MEAN ANNUAL RAINFALL AND TEMPERATURE
(mm and degrees centigrade)
Source: J.R. McAlpine, Gael Keig & K. Short 1975
* S.M.Latif, 1960
** R.J.Hnatiuk et al. 1976
GEOLOGY AND GEOMORPHOLOGY
The geology and the topography of New Guinea are complex; the latter is extremely young and is actively changing. To describe the geology and geomorphology, the accounts by Lea (1973) and Stevens (1989) are drawn upon. Additional information has been derived from Loffler (1972, 1974, 1977) and Pieters (1982).
Stevens divides New Guinea up into six areas or sub-regions, the southern plains, the central cordillera, the Vogelkop and islands, the northern basins (inter montane troughs) and ranges, the northeastern islands and the southeastern islands. Following Loffler (1977) the eastern metamorphic and the ultrabasic ranges, and the Cape Vogel basin are included here.
The northeastern islands, in the northern Bismarck arc, are a continuation of the Solomons and Vanuatu. The Solomons are divided into three geological provinces where major fault lines occur, the Volcanic province in the west, running in a NW-SE, the Central province with a W-E trend in its direction and the third, the Pacific province with a NE trend. The first province is considered to be a young extension of the Central province and the area is seismically fairly active. Information on the geology of the Solomons is based on Coleman (1970).
The Southern Plain
This area includes the islands of the Torres Strait and is of low relief with two large rivers, the Digoel in Irian Jaya and the Fly in Papua New Guinea, as well as numerous smaller rivers. The northeastern part has a large area of exposed limestone and karst topography. Fairly recent volcanoes are present, but the area is seismically stable being situated on the Australian shield.
The Central Cordillera
Inland from the southern plain is a zone of foot hills often rising in tiers to the central cordillera, a band of extreme relief that runs almost the whole length of the island. It consists of a series of steep and rugged ranges which are roughly parallel to each other. According to Davies (1985) the geology of the region is characterized by metamorphics, intrusives and volcanics with strong faulting and structural complexity. A considerable portion of the main range is comprised of heavily folded rock of the New Guinea Mobile belt.
Granodiorites are found on Mt Wilhelm, (at 4,510 m this is the highest point in Papua New Guinea) and limestone on Pegunungun Sudiman of which Mt Jaya (Mt. Carstenz), at 4,880 metres, is the highest point in the whole of New Guinea. Over its length the cordillera rarely drops below 2,000 m in altitude for a distance of over 1,600 km. In its western half it has escarpments which rise to over 1,000 m and sharply demarcate the southern plain in that section. At about 146 degrees E in a northerly direction the range drops below 2,000 m and the rocks here are formed from sediments of the Aure trough.
The main range passes southeasterly through Papua where it becomes lower than the western portion and is known as the Owen Stanley Range. Further to the east the cordillera continues to dip. The D'Entrecasteaux Islands and the islands of the Lousiade Archipelago are peaks of submerged mountains and are considered an extension of the cordillera.
New Guinea is young and tectonically dynamic and has been subject to abundant Cainozoic vulcanism (Galloway and Loffler, 1972). There are numerous, mostly inactive volcanoes between 143 and 146 degrees E including Mt. Giluwe (4,088 m.a.s.l.) in the highlands of Papua New Guinea (Map 7). Volcanic activity virtually ceased in the highlands about 50,000 years ago (Bleeker, 1983). East of 146 degrees E. there are other volcanoes in the extreme southeast both to the north and the south of the main range. Two of the higher mountains, Gunung Doorman and Mt. Suckling, have substantial ultramafic rock with the latter being part of the East Papuan Ultrabasic Belt. Traces of wide spread glaciation occur above 3,500 m and some glaciers are still to be seen on Mt Jaya, Mandala and Trikora in Irian Jaya. Extensive intermontane valleys are found in both Irian Jaya and New Guinea.
Northern Basins and Ranges
This area runs more or less parallel to the central cordillera and mainly consists of the inter montane trough comprised of the wide valleys of the major streams, the Sepik, the Ramu and the Markham rivers. The latter two are enclosed on the seaward side by the Finisterre- Sarawaged - Adelbert mountains in the east and the Sepik on the seaward side by Torrecelli-Bewani mountains rising between 1,000 to 1,500 m. The large river system of the Mamberamo in Irian Jaya are enclosed in a similar way by the van Rees and the Peg ranges.
