Plant formation of the dry forest
 


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Diversity of the plant formation of the dry forest

The dry forest (or sclerophyllous forest) as defined in the atlas of New Caledonia, covers all the forest or shrub-like formations developing in a dry and luminous climate area. Taking into the originality of the soil on ultramafic parent rock, it has been described only on a ground of sedimentary or volcanic origin. The formations of the same climatic areas on ultramafic parent rock have been classed with “maquis des terrains miniers”.

Today, in New Caledonia, intact dry forests no longer exist, as they might have existed about 4000 years ago at the arrival of man on the Grande Terre.

The plant formations that can be found in the climatic dry forest area are at different levels of progressive evolution (return towards closed and dense formations) or regressive (loss of diversified arborescent elements, to profit mono-specific plant formations dominated by Acacia spirorbis, Leucaena leucocephala ), or different grasses. Furthermore the plant formations united under the term of dry forest in New Caledonia vary in function of the intensity of the hydrous stress that the vegetation undergoes during the dry season, the proximity of the coast line, and very probably (but this has not as yet been studied) the type of geological substratum.

In oversimplified way, without taking into account the substitution formations (savannas and various thickets) it is possible, with the knowledge that we have currently to distinguish it, namely in the case of the Botanical Conservatory of the dry forest of Tiéa (Pouembout).


Dry Forest: closed facies. Tiéa (Pouembout)

- - a closed dry forest facies where the re-covering of the arborescent stratum (from 8 to 12 m high) and shrubbery is near a 100%. The flora of this formation is characteristic of the dominance by a representative specie of the dry areas (Arytera arcuata, Cleistanthus stipitatus, Diospyros minimifolia, Homalium deplanchei, Ormocarpum orientale, Maytenus fournieri, Premna serratifolia, Psydrax odorata …). It shelters as well many rare species (Albizia guillainii, Captaincookia margaretae, Leptostylis sp., Pittosporum brevispinum, Turbina inopinata, …)


Dry Forest: closed facies. Tiéa (Pouembout)

- an open dry forest facie where the arborescent stratum and the shrubbery have a level of re-covering between 50 and 75%. The flora has most of the characteristic species of the dry forest, some of this species can locally disappear allowing the species less consumed by the cattle and deer to proliferate (Gardenia urvillei, Acacia spirorbis, Malaisia scandens, Croton insulare…).They are accompanied by several species which have been introduced (Acacia farnesiana, Leucaena leucoceplala, several types of grasses…….) The rare species mentioned above can survive in this formation.


Dry forest open type. Tiéa (Pouembout)

- A riverine type of dry forest is found along dry river beds and in flood plains. It has a 20m high arboreal stratum consisting in species which are also found in moist evergreen forests (Mammea neurophylla, Garcinia neglecta, Mimusops elengi, Semecarpus atra, Syzygium aggregatum, Diospyros fasciculosa… The shrubbery stratum is more characteristic of dry areas (Arytera arcuata, Cupaniopsis globosa, Diospyros minimifolia, Oxera sulfurea, Psydrax odorata) and include a few rare species: Captaincookia (Captaincookia margaretae) and the Pouembout rice (Oryza neocaledonica)...


Dry forest : riverine type (Pouembout)

These two types belong to what has been named “sclerophyll formations sensu stricto” in the Southern Province. The riverine type, on the other hand, belongs to “valley and riparian mixed sclerophyll formations” according to the Northern Province terminology.

Generally speaking, along the whole of the West coast and close to the shore, the height of dry forest decreases. Some trees (Planchonella cinerea, Mimusops elengi) and shrubs (Zieridium gracile, Eugenia oraria, Carissa ovata) become more widespread and mix with dune and behind dune species (Myoporum tenuifolium, Pandanus tectorius, Cerbera manghas, Crinum asiaticum…)


Dry forest substitution vegetation : “Gaiac” (Acacia spirorbis) monospecies type, Tiéa (Pouembout)

Vegetation types related to dry forest are found on calcareous substrates in the Poya and Koumac regions. The floristic composition of these forests resembles that of tropical rain forests on calcareous substrates in the Loyalty islands.

In valley bottoms, and frequently on scree, but always in less severe hydrous conditions than on exposed plains, a transitional forest type, intermediate between dry forest and moist evergreen forest has developed. It is often characterised by a predominance of Ficus, Aleurites moluccana (bancoulier), de Olea paniculata, as well as several fern and orchid species.

Sclerophyll forest phytogeography

Sclerophyll forest is of recent origin and is mainly made up of flora that appeared towards the end of the Tertiary and during the Quaternary, thus overlaying rare ante-Eocene species which survived the general peridotite deposition in NC. This conclusion is based on phytogeographic analysis of dry forest genera which show strong affinities with Australia as for NC flora as a whole. On the other hand, sf vegetation differs from other NC genera by an absence of flora of gondwanan origin (Gymnosperms, Balanopaceae, Cunoniaceae, Lauraceae, Proteaceae, Winteracea…)

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