Guinea is the northern quadrant of the Australian tectonic plate; thus this island is geologically one with the Australian continent. And yet in spite of geological linkages, there are considerable environmental differences. In particular, Australia today is dry and temperate, whereas New Guinea is tropical and perhumid. These two fundamental distinctions can explain much of the differences between these sister biotas, north and south.
Climatologically, Papua is remarkable mainly for its cloudiness. It is perhaps one of the cloudiest places on earth. Spanning latitudes from the equator to 12 degrees south latitude, Papua’s equatorial climate is seasonally dominated by the Northwest Monsoon and the Southeast Trade Winds. In most parts of Papua, the effects of the Northwest Monsoon dominate in the period from November to March, bringing rain and unsettled weather. The Southeast Trade Winds tend to bring cool and dry weather, and predominate from April until September. That said, Papua has many microclimates. Rainfall regimes range from low in the southeast (less than 2,000 mm/year) to extremely high on the southern scarp of the Central Cordillera (more than 5,000 mm/year). The highest rainfall on record for Papua is from Tembagapura town, which receives 7,500 mm/year on average. In the wetter areas, the typical seasons are reversed, and the most rain falls in the April–October period. In fact, the wettest sites receive rain from both the monsoon and the trades, and they tend to be found in the mountains along the southern front of the Central Cordillera. Moreover, annual accumulation in the very wettest areas tends to show great variability. This variability can exceed the mean annual accumulations recorded for typical medium-rainfall sites.
Seasonally, temperature varies little. Elevation is the key to temperature in equatorial zones. This "lapse rate" is equivalent to 5 C per 1,000 m elevation. Thus, at sea level, in the forests near Timika, one will encounter an unpleasant combination of high humidity and warm temperature day and night on all but the coolest days of the austral winter. By contrast, at 4,000 m in the Sudirman Range one must expect regular night frosts during the dry season, when the skies are clear. Above 4,500 m periodic snowfalls are common. And glaciers cap the highest peaks of the Jaya Mountains (formerly the Carstensz Range). These glaciers expanded outward and downward during the Pleistocene cooling, melted altogether by 6,000 years BP, and returned during the recent cooling, only to begin retreating again in the last century.
The elevation-temperature equation is a defining environmental phenomenon in mountainous Papua. This allows essentially distinct biotas to inhabit adjacent patches of land, separated only by elevation. It certainly explains much of the species-richness of Papua (beta diversity).
Rain shadows exist in some interior valleys (such as the Baliem), on the Bomb-erai Peninsula, and in the Trans-Fly of the far southeast. Rainfall is also slightly attenuated along the northern coast, from the mouth of the Mamberamo east to Jayapura. Much of the interior receives well in excess of 3,000 mm/year.
Papua is a land in flux. Significant chronic disturbance is produced by ongoing mountain-building in contest with rainfall-driven erosional processes, as well as by periodic vulcanism, human-caused and naturally occurring fire regimes, plus El Niño droughts. Over the long history of human occupation, swidden agriculture has disturbed large swaths of habitat, most of which is now regenerated forest. Thus historical disturbance is a dominant factor dictating the distribution and pattern of today’s vegetation. Much of what appears to be "virgin rainforest" is, in fact, the product of recent and not-so-recent patch disturbance. This is abundantly evident when conducting plot-based plant surveys in the forest. Thus any attempt to characterize forest types is a rough generalization, and at best a qualitative assessment with minimal predictive power at the taxonomic scale.
FOREST TYPES
Closed forest is the default vegetation type over virtually the entirety of Papua except perhaps in the southeast (although the fire regime that produces savannas there may be anthropogenic). Papua’s forests are highly species rich, with minimal stand dominance by particular tree species, and with remarkable history-driven variation from site to site, even within single catchments. One-hectare stands of forest typically support between 70 and 200 species of trees larger than 10 cm diameter breast height (dbh). It is thus difficult to characterize the forest types of Papua taxonomically. Instead, forest types are delineated by elevation, rainfall, and structure. In general, New Guinea’s forests can be termed "tropical humid forests." Tree species of the following families are important components of this tree flora: Podocarpaceae, Fagaceae, Moraceae, Lauraceae, Meliaceae, Myristicaceae, Sapindaceae, Euphorbiaceae, Combretaceae, Sapotaceae, Annonaceae, Clusiaceae, and Rubiaceae, among others (Oatham and Beehler 1997).
