Aspects of the Ecology and Biogeography of Lowland Cushion Communities in New Zealand with Special Reference to Fitzroy Bay, Lower North Island

Abstract

The coastal cushion plant communities at Fitzroy Bay, Wellington with high conservation values are the focus of a wider study of lowland cushion communities. The context of the Fitzroy Bay plant communities is established by an ecological and biogeographical comparison with some coastal and non-coastal South Island lowland cushionfields. The mosaic of plant communities at Fitzroy Bay reflects the geomorphology of the site and its proximity to the sea. The sea has modified the distribution of the Fitzroy Bay plant communities through inundation, salinity and its contribution to the geomorphological dynamics of the gravel barrier. The onshore wind has also contributed to the vegetation mosaic through sand deposition on the seaward side of terrace risers (and other barriers) facilitating the persistence of dune plant communities. The two plant species that characterise the Fitzroy Bay dune community are Desmoschoenus spiralis and Spinifex sericeus whilst the cushion plant communities are dominated by Raoulia hookeri and Pimelea urvilleana. The predicted parallel zonation of the plant communities at Fitzroy Bay was shown to be inaccurate but the relationship between site topography and distribution of plant communities was quite clear. The two South Island coastal cushion plant sites, Cape Campbell and Chrystall's Beach also reflect the proximity of the sea but were more limited in their extent and thus interpretability. The floristic differences between the Cape Campbell, Chrystall's Beach and Fitzroy Bay sites can be partially attributed to biogeography and partly the nature of the predominant landforms at each site. The Fitzroy Bay cushion communities are found on the prograding Lake Kohangapiripiri gravel barrier while Cape Campbell has one substantial Raoulia community on dunes. The Chrystall's Beach cushion plant community reflects it's dune hollow locality and its more southern location. The Fitzroy Bay and Cape Campbell communities were identified as have the clearest similarities. The non-coastal Central Otago cushion plant sites included Pisa Flats, Cromwell Chafer Beetle Reserve and a Luggate site. These sites have unique plant species including the undescribed Galium sp, Leptinella sp and Craspedia sp at Pisa Flats. There were nine native plant species and several genera including Raoulia, Pimelea, Colobanthus and Craspedia found at both the Central Otago sites and the coastal sites suggesting possible coastal origins. Moth species previously found in Central Otago and coastal Otago support this theory (Patrick 1989c). However the biogeographical significance of the plant species common to both Central Otago and coastal sites remains indeterminate due to the age of the outwash terraces that support the Central Otago cushion plant communities and the unknown pre-human distribution of the plants. The occurrence of salt pans at the Pisa Flats site is probably coincidental to the presence of cushion plant communities at the same site. Atriplex buchananii is the main saline native plant species at Pisa Flats which is characteristically coastal. Many other native plant species found at the Central Otago sites as well as the coastal sites are widespread colonisers of relatively dry open sites rather than obligate coastal species. There are some similar nature conservation management criteria for the coastal and non-coastal cushion plant communities. Perhaps not surprisingly, the effects of grazing by feral animals and stock and the proliferation of exotic plants are two common factors that require monitoring. The potential for restoration of coastal sites following mining needs a careful management approach. Mining should only occur where environmental modifications are already extreme. The remaining native coastal plant communities are too vulnerable to permanent alteration through major disturbance to risk any experimentation on the few relatively unmodified coastal ecosystems. The very restricted distribution of some of the Central Otago cushion plant species highlights the need for close monitoring and effective management of the cushion plant community sites.