Focalización de la demanda

The type of wetland that forms in a depression is largely determined by the fertility of the water (Burrows 1990). The fertility changes over time causing the composition of the vegetation to change also. As noted previously, swamps are fertile wetlands where fresh nutrients are derived from drainage of the surrounding land, while bogs are infertile with nutrients supplied by rain or groundwater only.

A luxuriant forest dominated by tall trees has now invaded the Maungatapere wetland. The formation of the swamp forest has been enabled by eutrophic conditions where there is an abundance of plant nutrients and a favourable environment. As stated by Harris (1968), mesotrophic or even oligotrophic wetlands can occur in areas with fertile soils. The fertile Papakauri clay loam is the dominant soil type on the Rawhitiroa scoria cone, but currently the crater contains a natural wetland dominated by moss, rushes and sedges. The presence of short, low-fertility-demanding species such as Sphagnum moss and Machaerina teretifolia indicates that the wetland is low in plant nutrients and that oligotrophic conditions currently prevail.

Although the Maungatapere wetland is currently fertile, the presence of rare grains of Drosera in some of the samples, suggest that infertile conditions

occurred there occasionally in the past. Carnivorous plants such as Drosera often live in acidic bogs, where they capture and digest small animals to increase their supply of nutrients (Burrows 1990). Drosera was not identified in any of the Rawhitiroa samples, but the current infertile conditions are confirmed by the presence of a wetland dominated by Sphagnum and other low-fertility-demanding species.

So Maungatapere, a swamp forest, and Rawhitiroa, a bog dominated by

Sphagnum and sedges represent the two extremes of the fertility spectrum.

Yet the wetlands are relatively similar in age and the volcanic cones in which they lie, both consist of basaltic scoria. Reasons for this dissimilarity are explored further below.

The pollen diagram for Rawhitiroa indicates repeated episodes of burning since the arrival of humans about 800 years ago. The burning of forests induces wetter conditions (McGlone 2009), but provides a brief supply of fresh nutrients (Burrows1990) which encourages the growth of herbaceous vegetation and the formation of new wetlands (Burrows1990) if drainage is impaired. However, as the peat continues to build-up it limits the availability of nutrients to plants growing in the wetland, and together with the loss of nutrients through drainage, the wetland becomes more acidic and infertile forming a “raised bog” (Burrows 1990). It appears that this is what has happened at Rawhitiroa. Waterlogging was enhanced by the loss of tall forest trees through fire while fresh nutrients and higher water levels stimulated the growth of wetland plants contributing to the build-up of peat and ensuing infertile conditions.

Swamp forests, such as the one growing in the Maungatapere crater, are not normally associated with deep peat (McGlone and Neall 1994). However, the presence of sandy peat and abundant volcanic lapilli in Eltham Swamp, Taranaki, may have enabled the establishment of a swamp forest there by

providing nutrients and drainage (McGlone and Neall 1994). Similarly the presence of nutrient-rich, porous scoria forming the volcanic cone is a likely factor in enabling the establishment of a swamp forest at Maungatapere. Newnham (1992) noted a transition from an infertile poorly drained swamp, through a period dominated by Dacrydium cupressinum to a swamp forest dominated by Dacrycarpus dacrydioides at Otakairangi; this probably followed changes in drainage. He notes that this is a similar pattern to what has occurred in modern vegetation in the south-west of New Zealand. This may have also occurred at Maungatapere. Although the core did not reach the base of the peat, the pollen diagram indicates that the swamp was much wetter in the early Holocene than it is currently. As suggested previously, it may have become drier over time due to a decrease in rainfall or to the increased uptake of water by an encroaching forest. Another possibility is that the build up of sediments has enabled better drainage through the more porous rocks in the higher levels of the cone where fines were washed away into the central floor of the crater and thus below the base of the core.

Dacrydium cupressinum has increased in abundance during the Holocene and

probably grew on the swamp prior to the arrival of humans. The paucity of charcoal fragments suggests that the site was not repeatedly burned by Maori, thus maintaining fertility and drainage and enabling the establishment of a swamp forest. It is suggested that at Maungatapere, Maori cleared the outer flanks of the cone for horticulture rather than using fire to stimulate the growth of Pteridium esculentum as is likely at Rawhitiroa. Thus when the site was finally deserted by the local Maori population a new fertile wetland developed rather than a peat bog.

Concluding remarks on the findings of this thesis and suggestions for future research are the focus of Chapter Nine.