The Finisterre- Sarawaged- Adelbert mountains represent an old island arc that has accreted onto the New Guinea mainland and uplifted. However, the intermontane trough they border may still be sinking (Lea, 1973; Stevens, 1989) and the area is still seismically active.
The Eastern Metamorphic Ranges
This eastern portion of the central cordilleras is made up of the Owen Stanley Ranges and its flanks. They are dominated by massive ridge and valley landforms formed on low grade metamorphics and volcanics. The ranges reach an altitude of 2,800 to 3,400 m near Mt. Albert Edward and Murray Pass, dropping north west to 1,500 m and 2,000 m at Lake Trist and Mt. Missim near Bulolo and Wau. The highest peaks in the Owen Stanley ranges are Mt. Albert Edward, Mt. Scratchley and Mt. Victoria and these display glacial land forms.
The Ultra Basic Ranges
These are separated from the Owen Stanley ranges by a fault trough formed during the Eocene. The landforms are similar to the Owen Stanley ranges and dissected by erosion that has produced steep relatively uniform slopes and narrow crests. Near Mt. Lamington a thick layer of volcanic ash has formed volcano plateaux.
The Northeastern Islands Region
Two structural units are recognised by geomorphologists in this region. They are the Southern Bismarck Island Arc and the Northern Bismarck Island Arc. They are also referred to as the Finisterre - New Britain Volcanic Arc and the Bougainville - New Ireland Volcanic Arc. (Loffler, 1977).
(a) The Southern Bismarck Arc
This includes New Britain and a chain of volcanic islands off the north coast of Papua New Guinea. The area is seismically active with a belt of volcanoes along the concave north coast of New Britain, ending in the Vulcan and Tavurvur volcanoes near Rabaul. There are over 50 volcanoes in the region (Maps 4 and 5), and these are mainly andesitic strato-volcanoes. The tallest volcanoes, Ulamona and Bamus, rise to 2,000 m and dominate the landscape. The Cape Hoskin volcanoes have been periodically erupting over the past 900,000 years. Other noteworthy active volcanoes in this arc are Manam (1,300 m), Karkar and Long Island.
The central and southern part of New Britain consists of rugged mountains that do not exceed 1,500 m, and extensive karst areas. The coastal areas are formed from raised coral terraces and volcanic foot slopes and fans.
(b) The Northern Bismarck Arc
This consists of Bougainville, Buka, New Ireland, New Hanover, the St. Matthias group and the Admirality islands. The island of Bougainville is the largest in the arc and it has 3 active volcanoes, Balbi, Bagana and Loloru. These are accompanied by extensive volcano-alluvials foot slopes and fans.
New Ireland is mountainous in its southeast part. The remainder consists of karst land forms and raised coral reefs along the coast.
New Hanover and the islands of Feni, Tanga, Lihir and Tabar are of volcanic origin as is Mussau in the St. Matthias group and the islands south of Manus. The remainder were formed from raised coral reefs.
The Southeastern Islands Region
There are two groups of islands situated at the tip of the mainland. These are the D'Entrecasteaux group and the Louisiade Archipelago and they are structurally an extension of the mainland. Volcanic cones occur on Goodenough Island (the highest at 2,400 m) and Fergusson Island (with 3 volcanoes which rise over 1,820 m). Thermal activity occurs on these islands while raised coral platforms are evidence of recent uplifting.
Beyond these two groups are the low lying coral atolls of the Trobriand-Woodlark groups. Woodlark Island has a core of metamorphic rock. The highest island is no more than 142 m.
PHYTOGEOGRAPHY AND VEGETATION
In the Early Cretaceous, around 125 million years before present (BP), nearly all of New Guinea and much of Australia was inundated by sea. Maximum transgression is thought to have lasted to around 110 million years BP. However some islands may have existed in the Kubor Range region of present day Papua New Guinea (Doutch, 1972). More islands began to appear in the Eocene period, between 53.5 and 35 million years BP in Papua New Guinea (e.g. the Owen Stanley ranges) and in Irian Jaya (Japen island and the Arfak region). At this time very little land would have been available for plants to colonize but never-the-less there may have been sufficient to permit the ancestors of the Spathoglottis to intrude by island hopping as far south as the Australian land mass.
