Vegetation Zones

New Guinea Orchid Vegetation Zones

GEOGRAPHIC EXTENT

The Papuasian region is defined here as the area extending from Waigeo Island and the Vogelkop Peninsula in the west to the Solomon Islands in the east, but excluding Santa Cruz. The major landmass is the island of New Guinea, which is the largest tropical island and the second largest island in the world covering an area of 800,000 km2. The Island is 2,400 km long and 720 km wide. It is one of three only tropical areas with glaciers, and the highest peak is Mt. Jaya, which is approximately 5,000 m high. The Island stands on the Sahul shelf, a submarine extension of the Australian continent between latitudes 0 degrees 19 minutes South and 10 degrees 43 minutes South, longitudes 130 degrees 45 minutes East and 150 degrees 48 minutes East.

CLIMATE

Although New Guinea is close to the equator, the large altitudinal ranges that exist throughout the region ensures a variety of climates. A great part of Papua New Guinea, Irian Jaya and the islands from the Vogelkop to the east of Papua New Guinea, has a very heavy rainfall, with parts of the southern edge of the cordillera receiving up to 15,000 mm per year. The lowland areas are hot and humid, but ground frosts may occur above 2,600 meters elevation. Ground frosts are usually associated with dry days and clear nights.

PAPUA NEW GUINEA

The climate of Papua New Guinea is influenced by two pressure systems. From about May to August the country is influenced by the trade winds, which originate from the sub-tropical high-pressure system that is located 25 -30 degrees South. From January to April, the country is influenced by the Inter Tropical Convergence Zone (ITCZ) which represents the junction between air streams originating in the northern and southern hemispheres. This zone passes over New Guinea twice each year and as it passes from north to south the moist northwesterly winds commence to influence the country. The transition between the two periods occurs in September (Ho, 1960).

The topography has a considerable influence on precipitation. Low relief parallel to the prevailing winds produces little rainfall while the presence of the central cordillera zone on the main island forces the wind upwards which induces instability and causing rain to fall on the windward slope. On the leeward slopes the descending air, which periodically includes the air from the higher trade winds, induces dry conditions. Reverse conditions occur when the north westerlies develop.

In the highlands rain tends to have a distinct spatial and temporal distribution due to the influence of local factors on air circulation and consequently the mountain valley wind systems. The rainfall in Papua New Guinea has been divided into six categories, which are defined as follows by Spenceley (1985):

1. Continuous heavy rain: large falls throughout the year. Local factors may cause precipitation in excess of 10,000 mm per year. The southern flanks of the cordillera and the higher elevations of the adjacent islands may be subjected to these rains.
2. Heavy to intermediate rains (moderate range): A seasonal trend is present and the heavier falls are associated with the passage of the ITCZ.
3. Continuously moderate rain: occurs on the margins of the dry regions in small areas. Droughts and heavy rains are uncommon in these areas.
4. Heavy to light rains, heavy dominant (large range): Seasonal contrasts and heavy rainfalls predominant. Influenced by the ITCZ.
5. Heavy to light rains, light dominant (large range): Seasonal contrasts and light rainfall predominant. Influenced by the ITCZ.
6. Intermediate to light rains (moderate range): a seasonal contrast between light rains during a low sun period and the intermediate intensity rains when the ITCZ lies to the south of the country.

Temperatures, throughout the country, are generally high with little variation throughout the year. Differences generally are not seasonally large. The average minimum temperature for the lowlands exceeds 20 degrees C with the daily temperature range exceeding the seasonal temperature range. The absolute and average temperatures decrease with the altitude of the recording stations. Frosts occur mainly in the upper regions of the high mountains. Mean annual rainfall and temperatures for a selection of sites in New Guinea are provided in Table 4.

IRIAN JAYA

There is considerably more readily available information on climate in Papua New Guinea than for Irian Jaya. However, according to Mitton (1983), most of the climate for Irian Jaya resembles the climate of the Southern and Western Highlands of Papua New Guinea. The steep southern aspect of the cordillera receives the highest precipitation; examples quoted by Mitton (1983) were Mapenduma which received an average of 12,000 mm per year over nine years and Gunung Bijih (Ertzberg Mt.) which received over 5,000 mm in the first half of 1973. It was suggested that the climate for the Balim Valley may have been modified by extensive clearing. Snow and ice fields occur on many of the main range peaks. There is a distinct climatic seasonality, with a dry period generally in the months between May and October. Thunderstorms occur in the high regions and these can result in large temperature drops even in the warmer seasons.

THE NORTHEASTERN ISLANDS

These are comprised of the larger islands of New Britain, New Ireland, Bougainville, Buka, New Hanover, the St. Matthias Group and Admirality islands and adjacent smaller islands as well as the chain of islands off the north coast of the mainland.

The East New Britain region is influenced by the northwest monsoons from November to March and again by the southeast trades in May. The annual rainfall varies from the central mountains, which rise as high as 2,110 m, to the coastal regions. The highest rainfall occurs along the southwest coast. Rabaul in the north east has an annual rainfall of around 2,003 mm and nearby Kokopo has 1,946 mm. The rainfall increases along the south coast at Jacquinot Bay with an annual rainfall of 5,709 mm.

The West New Britain area is similar in that the climate is governed by the trade wind with the annual rainfall pattern ranging from 3,271 in the north to 6,541 mm on the south coast.

THE SOUTHEASTERN ISLANDS

The southern islands commonly referred to as the Papuan islands, represent the higher sections of a submerged extremity of the New Guinea central mountain system. They are comprised of the D'Entrecasteaux group, and a long chain of islands off the eastern tip of the mainland known as the Lousiade Archipelago. Its major islands are Misima, Tagula and Rossel. To the north are the Trobriand Islands.

Three islands in the D'Entrecasteaux group consist of metamorphic rock and volcanics surrounded by raised coral limestone formations. These are Goodenough, Fergusson and Normanby Islands. Goodenough is the most mountainous and the highest point is around 2,400 m. Some islands have alluvial plains. The volcanic region of SE Papua includes the D'Entrecasteaux group. Fergusson island has three volcanic masses over 1,828 m high. There is considerable thermal activity in the region.

There are three distinct rainfall patterns in the province which includes these islands, however the islands themselves are classified as continuously wet, with an average annual rainfall ranging from 3,000 mm to 5,120 mm. Some of the islands have both lowland rainforest and patches of savannah which appear to be in a slight rain shadow.

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 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
Phytogeography

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).

Vegetation

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.