Broadleaved forests

Mangrove Swamp

Sheltered onshore mudflats, mainly on Tongatapu and in Vava´u, support a depauperate mangrove swamp community dominated by the mangroves Bruguiera gymnorrhiza, Rhizophora samoensis and/or R. stylosa. Farther inland, where the water changes from saline to brackish or fresh, occur buttressed trees such as Excoecaria agallocha, Inocarpus fagifer and Xylocarpus granatum. Paspalum vaginatum often forms a dominant grass cover on the inland margin of the swamp.

Littoral Forest

This forest type grows inland from the harsh beach vegetation zone. On the sheltered western and northern sides of the islands, these forests are typically dominated by Barringtonia asiatica, Hernandia nymphaeifolia, Hibiscus tiliaceus, Neisosperma oppositifolia, Terminalia catappa and Thespesia populnea. On exposed windward coastal sites the strand scrub zone is often backed by dense thickets of Pandanus, and further inland by a taller forest of Barringtonia asiatica, Calophyllum inophyllum, Casuarina equisetifolia, Guettarda speciosa, Tournefortia argentea and other widespread littoral trees and shrubs. The following two subtypes occur on Eua:

  • Excoecaria-Tournefortia Forest: Occurring on raised limestone substrates lacking sand or soil, this species-poor community forms narrow bands along the coast. Excoecaria agallocha and Tournefortia argentea dominate the canopy (up to 12 m high). Hibiscus tiliaceus forms a lower middlestory, often along with Morinda citrifolia, Neisosperma and Schleinitzia insularum. The shrub layer consists of scattered individuals of Bikkia tetrandra, Clerodendrum inerme, Scaevola taccada and Wollastonia biflora. This littoral forest type is widespread in the Pacific except for the dominance of Excoecaria.

  • Hernandia-Terminalia Forest: This forest type occurs on sand substrates and is more species-rich than the previous community. The upper canopy is dominated by Hernandia nymphaefolia and/or Terminalia catappa, with the subcanopy largely composed of Neisosperma oppositifolia. Other canopy trees include Guettarda speciosa, Hibiscus tiliaceus, Myristica hypargyraea, Pisonia grandis, Planchonella grayana, Schleinitzia and Terminalia litoralis. Microsorium scolopendria is a common terrestrial herb.

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On the dormant Late volcano, on a substrate of cinder and lava rock, the main forest type up to 300 m is dominated by Casuarina equisetifolia, often with smaller trees of Alphitonia, Elattostachys, and Rhus, along with a variety of shrubs and lianas.

Tropical Lowland Rain Forest

Once the most common forest type in Tonga, but now largely displaced by agricultural practices, only remnant patches of lowland rain forest can still be found on Tongatapu but are still common on 'Eua and the upper slopes of Vava'u. These dense forests have a canopy 20 to 30 m high, a variety of lianas and epiphytes, and a sparse understory. The main tree species include Aleurites moluccana, Alphitonia zizyphoides, Calophyllum neo-ebudicum, Dysoxylum spp., Elaeocarpus tonganus, Elattostachys falcata, Ficus spp., Garcinia myrtifolia, Grewia crenata, Myristica hypargyraea, Rhus taitensis, Santalum yasi, Syzygium clusiifolium, Tarenna sambucina, Vavaea amicorum and Xylosma simulans. A few subtypes have been described for 'Eua:

  • Maniltoa-Pleiogynium Forest: Restricted to the north-western part of 'Eua above the littoral forest on volcanic soils overlying limestone, the canopy is dominated by Maniltoa grandiflora and Pleiogynium timoriense. Myristica hypargyraea and Neisosperma oppositifolia are conspicuous by their absence in this forest type. Other large trees include Aleurites moluccana and Sapindus vitiensis. Lianas comprise 29% of the species in this forest and include Alyxia stellata, Entada phaseoloides, Gynochtodes epiphytica, Jasminum spp. and Malaisia scandens.

  • Myristica Forest: This forest type dominates the ecological zone above the littoral forest up to 110 m elevation, except for the previously described forest. It is heavily dominated by Myristica hypargyraea, which grows up to 25 m tall and is present in all size classes. Scattered trees include Calophyllum neo-ebudicum, Canarium harveyi, Diospyros samoensis, Guettarda speciosa, Maniltoa grandiflora, Planchonella grayana, and Pleiogynium timorense. The subcanopy includes Neisosperma oppositifolia and Xylosma simulans. Herbaceous ground covers are uncommon. Lianas comprise 25% of the species and include Alyxia bracteolosa, Entada phaseoloides, Epipremnum pinnatum, Gynochtodes epiphytica, Faradaya amicorum and Jasminum spp. Some areas are co-dominated by Dendrocnide harveyi, an apparent artefact of past disturbances.

  • Calophyllum-Mixed Species Forest: Occurring on volcanic soils overlying limestone, this forest type appears mostly at elevations of 100 to 180 m. No tree species is dominant in this forest; Calophyllum neo-ebudicum is the most consistently abundant and reaches a height of 35 m, while Dysoxylum tongense is the most common co-dominant. Other canopy trees include Alphitonia zizyphoides, Canarium harveyi, Dysoxylum mollissimum ssp. molle, Elattostachys falcata, Litsea mellifera, Maniltoa grandiflora, Myristica hypargyraea, Neonauclea forsteri and Semecarpus vitiensis. The subcanopy is dominated by Citronella samoensis, Cryptocarya hornei, Diospyros samoensis and Garcinia myrtifolia. Macropiper puberulum is an abundant shrub, and the herb and liana layers are well developed. Some areas are dominated in the upperstory by Dendrocnide harveyi and Bischofia javanica, or by Rhus taitensis alone, an apparent artefact of past disturbance as these species are absent from the smaller size classes.

  • Calophyllum-Garcinia Forest: Occupying the highest elevations on 'Eua at 190 to 300 m, again on volcanic soils overlying limestone, this forest type is best developed on steep slopes near the summit of the eastern ridge of 'Eua. Here Calophyllum neo-ebudicum more clearly dominates the canopy and Garcinia myrtifolia the undercanopy. Other common canopy trees include Canarium harveyi, Dysoxylum tongense, Elaeocarpus graeffei, Hernandia moerenhoutiana, Homalium whitmeeanum, Myristica hypargyraea, Neonauclea forsteri, Podocarpus pallidus and Rhus taitensis. The herb and liana layer are well developed. Apparently disturbed sites are dominated by trees of Alphitonia zizyphoides and Elattostachys falcata. On dormant Late, with its substrate of cinder and lava rock, an immature lowland rain forest dominated by Elattostachys falcata, and Rhus taitensis is developing. Casuarina equisetifolia, dominant in earlier stages of succession, occurs as 30 m tall emergent trees.

    Secondary Forest

    Now the dominant forest type in Tonga, secondary forest is a mixture of many native and introduced plant species, an artefact of land clearing. This high forest is dominated by shade-intolerant trees that replace secondary scrub. The dominant overstory tree species are Alphitonia zizyphoides, Elattostachys falcata and Rhus taitensis, however these species are rare in the understory , indicating that they will eventually be replaced by other secondary and primary forest species. Other common secondary forest species include Adenanthera pavonina, Bischofia javanica, Cananga odorata, Dendrocnide harveyi, Dysoxylum spp., Hibiscus tiliaceus, Kleinhovia hospita and Neonauclea forsteri.

last updated: Monday, January 12, 2004