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Vegetation history
This information together with information on the current distribution and composition of forest remnants suggests that the predominant pre-human vegetation would have been mixed podocarp-broadleaved forest with totara and matai occurring as emergent trees over a diverse canopy including kowhai, lemonwood, kohuhu, lowland ribbonwood, mahoe, five-finger and broadleaf amongst others. Examples of this forest type can be seen today on the southeast facing slopes under the Mt Cass-Totara ridge 2 km northwest of Kate Valley. This forest type would have occurred on both limestone and loess substrates. In coastal sites, ngaio would have been the dominant canopy species (e.g., as seen today in the lower part of Kate Valley). Black beech forest is likely to have been of more limited extent, being confined to well-drained ridge crests and terrace faces and to substrates formed on low-fertility sediments.
The fossil evidence suggests that deforestation of the Motunau Ecological District was primarily by fire. This appears to have been the case throughout the eastern South Island, with a peak in burning occurring 500-700 years before present (Ogden et al. 1998). All the scientific evidence suggests that these fires were associated with early Maori settlement (Molloy et al. 1963, McGlone 1983, Ogden et al. 1998). This deforestation would have resulted in a dramatic reduction in forest cover and an expansion of tussock grassland and shrubland. While some forest regeneration would have occurred after fire, subsequent fires would have limited this. The vegetation encountered by the first European visitors to the Motunau Ecological District most likely comprised a mixture of short tussock grassland, cabbage tree treeland, mixed shrubland, kanuka regenerating forest, mixed podocarp-broadleaved forest, coastal broadleaved forest, and black beech forest.
One remnant of black beech forest occurs on a steep face in an upper tributary of Kate Stream. Large black beech trees occur with kanuka and various broadleaved trees (e.g., five-finger, mahoe, lemonwood and lancewood). There are plenty of light gaps for seedling development, although beech seedlings are sparse presumably reflecting past grazing impacts.
Broadleaved forest is most common in the lower reaches of Kate Valley where ngaio is usually the dominant canopy tree, although a number of other tree species are present (e.g., broadleaf, mahoe, golden akeake, cabbage tree). Higher in Kate Valley, small areas of broadleaved froest trees are present usually close to streams within the larger patches of kanuka low forest. Kowhai, cabbage tree, lemonwood and mahoe are often present, while kawakawa is common in sites that have escaped stock grazing.
Kanuka low forest occurs extensively in the middle reaches of Kate Valley and on the coastal faces. Kanuka is usually the only canopy species and the understorey can be quite bare, although thickets of prickly mingimingi and Helichrysum aggregatum can be almost impenetrable in places. With the cessation of grazing, mahoe regeneration can be locally prolific.
Native shrubland is widespread and dominated by matagouri, but with significant amounts of other native species including Coprosma crassifolia, Coprosma propinqua, native broom, shrub pohuehue, native convolvulus and native jasmine. Gorse shrubland (and to a lesser extent European broom shrubland) is becoming increasingly dominant as a result of the removal of domestic grazing animals and it is likely that gorse shrubland will become a very widespread vegetation type in the next few years.
Exotic pasture is dominated by a range of exotic grasses and herbs including cocksfoot and browntop. The native silver tussock is occasionally present, but as the grass sward is thickening up after grazing removal the silver tussocks are slowly disappearing. As both native and gorse shrubland expands, the area of pasture will decrease.
Recent vegetation surveys of Tiromoana Bush have recorded 172 native vascular plant species including 49 trees and shrubs, 28 ferns, 14 climbers, 58 herbs, and 23 grasses, rushes and sedges. Amongst the more interesting species recorded are matai and totara (single records) which would have been dominant in this area historically, and two species of leafy mistletoe. Certainly the elements for successful forest restoration are present and it is likely that as further study of Tiromoana Bush occurs more species will be added to the list.
Leathwick, J. et al. 2003. Land Environments of New Zealand. Bateman, Auckland.
Information from pollen analyses and from a reconstruction of the potential vegetation cover of New Zealand, suggests that the Motunau Ecological District would have been covered by an almost continuous mantle of forest prior to human settlement. The only parts that would have lacked forest would have been bluffs, recently disturbed sites (e.g., slips along the coastal faces) and wetlands. The most specific information on the pre-human vegetation of the Motunau Ecological District comes from a pollen diagram from Swinton Park Amberley (Moar 1971), some 10 km southwest of Kate Valley. This pollen diagram shows the dominance of Podocarpus and Nothofagus pollen in the regional pollen rain and based on this it was suggested that the pre-human vegetation of the surrounding landscape was forest dominated by totara, matai and black beech. The more general reconstruction of the potential vegetation cover of New Zealand (Leathwick 2001) shows the Motunau Ecological District as supporting a mixture of lowland conifer forest and conifer/Nothofagus solandri forest.
Tiromoana Bush comprises a mosaic of vegetation types including black beech forest, broadleaved forest, kanuka low forest, native shrubland, gorse shrubland, pasture and wetland. Pasture is still the most extensive vegetation type, but with removal of grazing pressure, shrubland and forest vegetation types are slowly expanding.
McGlone, M.S. 1983. Polynesian deforestation of New Zealand: a preliminary synthesis. Archaeology in Oceania 18:11-25.
Moar, N.T. 1971. Contributions to the Quaternary history of the New Zealand flora 6. Aranuian pollen diagrams from Canterbury, Nelson, and North Westland, South Island. New Zealand Journal of Botany 9: 80-145.
Molloy, B.P.J. et al. 1963. Distribution of subfossil forest remains, eastern South Island, New Zealand. New Zealand Journal of Botany 1: 68-77.
Ogden, J., Basher, L. & McGlone, M. 1998. Fire, forest regeneration and links with early human habitation: evidence from New Zealand. Annals of Botany 81: 687-696.