Tongan Tropical Moist Forests

The Tongan tropical moist forests is a tropical and subtropical moist broadleaf forests ecoregion that includes the Tonga archipelago and Niue.

Geography

The ecoregion includes the Tonga archipelago, a group of 170 islands that extends 800 km north to south between 15º to 23ºS latitude and 173º to 177º W longitude, and comprise the nation of Tonga. It also includes Niue, which lies 400 km east of Tonga at 19º S latitude and 169º W longitude.

The Tonga archipelago lies at the boundary of the Pacific and the Australian tectonic plates, and the islands were created by the subduction of the Pacific Plate under the Australian Plate. The islands on the eastern side of the chain are mostly made up of uplifted coraline rocks lying atop blocks of oceanic or continental crust. Some of the larger eastern islands are Foa, Lifuka, Vavaʻu, 'Eua, and Tongatapu. Several of the islands on the western side of the chain are of stratovolcanos, including 'Ata, Tofua, Kao, Late, and Fonualei in the south, and Niuafo'ou, Niuatoputapu, and Tafahi in the north. The highest peak in the islands is 1,030 metres on Kao. Many islands have fringing coral reefs, and others have become coral atolls as the central islands eroded away or subsided.

Niue is an uplifted coral atoll, with cliffs along the shoreline and without a fringing coral reef. It covers an area of 259 km².

The islands have been inhabited for about 3000 years.

Climate

The ecoregion has a humid tropical climate. Mean daily temperatures generally range from 24º to 30 °C in February, the warmest month, to 20º to 26 °C in August, the coolest month. Average annual rainfall is 2000 mm or more, and higher on windward slopes and at higher elevations. Rainfall is generally higher between December and April. Leeward lowlands can experience seasonal droughts between May and November.

Flora

The natural vegetation is moist broadleaf forest.

Tropical lowland rain forests predominate below 500 meters elevation. Evergreen trees form a continuous canopy up to 30 meters in height. Common canopy trees include ''Diospyros'' spp., ''Rhus taitensis, Alphitonia zizyphoides, Calophyllum neo-ebudicum, Elattostachys falcata, Litsea mellifera, Pisonia grandis, Pittosporum arborescens, Garuga floribunda, Ficus obliqua, Pleiogynium timoriense, Cryptocarya turbinata'', and '' Maniltoa grandiflora''. The understorey includes dense clumps of the fern ''Dicranopteris linearis'' and the shrubs '' Macropiper puberulum'' and ''Psychotria insularum''.

On coastal dunes and relatively recent lava and ash deposits where soils are thinner and drier, forests of ''Casuarina equisetifolia'' predominate, with low trees and shrubs including ''Pandanus tectorius, Syzygium dealatum, Hibiscus tiliaceus'', and ''Scaevola taccada''.

Mangrove scrub and mangrove forest are found in saltwater wetlands on coral atolls and around the shores of the larger islands' coastal lagoons. Mangrove scrub is found on estuaries and reef flats and on the edges of mangrove forests. The small- to medium-sized trees ''Rhizophora mangle, Rhizophora stylosa'', and ''Lumnitzera littorea'' are dominant. Where conditions are favorable mangrove forests dominated by ''Bruguiera gymnorhiza'' with ''Xylocarpus moluccensis'' and '' X. granatum'' replace the mangrove scrub.Whistler, W. Arthur (1992). "Vegetation of Samoa and Tonga". ''Pacific Science'' (1992), vol. 46, no. 2: 159-178

Montane forests are found above 500 metres elevation on the island of Tafahi. The climate is cooler and more humid than that of the lowland forests. Characteristic montane forest tree species include '' Syzygium samoense, Syzygium'' spp., ''Weinmannia'' spp., ''Fagraea berteroana, Bischofia javanica, Hernandia moerenhoutiana, Trichospermum richii, Spiraeanthemum samoense, Astronidium'' spp., and '' Reynoldsia'' spp. Other montane plant communities are upland volcanic scrub on Kao's young volcanic slopes, and fernlands dominated by ''Dicranopteris linearis'' with ''Lycopodium cernuum'' and ''Miscanthus floridulus'' in highland areas of 'Eua disturbed by human-caused fires.

There are 419 native species of vascular plants in the Tonga, and approximately 3% are endemic.

Fauna

There are no native land mammals in the ecoregion. Native bats include the Pacific flying fox (''Pteropus tonganus'') and Pacific sheath-tailed bat (''Emballonura semicaudata''). The black rat (''Rattus rattus'') and brown rat (''R. norvegicus'') were introduced to the islands by humans, and have decimated the island's birds.

Tonga has native 20 species of land and freshwater birds. The Tongan whistler (''Pachycephala jacquinoti'') is an endemic species found the Vava'u Group. The Niuafo'ou megapode (''Megapodius pritchardii'') is endemic to Niuafo'ou island. Other native birds include the red shining parrot (''Prosopeia tabuensis'') and blue-crowned lorikeet (''Vini australis'').

The islands are home to numerous seabirds, including brown noddy (''Anous stolidus''), Phoenix petrel (''Pterodroma alba''), and bristle-thighed curlew (''Numenius tahitiensis''). Seabird breeding colonies are mostly limited to offshore islets free of rats, which prey on eggs and chicks.

Niue has 12 native species of land birds and six species of seabirds, including two endemic subspecies.

There are 20 known species of reptiles in the ecoregion. They include the Lau banded iguana (''Brachylophus fasciatus''), the boa ''Candoia bibroni'', nine species of gecko, and nine species of skink. The skink '' Tachygia microlepsis'' is now considered extinct. Fossil remains of an giant land iguana have been found, which became extinct after the first humans arrived.

Protected areas

A 2017 assessment found that 117 km², or 21%, of the ecoregion is in protected areas.

References

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Oceanian ecoregions
Tropical and subtropical moist broadleaf forests
Biota of Tonga
Geography of Tonga
Environment of Tonga
Geography of Niue
Environment of Niue
Ecoregions of Tonga
Ecoregions of Niue