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New Zealand´s flora is still not fully understood. It is estimated that between 15 and 20% of the vascular plant flora of New Zealand remains unnamed.

Research in this area aims to investigate a number of problematic genera. Genera such as Cardamine, Craspedia, and Myosotis probably contain large numbers of unnamed species; these genera require detailed research to understand relationships and variation in the genus. We also study genera with fewer species or smaller and more easily defined taxonomic issues that can be resolved without having to undertake a major study of the whole genus. Since 1995 research has addressed many of these smaller taxonomic problems in the indigenous and naturalised flora.

Research may include detailed morphological, anatomical, chemical and molecular investigation. It involves extensive fieldwork and growing plants in uniform conditions in cultivation to fully understand the variation in the group. The resulting monographs provide comprehensive description of the taxa in New Zealand including identification keys and detailed description, synonymy and analyses of taxa and their variation.

Why does New Zealand have so many indigenous species?

New Zealand is renowned for its exceptional ecological richness and disharmonic biota, which may have been brought about by its long isolation, the great range in altitude and rainfall, and its diverse landforms, rocks, and soils.

Following the separation from Australia, New Zealand drifted northwards into warmer climates but remained close enough to other land masses such as New Caledonia to gain new plants by long– distance dispersal. A distinctive tropical, woody element of New Zealand’s flora can be traced to this period. Many of its members, including palms, pittosporums, coprosmas and araliads, adapted to cooler climates or survived the following cool periods only in the far north. Beginning 10 million years ago, movement of the two continental plates thrust up tall mountain chains, thus creating new alpine habitats. There is much debate over the origin of our alpine flora. Undoubtedly, some plants already in New Zealand adapted to the new alpine areas whereas others arrived from or via Australia or Antarctica. The Apiaceae, Asteraceae, Plantaginaceae, and other families rapidly diversified in the alpine zone. Dispersal of plants to New Zealand by strong westerly winds, by floating in seawater, or on the feet of waterbirds still provides a continual supply of new species.

Features of New Zealand flora

New Zealand’s flora shows many unusual features. For example, New Zealand has a high proportion of plants that have separate male and female individuals (Webb et al. 1999) and New Zealand has become a centre for research in plant sexuality. Godley (1975, 1979) showed that a high proportion of this gender dimorphism had probably not evolved in New Zealand, leading Lloyd (1985) to propose that immigrant selection was involved in about 85% of the examples of the evolution of New Zealand’s unisexual plant groups.

Compared with their overseas relatives, the flowers of native plants are often small, white, and regular, with short tubes and spreading petals. Such simple flowers are hypothesised to suit New Zealand’s pollinating insects (Lloyd 1985). In some seed plant families the ancestors of today’s natives probably had simple flowers when they arrived in New Zealand enabling these plants to diversify rapidly here. In other groups, e.g. Melicytus (Powlesland 1984), ancestors of New Zealand’s simple–flowered species lost their complex pollination mechanisms during later evolution in New Zealand (Newstrom & Robertson 2005).

Some families have shown a remarkable adaptability in evolving forms to cope with new habitats. This is particularly true of New Zealand’s alpine cushion plants, mountain scree plants, and lowland divaricating shrubs. New Zealand’s flora has few annuals, deciduous plants or geophytes, and this is thought to result from an equable and unpredictable climate (Godley 1975). The absence of the scleromorphic element so characteristic of Australia is thought to be a result of higher soil fertilities in New Zealand’s rapidly eroding landscapes (Flannery 1994; Groves 1994), which place slow–growing scleromorphic immigrants at a disadvantage in competition with mesomorphic species (Lloyd 1985).

Naturalised plants

Naturalised plants in New Zealand include those that have established themselves in wild habitats, and plants that are spontaneously seeding or spreading vegetatively in urban environments.

Newly naturalised plants are documented by the publication of checklists that update the Flora of New Zealand series. These lists serve as a biosecurity watchlist for plants currently spreading in urban environments, with the potential to become pest plants in wild environments.

Checklist publication is a collaborative project involving botanists from Landcare Research, DOC, Auckland Museum, and NIWA.

Origins of naturalised plants

Polynesian settlers, the ancestors of today's Māori people, brought plants with them intentionally or unintentionally, some of which like Sigesbeckia orientalis became established as wild plants. Some weedy plants characteristic of the northern hemisphere, such as Rorippa palustris, were established here when Banks and Solander collected them in 1769, and perhaps were able to establish following human disturbance.

Large–scale introduction of European crops and garden plants after 1840 brought many attendant weeds and the number has steadily grown year by year (e.g. Heenan et al. 1998, 1999, 2002b, 2004a, 2008) with only a few losses. Origins and biology of the naturalised flora were reviewed by Healy (1969, 1973), Esler (1988), and Webb et al. (1988).

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