community of interest through time. The montane swamp forests of New Guinea are readily identified in the pollen records by high percentage representation in the pollen assemblage of Myrtaceae (mainly Syzy- gium), Dacrydium and Pandanus (Haberle 1998b). The assumption here is that a loss or reduction of all or part of these key indicators from the pollen record is indicative of a loss or reduction in the extent of the associated vegetation community and therefore a loss in local biodiversity. A major uncertainty in this approach is the temporal constancy of the association between the key indicator types and their vegetation community which could lead to a potential over- estimate of biodiversity change.
(b) Study sites The island of New Guinea lies within the humid tropics and is strongly influenced by seasonal fluctuations of the major equatorial circulation patterns. During the austral winter, New Guinea is under the influence of deep tropical easterly air flow (southeast trade winds), while during the austral summer monsoon, equatorial north-westerlies dominate. Throughout the year, the region is a locus of airstream convergence known as the intertropical convergence zone (ITCZ) and, as a result, is one of the most persistently cloudy regions around the equator (McAlpine et al. 1983). Circulation patterns over the region are strongly affected by the Southern Oscillation, though the influence is strongest during the pre-monsoon from September to November (McBride 1999). The severity of the 1997–1998 El Nin˜o event was brought about by an anomalous eastwards displacement of the ITCZ from over the maritime continent towards the central Pacific and a subsequent failure of the austral summer monsoon (Webster et al. 1998).
The large highland valleys with complex agriculture based on root cropping centred on the altitudes of 1400–1850 m were a major discovery of the twentieth century (Brookfield 1964). The outer flanks of the mountains are perhumid, with precipitation more than double evaporation in almost all months. In the highland or intermontane valleys, the mountains cut off the orographic rain and local circulations dominate in most seasons. Here, air rises each day up the warmed slopes and descends over the valley, giving sunny conditions with adequate rainfall from afternoon thunderstorms. Away from the large highland basins, even small valleys may have this effect and may thus support small hamlets. In these marginal settlements, techniques of ditching and mounding are used to shed water from the fields. There is a northwest–southeast gradient from aseasonal precipitation to the appearance of a weakly defined dry season, and from relatively infertile limestones and mudstones in West Papua to richer soils fertilized by volcanic ash falls in Papua New Guinea east of the Strickland River (Hope & Haberle 2005).
Despite the present day restriction of forested areas to the surrounding mountainous slopes and degraded patches on highland valley floors, the structure and composition of lower montane forest found between 800 and 2200 m altitude is highly variable with the common canopy trees including Castanopsis,
Phil. Trans. R. Soc. B (2007)
S. G. Haberle
Nothofagus, Elaeocarpus, Beilschmiedia, Melastomata- ceae, Phyllocladus and the emergent gymnosperms Araucaria hunsteinii and Araucaria cunninghamii. Nothofagus is found as a scattered element of mixed lower montane forest, though it commonly forms a continuous forest between 2200 and 2700 m, possibly in response to persistent cloud cover and infrequent but large-scale disturbance (Read & Hope 1996). Grass- lands resulting from anthropogenic activity have replaced much of the lower montane forests through- out the major highland valleys, with Leersia and Phragmites common in wetter areas and the tall Miscanthus floridulus common in areas experiencing more aseasonal precipitation. Where precipitation has a more pronounced seasonality and fires are more frequent, particularly in the east, the short Imperata grassland communities are dominant. Using the two biodiversity indices discussed above (palynological richness and key biodiversity indicator taxa) as a means of tracking changes in biodiversity through time, selected pollen records from five highland valley floors where intensive agriculture is practised today are examined (figure 1).
3. SWAMP FOREST LOSS DURING THE HOLOCENE IN NEW GUINEA Pollen records from New Guinea spanning the Late Glacial to Holocene period show that vegetation, at least within montane and alpine regions, has a remarkable ability to track climate change (Hope 1976; Haberle 2003b). In highland valleys where human settlement and deforestation are prominent features of the contemporary landscape, palaeoeco- logical records are generally not continuous, perhaps due to anthropogenic-related disturbance and lack of detailed chronological control. One exception to this is the Tari Basin record which is the longest and most continuous record from the island of New Guinea (Haberle 1998b). Despite these problems, five sites have been selected from highland valleys extending from the central ranges of Irian Jaya to the eastern highlands of Papua New Guinea, representing the general nature of vegetation change through the Holocene. A summary diagram (figure 2) is presented for each site that includes the relative proportions of forest trees, swamp forest (Myrtaceae), a disturbance and arboriculture indicator (Casuarina), and charcoal particle concentration as an indicator of fire. The Casuarina pollen type is most probably associated with Casuarina oligodon which is a nitrogen fixing tree that is commonly planted by people throughout the highlands and used for multiple purposes (house/fencing material, crop shade, fallow tree, etc.). Loss of local forest cover is believed to have led to the adoption of agroforestry techniques to compensate for loss of forest resources (Haberle 1998a).
(a) Baliem Valley The western-most site in the transect is Kelela Swamp at 1400 m altitude in the Baliem Valley (Haberle et al. 1991), which has a long, possibly discontinuous, pollen record dating back to some time before 7000 yr BP. In the earliest phase, regional forest cover was dominated