Tanegashima in Kagoshima is well-known in Japan both as the first place where Western-style muskets were introduced by the Portuguese in 1543 and for the Tanegashima Space Center, which opened in 1988 and is located in the southeast corner of the island.
|A mangrove forest in Kumanoura on Tanegashima Island|
What people do not know (aside from a few interested specialists) is that Tanegashima is also home to the world’s northernmost mangrove forest. In fact, the mangroves on Tanegashima are much more extensive than those at the mouth of the Kurio River on Yakushima Island, 30 km to the south. The Yakushima mangroves are a World Heritage Site, so designated in December 1993.
Yet the Yakushima mangrove site is tiny and is in a bad state. The mangroves on Yakushima were much more extensive 30 years ago, but most were cut down.
|The mangrove creates a unique environment for other species.|
On Tanegashima, in contrast, the mangrove forest is in good condition, and yet, somehow, not one of the mangrove locations on Tanegashima has even prefectural park protection, let alone national or UNESCO recognition. When I inquired at the Tanegashima Development Center museum in Nishi-no-Omote (the island’s only city), I was informed that unfortunately there was not enough public interest to warrant protection.
In fact, Kazuo Odaki and the museum’s other staff members would only be too glad to see some sort of protection for these mangroves. Odaki has spent many years researching mangroves in Japan and has visited mangrove forests in tropical countries.
Tanegashima is located 40 km south of Cape Sata on the Osumi Peninsula. It is relatively flat and is long and narrow in shape, unlike its more famous sister island, Yakushima, which is round. Tanegashima’s highest point is only 282 meters above sea level. Its annual rainfall is 2,500 mm, and the temperatures range between 2.5 C and 33 C, giving an annual average of 18.5 C.
Some 1,156 species have been identified in this one island’s flora. Five species are found only on Yakushima and Tanegashima: the Armand pine (yakutane-goyo, Pinus armandii var. armandii); Yakushima crape myrtle (Yakushima sarusuberi, Lagerstroemia subcostata var. fauriei); kan-tsuwabuki (Farfugium hibernifolium), dwarf Yakushima violet (Yakushima hime-miyama-sumire, Viola sieboldii var. pseodo-selkirkii) and a terrestrial orchid, Yakushima ran (Apostasia nipponica).
Mangrove forests are very specialized ecosystems. Mangroves (Rhizophoraceae family) are saltwater-tolerant evergreens; they may be trees, shrubs or climbers. They grow in muddy silt at the edge of river estuaries, flooded by high tides twice per day. The trunks are supported by arching roots rising out of the mud; the root and mud provide a home for many species of fish and arthropod.
The arching roots, scientifically known as pneumatophores, grow out of the mud and have well-developed breathing holes known as lenticles through which vital oxygen enters. The oxygen is then transported down to the buried roots. The leaves on Kandelia candel are simple (i.e., have no lobes); they have a waxy surface to withstand high salt concentrations and they are also able to store water.
The water storage function is vital. Mangroves cannot use saline water for metabolic activity (breaking down of complex organic compounds into simpler compounds); instead they must use what river- and rainwater they can obtain.
Water is a rich source of nutrients. Detritus, organic debris from animals and plants, is washed down the rivers and accumulates around the pneumatophores. Bacterial action breaks down the nutrients, which are then released into the water. The soil is physiologically dry.
In Kandelia, the breathing holes (stomata) on the undersides of leaves are sunken deep so as to reduce loss of moisture. The white flowers are bisexual and borne in cymes. Mangrove reproduction is another interesting environmental adaptation: Seeds germinate while growing on the tree! The young root, or hypocotyl, elongates before dropping to the mud below. Of course the seedlings cannot tell whether the tide is in or out, but if they guess wrong, a percentage may still be washed back to the shore when the tide turns.
Mangroves forests are very delicate, vulnerable to minute changes in the chemical content of the water. They have survived virtually unchanged for thousands of years, but human mismanagement and lack of understanding has led to their decline in many areas, both in Japan and in tropical areas of the world.
Tanegashima is the most northerly natural mangrove forest in the world. There is a small mangrove patch in Kiire, a town between Kagoshima City and Ibusuki, but it is thought to be man-made. The Kiire mangroves are a national monument, and Odaki reckons the trees were planted around 1909. Now, due to encroaching sand, the Kiire mangroves are beginning to die.
There is also a mangrove plantation on the Izu Peninsula, the seed for which was collected from Tanegashima in 1958.
Tanegashima is too far north and too cool to support more than one species of mangrove, unlike Iriomote Island in Okinawa Prefecture, where seven species grow. The Tanegashima species is Kandelia candel, or Ryukyu kogai, also known as me-hirugi.
Tanegashima’s mangroves are found in two areas. The first is a tidal lagoon in Kumanoura, just before the space center. Fresh water is supplied by two small rivers, the Adake and the Oura. Due to the different salt concentrations in the soil, trees grow to different heights; where the salt concentration is highest, the mangroves only grow 50-60 cm. Where the water is fresher, the trees grow to 2 meters in height. This mangrove “forest” can easily be observed from the roadside. It forms a huge carpet of green.
More mangroves are found in the very north of the island, about 450-700 meters upstream from the mouth of the Minatogawa.
Trees here grow in three communities. Again, where the salt concentration is greatest the trees are smallest, but they are much taller than in Kumanoura. The smallest trees here are 4 meters and the tallest grow as high as 8 meters. Some are reckoned to be over 100 years old. The mangroves buffer extremes of temperature, the woodland staying a few degrees warmer or cooler than the area outside.
It would be great to see these two mangrove locations on Tanegashima protected for future generations to enjoy. This is a valuable natural resource. Next time you are planning a trip to Yakushima, maybe you should set aside some time for Tanegashima’s mangroves as well.