Broadleaved forests

Littoral Forest

Inland from the coastal herb zone, typical woody littoral species include Hibiscus tiliaceus, Pandanus tectorius, Scaevola taccada, Thespesia populnea and Tournefortia argentea. A tree assemblage of Barringtonia asiatica, Calophyllum inophyllum, Heritiera littoralis and Terminalia catappa can often be found growing together. On disturbed sites, Casuarina equisetifolia often fills in.

Mangrove Forest

Occurring in sheltered coastal bays and along river mouths, two structural types of mangrove forest can be distinguished: one is a low, stunted, 2.5 m tall forest dominated by Rhizophora apiculata; the other is up to 25 m tall and composed of Bruguiera parviflora, B. sexangula, Rhizophora apiculata and R. stylosa, with local populations of Dolichandrone spathacea. Other mangrove species include Ceriops tagal and Lumnitzera littorea, the latter sometimes forming pure stands. The palm Nypa fruticans is also present. Differing structure and composition are related both to habitat differences and to past habitat disturbance.

Freshwater Swamp Forest

There are extensive areas of freshwater swamp in the Solomons, especially in the New Georgia group of islands, and several distinctive types can be delineated. In different areas, the swamp is dominated by a closed canopy of the trees Campnosperma brevipetiolata or Terminalia brassii, or by a mix of species; Inocarpus fagifer and Syzygium tierneyana are generally restricted to these poorly drained sites, but other species, such as Barringtonia racemosa, Calophyllum vexans, Campnosperma brevipetiolata, Intsia bijuga, Pterocarpus indicus and Terminalia brassii can also be found on well-drained soils.

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Lowland Rain Forest

The most widespread vegetation type in the Solomons, the lowland rain forest contains about 60 common tree species, of which twelve are very common: Calophyllum kajewskii, C. vitiense, Campnosperma brevipetiolata, Dillenia salomonensis, Elaeocarpus sphaericus, Endospermum medullosum, Gmelina mollucana, Maranthes corymbosa, Parinari papuana ssp. salomonense, Pometia pinnata, Schizomeria serrata and Terminalia calamansanai. In studies on the island of Kolombangara, it was found that these 12 species could be placed in one of four groups based on their successional behaviour: (1) species shade-tolerant from seedling to mature size (Dillenia, Maranthes, Parinari and Schizomeria); (2) species able to establish in shade but needing at least a small tree gap to grow into the canopy (Calophyllum kajewskii, C. vitiense and Pometia); (3) same as (2) but needing a larger tree gap (Campnosperma and Elaeocarpus); and (4) species with seedlings rarely occurring in forest under the parent tree, but sometimes found elsewhere in large canopy gaps ("biological nomads") (Endospermum, Gmelina and Terminalia). The understory of the lowland rain forest contains a variety of short, thick-stemmed, low-statured trees such as Barringtonia papeh, Leea indica and Tapeinosperma spp., as well as palms (Areca catechu, Licuala lauterbachii and Strongylocaryum latius), Pandanus spp. and bamboos.

Some lowland rain forests in the Solomons have low diversity. On Choiseul, New Georgia, and Santa Isabel, there are almost mono-dominant stands of Campnosperma brevipetiolata, sometimes with Dillenia salomonensis as a co-dominant. Elsewhere, Pometia pinnata and Teysmanniodendron spp. are co-dominant with Campnosperma brevipetiolata. The simple composition of these forests is as yet unexplained. Associated with extreme soil conditions are other low-diversity forests, such as the combination of Pterocarpus indicus and Terminalia brassii on alluvial habitats subject to periodic flooding, and that of Pometia pinnata and Vitex cofassus on limestone soils. Ultramafic soils rich in chromium and nickel occur in small areas in the Solomons, such as at the south tips of Choiseul and Santa Isabel, in south-west Guadalcanal and north-central San Cristobal. These low-diversity forests are dominated mostly by Gymnostomsa papuana. Associated taxa include Dillenia crenata, Syzygium spp. and, in higher areas, the gymnosperms Dacrydium spp. Woody species exclusively found on ultramafic substrates include the palm Gulubia hombronii, Myrtella beccarii (only on Santa Isabel), Pandanus lamprocephalus (only on San Cristobal) and an undescribed species of Xanthostemon.

As mentioned, the Santa Cruz Islands are phytogeographically distinct from the rest of the Solomon Island group. Only in the Santa Cruz group (on Vanikoro and Santa Cruz) does the gymnosperm tree Agathis macrophylla appear in the Solomons. Also, of the twelve very common lowland rain forest tree species in the rest of the Solomons, only Calophyllum vitiense and Campnosperma brevipetiolata could be considered common in the Santa Cruz group. Besides the woody species already mentioned, the lowland rain forest also includes Dacrydium elatum (a second gymnosperm), Endospermum medullosum, Metrosideros ornata, Falcataria moluccana, Maranthes corymbosa, Pterocarpus indicus and Syzygium spp.

Montane Rain Forest

Except for summit areas on mountains over 1 000 m high, the elevational boundary between lowland and montane rain forests in the Solomons is indistinct. This common vegetation type can occur as low as 700 m in heavily clouded areas. Often, tall-statured 25 to 35 m lowland rain forest changes abruptly to a lower-statured 15 to 20 m tall montane rain forest on wet, cloudy, windy sites and on ridges with shallow soils. On Mt. Popomanaseu (2 230 m elevation) on Guadalcanal, tree species include species of Ardisia and Rhododendron, Metrosideros collina, M. salomonensis, several species of Ficus, Psychotria, and Schefflera, and the gymnosperms Dacrydium cf. xanthandrum and Podocarpus pilgeri. Scrub stands of bamboo are common.

last updated: Monday, January 12, 2004