By the early Miocene period (22.5 to 5.5 million years BP) these islands enlarged and the numbers increased. More of the New Guinea landmass became exposed and New Britain appeared. By the Pliocene, between 5.5 and 2 million years BP, more than two thirds of New Guinea is believed to have been formed with the recession of the seas and land uplift.
The New Guinea and Australian land mass made contact with, what is now known as, the Indonesian Archipelago about 12 million years ago, making it possible for plants to migrate from the super-continent Laurasia into New Guinea and northern Australia. The last contact of significance with Australia, lasted until about 8,000 years B.P. during which time rain forest and moist open forests were more extensive, ranging from what is now the more seasonal western province of Papua New Guinea to Australia where they extended down the eastern seaboard of the Australian land mass (Lavarack, 1981).
The island of New Guinea is of biogeographic importance because of its past and present geographic position between Asia to the west and Australia and Pacific to the south and east. Given its origin, its geomorphology and climate, the island not only supports its own high level of endemic biota with both southern and Asiatic affinities, but it also supports taxa whose distribution extends beyond New Guinea and which may have their centers of diversity outside of New Guinea (Balgooy, 1976; Gressitt, 1982).
Phytogeographers place New Guinea in the Indo-Malayan floral region. This is based on enumeration and comparison of taxa at family and genera level, which despite the southern and eastern affinities of the montane flora, shows a greater number of western-derived taxa. The latter are particularly dominant in the lowlands (Balgooy, 1976). On the other hand, based on the distribution of mammals and birds, zoo geographers place New Guinea in the Australian region. However, some research workers dealing with the invertebrate groups consider the distribution of these to be more in line with that of the flora (Balgooy, 1971, 1976; Gressitt, 1961).
A number of biogeographic demarcation lines have been drawn around New Guinea and phytogeographical regions have been proposed, most of which are incorporated in part by those as outlined in the work of Balgooy (1971, 1976). The demarcation lines outlined by him are: Ridley's line, which separates the Indochinese from the Indo-Malayan region; and the Merrill and Dickerson line which includes portion of the Wallace line to separate the Papuan and Pacific flora from the Indo-Malayan region. The Wallace and the Zollinger lines merge in part to create sub-regions of the Philippines in the north and Timor in the south, while two lines have been proposed for the demarcation of flora and fauna between northern Australia and southern New Guinea; the Torres Strait boundary and Good's line.
In all cases the phytogeographical regions or the sub-regions as outlined by Balgooy (1971), including those of his own work, have kept the Bismarcks and the Solomons apart from mainland New Guinea. On the other hand, according to Balgooy (1971), the zoo-geographical subdivisions of the Pacific compiled by Gressitt (1961) agreed in its broad outline with the subdivisions based on the distribution of phanerogam genera.
Gressitt's subregions, divisions and subdivisions of the Pacific provide a version of regional phytogeographical subdivision which, apart from including the Bismarck and the Solomons as one region with New Guinea, is much more subjective in establishing the distribution of endemic and non-endemic species. In addition to this, it satisfactorily indicates the region of overlap with Australia and the Western Province more or less including Good's demarcation line in New Guinea (Good, 1964).
The following account of the New Guinea vegetation is based on Johns (1977) and Paijmans (1976, 1985). Due to the great diversity of environments the vegetation of New Guinea has developed into one of the richest in the world. The diversity of its flora may also have been influenced by its geographic position, past and present, in relation to Asia, Australia and the South-west Pacific islands. This has placed it in the path of plant migrations to and from these regions.
The vegetation of New Guinea has been ranked in four major vegetation belts or zones (Table 5). These are the lowland zone which extends altitudinally from sea level to 300 m, the montane zone from 300 to 3,000 m, the sub-alpine zones and the alpine zones above 3,000 m. Johns (1977) splits the montane zone into three sub zones: the lower montane, the mid and the upper montane zone.