islands into the new Taiwan landmass. It is therefore logical to assume that Arenga existed first in the Ryukyu, which are very much older islands, and was then taken to Taiwan to evolve into Arenga engleri. It is not possible that there has been a land-bridge between Taiwan and the Ryukyu since that time.
Livistona chinensis var. subglobosa
I have written before about this species, so will not spend too much time discussing it here. Only to say that Livistona chinensis does not exist in its truly wild state anymore anywhere within the Ryukyu or Taiwan. In Japan, so-called virgin L. chinensis forest is now found only on the islets of Aoshima and Tsukishima in the Miyazaki prefecture, Kyushu, Japan. The islet of the gods on Aoshima is the extreme northern limit of the species, and this is also officially recognised as the largest single population of the species in Japan consisting of 4000 individuals. But as I have published before, this is not acurate, the largest virgin population of Livistona chinensis var. subglobosa is to be found on a tiny island called Uotsurijima (Japanese) or Diaoyudao (Chinese). This is the largest island of the Senkaku Islands (釣魚台群島 Diàoyútái Qúndǎo) or what we know as the Pinnacle Islands. Uotsurijima or Diaoyudao Island located at 25°44′39”N 123°28′26”E has an area of 4.3 square kilometres (1.7 sq mi) and a highest elevation of 383 metres (1,260 ft). L. chinensis is the dominant tree species on this island and I estimate this population to be over 100,000 individuals.
It is more often than not that the Japanese Livistona trunks are seen to be leaning, bent, twisted, and show irregular growth patterns. I’ve questioned this in Japan, and was told that the Ryukyu islands experience many typhoons, which bend the trees over. But, coastal Taiwan experiences the same typhoons, and the Taiwan trees tend to have much straighter, upright trunks. Another, difference is that many of the Ryukyu trees produce much more globose, almost round, seeds than do the Taiwan trees. The Taiwan Livistona produce more globose seeds than do the Chinese trees.
Nypa fruticans exists in Japan in one single, very isolated, and inaccessible population at Funaura on the island of Iriomote Jima.
This population of about 28 or more clumps are located a long way up a tidal tributary stream in a large mangrove swamp that surrounds a stunningly beautiful tidal estuary on the north of the island.
I must say that these Nypa palms are not good examples. Much better examples are to be seen, in much more accessible locations elsewhere in South-east Asia. The Japanese Nypa are located deep in an important mangrove, declared as a natural monument in 1959, well off the beaten track. This mangrove is an important habitat for wildlife, and is best left to the wildlife that inhabit it.
The population is the world’s northernmost natural occurrence, and it has been rapidly reduced in size. Its genetic diversity examined by the Random Amplified Polymorphic DNA(RAPD) method showed that all 28 individuals examined were genetically identical and had no diversity. They are thus considered clones derived from a single individual by vegetative propagation. Because flowers fail to set fertile seeds, the species is likely to be self-incompatible. The population at Funaura is at an extinction crisis.
Satakentia contains only one species, which is endemic to Japan in the far south of the Ryukyu Islands in the islands of Ishigaki Jima and Iriomote Jima. This genus was named by Harold Moore for Toshihiko Satake, who had noticed it was something special. There is now a museum built to honour Toshihiko Satake within the main population of the palms on Ishigaki Jima.
Trunks can be very tall, brownish/grey, and solitary, topped with a prominent, brown or reddish green crownshaft, which is very distinguishable. With large green pinnate leaves, 3 m. long.
The inflorescences are also distictive, which are branched to two orders, and are borne below the crownshaft.
Satakentia liukiuensis is in decline in its natural environment with no known cause, and it was once much more widespread throughout the two islands than it is today. However, plants are being raised in cultivation and are widely planted as a street tree in cities further north, notably Naha on Okinawa.
Satakentia is grouped in the subtribe Carpoxylinae, which comprises three genus - Carpoxylon, Satakentia, and Neoveitchia. Carpoxylon and Neoveitchia come from Vanuatu and Fiji, it is not clear how the natural distribution could extend to the Japanese Ryukyu for Satakentia.
There are two main wild populations, the main population on Ishigaki Jima, and a much smaller population on Iriomote Jima with a few individual trees scattered across Iriomote. The Iriomote trees are inaccessible, as they grow in a cemetery, or the isolated trees are remote in the hills.