St Helena Ascension Ecosystems

Plant Zones

The primary plant formations prior to human intervention are thought to have included about seven zones or communities all dominated by endemic plants. These can be summarized as following:

Tree-fern thicket (altitudes ranging from 700-800 m) dominated by Dicksonia arborescens, Melanodendron integrifolium (Asteraceae), Nesiota elliptica (Rhamnaceae), Nesohedyotis arborea (Rubiaceae) and Trimeris scaevolifolia (Campanulaceae)

Cabbage-tree woodland (altitudes ranging from 600-750 m) dominated by Commidendron spurium (Asteraceae), Lachanodes arborea (Asteraceae), Petrobium arboreum (Asteraceae), Plataroxylon leucadendron (Asteraceae) and Trochetiopsis erythroxylon (Sterculiaceae)

Moist gumwood forest (altitudes ranging from 500-600 m) dominated by Commidendron robustum, C. spurum (Asteraceae), Phylica polifolia (Rhamnaceae) and Trochetiopsis erythroxylon (Sterculiaceae)

Dry gumwood forest (altitudes ranging from 100-500 m) dominated by Trochetiopsis ebenus (Sterculiaceae), Commidendron robustum and C. rugosum (Asteraceae)

Scrubwood scrub (at altitudes ranging from 20-350 m) dominated by Commidendron rugosum (Asteraceae), Frankenia portulacifolia (Frankeniaceae) and Trochetiopsis melanoxylon (Sterculiaceae)

Saline semi-desert(altitudes ranging from 0-250 m) dominated by Hydrodea cryptantha, Hypertelis acida (Molluginaceae), Commidendron rugosum (Asteraceae) and thenon-endemic Suaeda fruticosa.

With the exception of Trochetiopsis melanoxylon, which is now thought to be extinct, the rest of the above-mentioned species are still extant, although the endemics Chenopodium helenense (Chenopodiaceae) and Mellissia begoniifolia (Solanaceae) are now thought to be close to extinction in the wild. Also most of the original vegetation has been destroyed or reduced to small areas and often invaded by introduced species. A few of the remaining remnants, such as tree-fern thicket and cabbage-tree woodland, occur along the high central ridge (mainly Diana’s Peak and High Peak).The endemics associated with these remaining patches of indigenous vegetation include Wahlenbergia linifolia (Campanulaceae), and ferns such as Asplenium compressum, A. platybasis (Aspleniaceae), Diplazium filamentosum, Dryopteris cognata (Dryopteridaceae), Elaphoglossum dimorphum, E. nervosum, (Elaphoglossaceae), Grammitis ebenina (Grammitidaceae), Hymenophyllum capillaceum (Hymenophyllaceae), Microstaphyla furcata (Dryopteridaceae), Pseudophegopteris dianae (Thelypteridaceae)and Pteris paleacea (Adiantaceae), several of which are epiphytic on tree ferns. In the wet flushes below cabbage trees (Lachanodes arborea or Pladaroxylon leucadendron) can found together with various endemic hygrophytes such as Carex dianae, C. praealta (Cyperaceae) and Sium bracteatum (Apiaceae). Other habitats supporting endemic species include the sea cliffs, upland crags and rocks, and semi-desert and scrub zones.

St Helena Sea Cliff Formations

The cliffs on St Helena can be up to 500 m high, but most of them are composed of layers of eroding rock and loose volcanic ash. The dry western cliffs are virtually devoid of vegetation, but the wetter eastern and southern cliffs support a scattering of plants, including number of endemics that have become extinct in other parts of the island such as Pelargonium cotyledonis (Geraniaceae) and Plantago robusta (Plantaginaceae). Sea cliffs are therefore considered to be of great importance for conservation.

St Helena Upland Crag and Rock Formations

The rocks and cliff above about 600 m provide refuge for a few rare or local endemics such as Sium burchellii (Apiaceae), Wahlenbergia angustifolia (Campanulaceae) and the ferns Ceterach haughtonii (Aspleniaceae)and Dryopteris napoleonis (Dryopteridaceae).

St Helena Semi-Desert Formations

In some of the lowland areas there are barren volcanic lands extending down to sea level. These areas receive very little rainfall and support only a scattering of plants. Many of these are introduced, but there are also various endemics such as the tufted grass Erogrostis saxatilis, and other flowering plants such as Bulbostylis lichtensteiniana (Cyperaceae), Euphorbia heleniana (Euphorbiaceae) and Osteospermum sanctae-helenae (Asteraceae).

St Helena Scrub Formations

Between altitudes of about 200 and 400 m much of the northwestern part of the St Helena is covered in scrub. Most of this is made up of introduced species such as Chrysanthemoides monoliferana, Diospyros dichrophylla, Lantana camara, Schinus terebinthifolius and species Opuntia. However, since intensive grazing by donkeys and goats has been controlled the endemic scrubwood Commidendrum rugosum (Asteraceae) is starting to spread back into its former areas.

Further information required.

References

Ashmole, P. & Ashmole, M. 2000. St Helena and Ascension Islands: a natural history. Anthony Nelson.

Balgooy, Van. M. M. J. 1969. A study of the diversity of island floras. Blumea, 17: 139-178.

Cronk, Q. C. B. 1980. Extinction and survival in the endemic vascular flora of Ascension Island. Biological Conservation, 17: 207-219.

Cronk, Q. C. B. 2000. The endemic flora of St Helena. Anthony Nelson.

Duffey, E. 1964. The terrestrial ecology of Ascension Island. Journal of Applied Ecology, 1: 219-251.

White, F. 1983. The Vegetation of Africa. UNESCO.