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Abstract
All vascular plants recorded in the lower Orongorongo Valley DSIR study area (1100 ha) are listed with notes on their status and distribution. Of the 501 taxa, 355 are native, 128 are naturalised, and 18 have probably been deliberately introduced or cultivated. Ofthe native taxa, 35% are woody plants, but only 75 of these (or 60% of woody native taxa) are common and thus make up the bulk of the vegetation structure. By contrast, less than 9% of the naturalised species are woody plants, and all except two — gorse and buddleia — are either uncommon or rare. Annuals comprise 5.6% of all native dicotyledon herbs and 49% of the naturalised dicotyledon herbs. The history of the study area, public use, introduced animals, and tectonic events, particularly the earthquake-induced landslides of the mid-nineteenth century, are outlined and discussed in relation to the native and adventive flora. Exotic herbivores, especially possums (Trichosurus vulpecula), have greatly reduced the abundance of some plants which were common 30 years ago, including Alectryon excelsus, Fuchsia excontcata, Coriaria arborea, and Pseudopanax arboreus. Some other species, although probably never common, have been orought to the verge of extinction. Naturalised plants — especially daisies and grasses — have been able to colonise extensive open habitats created by earthquake slipping in 1855, and the reactivated terraces and river flood plain. If current selection pressures continue, it is predicted that other species such as Metrosideros robusta. Weinmannia racemosa, and Sphaeropteris medullaris will become rare, at least in lowland forest. Spread of Pinus radiata into mid-altitude communities will accelerate as exotic forests to windward mature.