In the lowlands, one finds tall alluvial forests in well-drained catchment basins, as well as various types of periodically inundated swamp forests in the more poorly drained areas. The finest alluvial forests are grand, indeed, with emergent species reaching 60 meters (e.g., Octomeles sumatrana), and canopy height topping 45 meters. The canopy of this alluvial lowland forest is often irregular and broken, except where there has been uniform regeneration after some disturbance. Typical canopy tree genera of the wooded swamps include Barringtonia, Terminalia, Alstonia, Diospyros, Carallia, Syzygium, and Campnosperma. Palm swamps, dominated by sago, pandanus, or nipa are commonplace in the vast deltaic areas of the major rivers (e.g., Digul). These grade into herbaceous swamplands where inundation is the prevalent condition. Coastally one finds small strips of mangrove or large and extensive mangroves forests, depending upon conditions. These comprise species of Sonneratia, Xylocarpus, Brugiera, Rhizophora, and Avicennia. Mangrove formations are dominant in the south and southeast, between the southern Vogelkop and Bomberai Peninsula, and along the Waropen coast (northeastern Cenderawasih Bay). In the far southeast one encounters closed monsoon forest that grades southward into Melaleuca woodland and Eucalyptus savanna.
Much of Papua is hilly, and here forests are on well-drained soils and tend to be less grand, with smaller-boled trees of lesser height. In the low hills on the southern side of the Central Cordillera above Timika one finds a very poor "white sand" and heath forest that is both structurally bizarre and taxonomically distinct. Above 1,000 meters one encounters submontane forests that in places have a strong representation of oaks (Castanopsis acuminatissima, Lithocarpus spp.) and several genera of Lauraceae. A cloud line settles on the mountains at varying elevations, depending on local conditions. This produces cloud forest conditions, which are typified by the abundance of moss on tree trunks as well as an effusion of epiphytes. This cloud line most typically can be found between 1,500 and 2,500 meters elevation. Midmontane forests are more species-poor and can be dominated by the Antarctic beech Nothofagus as well as several genera of gymnosperms from the family Podocarpaceae (Podocarpus, Dacrycarpus, Dacrydium, Phyllocladus). Above 3,000 meters, one encounters an elfin woodland that is low in stature (15 m), and small-boled (10–30 cm) and dense, with heavy mossing and with tangled moss-laden root mats on the ground in the place of soil. Climbing higher into the mountains, this leads to areas where patches of dense thicket-like dwarf forest is interdigitated with open boggy grasslands in the more poorly-drained and frost-prone areas. In these areas one can find prominent stands of large Dacrycarpus compactus as well as the more conifer-like Papuacedrus papuanus. On the summit areas above 4,000 meters one encounters a mix of tussock grasslands, rocky areas, low ericaceous thickets, and a variety of tropical alpine herbaceous vegetation.
Botanically, Papua is remarkable, estimated to house more than 15,000 species of vascular plants, notably some 2000 species of orchids, more than 100 rhododendrons, one species of the great and ancient Araucaria conifers—Papua’s tallest tree, as well as the magnificent and valuable kauri pine (Agathis labillardierei). Dipterocarp trees are relatively uncommon, but appear in abundance in certain patches, the result of some natural disturbance regime. Other important timber trees include Intsia bijuga ("merbau"), Pometia pinnata ("matoa"), Pterocarpus indicus ("rosewood"), and Dracontomelon ("black walnut"), among others.
Fauna
VERTEBRATES
Birds dominate the Papuan vertebrate fauna, with more than 600 species recorded. This includes more than 25 species of birds of paradise,
