Micronesian Lowland Rainforest

There is little doubt that the coastal plains and slopes up to an altitude of about 300 m on all the volcanic islands of the Carolinas were once covered in tropical rain forest. The largest continuous stands today are found on Pohnpei, but there are also remnants on the west coast of Babeldaob, on the north coast of Arakabesang in the Belau group, on the upper slopes of Kosrae and on the low mountains of Chuuk. Many of the forest trees had great-buttressed trunks clothed in epiphytic ferns and orchids. Characteristic tree genera included Aidia, Calophyllum, Campnosperma, Clinostigma, Cynometra, Dysoxylum, Elaeocarpus, Eugenia, Fagraea, Ficus, Horsfieldia, Ptychosperma, Parinari, Pittosporum, Semecarpus and Sysygium. Many of the remaining species are endemic to Micronesia. In the remnant forest on Chuuk the two endemic species Schefflera kraemeri (Araliaceae) and Semecarpus kraemeri (Anacardiaceae) are now considered endangered.

Micronesian Limestone Forest

Forest on limestone that has been significantly elevated above sea level is one of the most important vegetation types in Micronesia. Large stands occur on Guam and other islands in the southern Marianas, in the southern half of the Belau Islands and Fais in the Carolinas, and on Banaba and Nauru west of the Gilbert Islands. Their canopy heights vary considerable but can exceed 30 m and there is often a sub-canopy layer that may reach 15 m or more.  On Guam the characteristic trees are largely composed of endemic species such as Aglaia mariannensis (Meliaceae), Atrocarpus mariannensis (Moraceae), Bleekeria mariannensis (Apocynaceae), Elaeocarpus sphaericus (Elaeocarpaceae), Glochidion marianum, Macaranga thompsonii (Euphorbiaceae), Guamia marianum (Annonaceae), Jasminium marianum (Oleaceae), Pandanus fragrans (Pandanaceae) and Psychotria hombroniana (Rubiaceae). Epiphytes are well represented with many bryophytes, ferns and orchids, and the strangling habit is common among species of Ficus. These begin life epiphytically then send down aerial roots, which eventually become so numerous that they kill the host tree.  The undergrowth also includes a wide variety of species including endemic shrubs like Discocalyx megacarpa (Myrsinaceae), Eugenia palumbis (Myrtaceae), Maytenus thompsonii (Celastraceae) and Piper guahamense (Piperaceae).  In fact, something like 36% of species in these forests are endemic, and usually have a higher level of endemism than any other habitat. Although certain trees such as Atrocarpus mariannensis, Cordia subcordata, Ficus prolixa, Mammea odorata, Merrilliodendron megacarpa and Pandanus tectorius may form local dominants, in general species distribution is so equitable that they are hard to characterize by reference to dominant species.

Micronesian Volcanic Savannas

The upland volcanic parts of many of the higher islands support grasslands of varying extent. These are generally referred to as savannas although they are normally not flat and trees may be absent. On Guam the dominant grasses include the endemic Dimeria chloridiformis (Poaceae), a tufted, soft bluish-green grass that occurs on the more level or gently sloping areas, or on the steeper slopes and ridges, Miscanthus floridulus, a coarse, sharp edged, cane like-grass that can reach heights of 3 m. The few trees may include Casuarina equisetifolia or the endemic Xylosma nelsonii (Flacourtiaceae).  Other associated endemic species include the shrubs Alyxia torresiana (Apocynaceae), Melastoma marianum (Melastomataceae), Myrtella bennigseniana (Myrtaceae), Pandanus fragrans forma savannarum (Pandanaceae), Timonius nitidus (Rubiaceae) and the herbaceous species Hedyotis megalantha (Rubiaceae). However, many of the characteristic savanna plants are pioneer species that rapidly colonize any denuded areas. The deeply weathered volcanic rocks on Guam often undergoes extensive erosion leaving bare scars for colonization by these species, while the more mature areas may be composed of almost monospecific grasslands.

Micronesian Montane Cloud (Moss) Forest

Botanically these dripping, soggy forests are some of the most interesting vegetation types of tropical islands with their gnarled, stunted trees covered in mosses and liverworts. In Micronesia cloud forest is confined to Pohnpei (Ponape) and Kosrae (Kusaie) in the Caroline Islands. On Pohnpei it is limited to a few hectares on the island’s summit, Mt Nahnalaud (772 m), where rainfall exceeds 5000 mm/year and ground level orographic clouds shroud the area on a daily basis. The forest canopy can be as low as 6 m, but canopy emergents can tower up to 25 m. The typical species are largely endemic and include Maesa carolinensis (Myrsinaceae), the palm Clinostigma ponapensis (Arecaceae), and the tree fern Cyathea ponapeana (Cyatheaceae), while on the more rocky terrains the endemic Astronidium ponapensis (Melastomataceae) and Garcinia ponapensis (Hypericaceae) predominate. The summit also includes several almost pure stands of the endemic Pandanus patina (Pandanaceae), which is frequently covered in the endemic liana Freycinetia ponapensis (Pandanaceae). Other common endemics include Cyrtandra urvillei (Gesneraceae) and Syzygium carolinese (Myrtaceae). On Kosrae the main canopy species include three ferns (Cyathea) and the endemic Astronidium kusaianum (Melastomataceae) and Elaeocarpus carolinensis (Elaeocarpaceae), while other common endemics include Cinnamomum carolense (Lauraceae) and Ptychosperma kusaiensis (Arecaceae). In the lower canopy Eugenia stelechanthoides and Polyscias subcapitata are common, while the many epiphytes include the endemic Elaphoglossum carolinense (Elaphoglossaceae) and Peperomia kusaiensis (Piperaceae).

Further information required.

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