S. G. Haberle
Figure 4. Remnant swamp forest on the valley floor surrounded by anthropogenic grasslands south of the Kainantu Valley, Papua New Guinea.
The composition of swamp forests that covered these valley floors is similar across the highlands, with all pollen records that contain pollen assemblages from the Early Holocene depicting a mixed montane swamp forest with Myrtaceae (mostly Syzygium), Pandanus and to a lesser extent Dacrydium, dominating the forest taxa assemblage. The dominance of the latter two taxa appears to be determined by increased frequencies of disturbance that may have impacted upon the ecosys- tem through natural processes (climate, competition) or human impact (selective exploitation of useful plant species, fire). What is clear from this analysis is that the present day swamp forests are remnants of a once much more extensive forest community.
(b) How rapidly did the transformation from swamp forest to grassland occur and was the change ubiquitous in time and space? The pace at which this change took place appears to vary from as much as around 1500 years in the Baliem Valley to less than 500 years in the Kainantu Valley (figure 2). A comparison of the timing of swamp forest loss between the five valley records in figure 2 shows that at this scale the loss is neither strictly time transgressive nor is it synchronous along the transect. This is not consistent with what might be expected under a ‘Neolithic Transition’ model where diffusion of agricultural techniques and crops may have been rapid and the impacts widespread. Nor is it consistent with a climate change model in which a shift to more frequent and intense El Nin˜o-related drought events around 4500 yr BP (Gagan et al. 2004) may have contributed to increased disturbance, burning and water loss which could have led to widespread loss of swamp forest cover after this time. A more complex model of landscape history is suggested by the palaeoecological results for swamp forest loss in which an increased focus on wetland agriculture during the Mid–Late Holocene (Denham et al. 2004), perhaps in combination with ongoing climate changes, led to rapid clearance of the swamp forest type at different times across the highlands.
Figure 5. Pandanus and conifer (Dacrydium and Araucaria) dominated swamp forest in the Erave District of the Southern Highlands, Papua New Guinea.
(c) What influence did anthropogenic fires and forest clearance activities have upon shaping the present day landscape? While the timing and nature of these changes can be identified, the reason for these changes at different times is not clear. Part of the reason may lie in the relative intensity of exploitation of each site and its vulnerability to the external forces of change. These
Phil. Trans. R. Soc. B (2007)