Western Polynesian tropical moist forests

The Carolines tropical moist forests cover the central and eastern Caroline Islands of the Federated States of Micronesia, spanning the high volcanic islands of Chuuk, Pohnpei, and Kosrae together with their surrounding atolls. Originally clothed mostly in tropical rainforest, with mangroves along the shore, the islands grade from lowland forest up through montane forest to summit cloud forest, supporting tree and shrub genera such as Syzygium, Glochidion, Myrsine, Elaeocarpus, and Psychotria alongside Cyathea tree ferns, Pandanus, and native palms. The climate is humid and tropical with little seasonal temperature variation and very heavy rainfall that rises toward the east, where Pohnpei averages more than 4,500 mm a year and parts of Kosrae exceed 6,400 mm. Remarkably, cloud forest forms here at only about 300 to 450 meters elevation, among the lowest-elevation tropical cloud forests in the world, and thirteen bird species are endemic to the ecoregion, including the flagship Pohnpei lorikeet. For gardeners, the region is the native home of ornamental tropicals including Pandanus screwpines, Cyathea tree ferns, and palms in genera such as Clinostigma and Ptychosperma.

The Central Polynesian tropical moist forests stretch across the scattered low islands of the central Pacific, spanning the northern Cook Islands, the Line Islands of Kiribati, and the United States possessions of Johnston Atoll, Jarvis Island, Palmyra Atoll, and Kingman Reef. Almost all of these are coral atolls with open or closed lagoons or raised reef platforms, most lying only a few meters above sea level, so their coastal forests are dominated by salt- and wind-tolerant Indo-Pacific trees such as Pisonia grandis, Calophyllum inophyllum, Pandanus tectorius, Cordia subcordata, and Guettarda speciosa, over an understory of Scaevola taccada, Morinda citrifolia, and Pemphis acidula. The climate is uniformly warm and tropical but ranges from continually wet to drought-prone depending on each island's position relative to the equator and the trade-wind belt. With no native non-marine mammals or amphibians, the ecoregion's wildlife centers on seabirds and endemic land birds like the bokikokiko reed warbler, and several islands hold notable populations of the coconut crab. For gardeners in warm coastal climates, native genera here, including Pandanus, Scaevola, Cordia, and Calophyllum, are familiar salt-tolerant ornamentals.

The Cook Islands tropical moist forests cover the Southern Cook Islands, a chain of extinct volcanic and raised-limestone islands in the South Pacific that includes Rarotonga, Aitutaki, Atiu, Mangaia, Mauke, Mitiaro, Manuae, Palmerston, and Takutea. Vegetation grades by elevation from lowland Homalium and Fagraea-Fitchia forest into low-stature Metrosideros cloud forest above about 400 metres on Rarotonga's steep slopes, while the surrounding makatea limestone belts carry Elaeocarpus, Hernandia, and Pandanus. The climate is humid and tropical, shaped by the southeast trade winds, with windward sides wetter than leeward ones and peak rainfall around November and December. Though small, the archipelago is rich in endemics: the blue lorikeet is the flagship species, and six landbirds are strictly endemic, among them the Rarotonga monarch and Mangaia kingfisher, with the Pacific flying fox the only native non-marine mammal. The ecoregion is considered critically endangered, with only a small fraction protected, chiefly Rarotonga's Te Manga cloud forest, and native plants and snails remain pressured by introduced species. Gardeners may recognize native genera such as the screwpine Pandanus and the ornamental Scaevola and Heliotropium of the coastal flora.

The Eastern Micronesia tropical moist forests stretch across the low coral islands of the central Pacific, spanning the Marshall Islands and the Gilbert Islands and Banaba of Kiribati, plus the single-island outliers of Nauru and Wake Island. Most of these landmasses are atolls of coralline sand encircling a lagoon, or raised platforms of coralline limestone, with the wetter islands carrying tropical moist forest. Characteristic trees and shrubs include Pisonia grandis, which can tower to around thirty meters and shelter seabird rookeries, alongside Pandanus tectorius, Guettarda speciosa, Cordia subcordata, Scaevola, and Calophyllum inophyllum. The climate is tropical with little seasonal temperature change; islands in the trade-wind belt receive the heaviest rainfall while the northern Marshalls, Wake, and southern Gilberts stay drier. The ecoregion's flagship is the coconut crab, the world's largest land-living arthropod, and it harbors endemics such as the Nauru reed-warbler, though it is highly threatened by invasive species and rising seas. For coastal or tropical gardeners, several of its natives, including Cordia subcordata, screw pine (Pandanus tectorius), and Calophyllum inophyllum, are familiar ornamental and shade plantings.

The Fiji tropical moist forests cover the wetter windward sides of Viti Levu and Vanua Levu, Fiji's two largest islands, along with the smaller Fijian islands and the three islands of Wallis and Futuna, a French overseas territory in the South Pacific. Across an elevational gradient the ecoregion grades from lowland rainforest into montane forest and, on the higher ridges, cloud forest, with characteristic trees including Degeneria, the southern conifers Agathis and Podocarpus, Pandanus, Calophyllum, and Metrosideros, plus tree ferns such as Cyathea. The climate is warm and consistently wet, with most areas receiving over 2,500 mm of rain a year and the windward mountains far more. The forests are exceptionally rich in endemics, harboring the primitive flowering-plant family Degeneriaceae found nowhere else and many island-restricted species, yet they are considered critically endangered with only a small fraction held in formal protected areas.

The Hawai'i tropical moist forests blanket the windward lowlands and montane slopes of the Hawaiian archipelago in the Pacific. The ecoregion is a mosaic of coastal and mixed mesic forests, montane rainforests, wet shrublands, and bogs, with 'ohi'a lehua (Metrosideros polymorpha) and koa (Acacia koa) forming the dominant canopy alongside genera such as Cheirodendron, Melicope, Myrsine, and Syzygium. Its climate is shaped by moisture-laden trade winds off the windward slopes, and it harbors one of the world's wettest places on the slopes of Mount Waiʻaleʻale on Kauaʻi, which averages roughly 9,500 mm of rain a year. Tens of millions of years of oceanic isolation have driven extraordinary endemism here, from fungi and land snails to the spectacular radiation of Hawaiian honeycreepers, and the forest's flagship is the scarlet I'iwi. That heritage is now imperiled: of more than 110 native bird species the forests once held, only 48 remain, and the ecoregion is considered critically endangered. Native ornamentals from these forests include the loulu fan palms (Pritchardia) and hala (Pandanus tectorius).