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Dune deflation hollow at Tiwai Point, Southland (Rowan Buxton)

Dune deflation hollow at Tiwai Point, Southland (Rowan Buxton)

Dune deflation hollows are where wind has removed sand down to a level where a layer of particles too heavy for the wind to move (an armoured surface) stabilises the sand and prevents the surface being lowered further. They often form between a series of sand dunes and when the dunes move further inland they enlarge the terminal deflation hollow behind. Initially the hollows are colonised by small plants such as sand sedge (Carex pumila), Zoysia pauciflora, and remuremu (Selliera radicans) and then by progressively taller plants over time such as knobby club rush (Ficinia nodosa).This system may grade into damp sand plains where the water table is permanently near the surface. Dune deflation hollows can be the most species rich sites within the larger sand dune system.

Notable flora and fauna

Threatened plants include the nationally critical Sebaea ovata, the declining Gunnera arenaria, sand tussock (Poa billardierei) and sand daphne (Pimelea villosa) and also the naturally uncommon Chatham Island geranium (Geranium traversii).

The endemic dune snail (Succinea archeyi) occurs in this habitat. While not endangered, small butterflies in the genus Lycaena are common.

Threat status

Endangered (Holdaway et al. 2012)

Threats

Many exotic species are present including hawkbit (Leontodon taraxacoides), cat’s ear (Hypochaeris spp.), several clovers (Trifolium spp.) and trefoils (Lotus spp.), buttercup (Ranunculus repens), creeping bent (Agrostis stolonifera), Yorkshire fog (Holcus lanatus), and hare’s tail (Lagurus ovatus). In some places, the large pampas grasses (Cortaderia spp.) are invasive. Coastal development poses a threat: dune systems have been converted into agricultural land and demand for coastal property is increasing. Planting and spread of exotic species, particularly marram grass (Ammophila arenaria) for stabilisation against erosion, has been common, which alters natural vegetation and geomorphic patterns and processes and prevents creation of new deflation hollows. In some areas, trampling and vehicle traffic (ATVs, motorbikes and off-road vehicles) and sand mining are a problem. Climate change poses a threat due to increased erosion from rising sea levels.

Where do they occur?

Dune deflation hollows are found in association with active sand dunes. They are particularly common in Northland, the western North Island from the Manawatu region to Wanganui, especially between the Manawatu and Rangitikei rivers where an extensive flat hinterland has allowed the dunes to spread inland, on Kaipara Peninsula, and on Kaitorete Spit in Canterbury, South Island. They also occur in Southland, on Stewart Island, and on Chatham Island.

Further reading

Brook FJ 2000. Holocene distribution, ecology and local extinction of the endemic New Zealand dune snail Succinea archeyi Powell (Stylommatophora: Succineidae). Journal of the Royal Society of New Zealand 30: 209-225.

Enright NJ, Anderson MJ (1988): Recent evolution of the Mangawhai Spit dunefield . Journal of the Royal Society of New Zealand 18:4, 359-367.

Esler AE 1969. Manawatu sand plain vegetation . Proceedings of the New Zealand Ecological Society 16: 32-35.

Esler AE 1970. Manawatu sand dune vegetation . Proceedings of the New Zealand Ecological Society 17: 41-47.

Lim D 2011. Marram grass seed ecology: the nature of the seed bank and secondary dispersal . MSc thesis, Otago.

Ogle, CC 1989. Sebaea ovata and its habitat near Wanganui. Wellington Botanical Society 45: 92-99.

Partridge TR 1992. Vegetation recovery following sand mining on coastal dunes at Kaitorete Spit, Canterbury, New Zealand . Biological Conservation 61: 59-71.

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