Sand Dunes¹²⁵

Sandy beaches built up from the sea and backed by sand dunes that have been formed by wind are a feature of much of the coastline of both islands. These beaches, regularly disturbed by tides, rarely support any plants and even the dune ridge immediately behind the beach is a difficult habitat for them. The sand is unstable, frequently dry near the surface and with a high salt content from wind and spray. A few plants page 157however, known as sand binders, colonise such dunes and often completely cover them with foliage. The sedge pingao (Desmoschoenus spiralis) (Fig. 88), now becoming rare, is the most robust and colourful of these with its coarse, narrow, curving leaves coloured yellow to orange-brown. It has widely spreading and branching subsurface rhizomes, which give rise to cord-like roots penetrating deep into the sandhill. The silvery grass Spinifex sericea is equally common, but in this case the more slender rhizomes spread and branch over the surface of the sand. The seed heads of Spinifex are distinctive in that they bear quite long, spherically arranged, flexible spines, which enable the heads to bowl along with the wind over the beaches and dunes for considerable distances. The introduced marram grass (Ammophila arenaria) is now also common as a sand binder.

As a dune becomes consolidated by the sand binders and particularly

Figure 88 Robust leading shoots of pingao (Desmoschoenus spiralis) spreading through a sand dune. Photo: J. Miles.

page 158when new sheltering dunes are formed seaward of it, other species less tolerant of instability enter in. These include the herbaceous Tetragonia trigyna (New Zealand spinach) and the Convolvulus-like Calystegia soldanella; the low shrubs Pimelea arenaria and Coprosma acerosa, which has interlaced mattress-like orangey stems bearing minute leaves.

Eventually a variety of taller shrubs, robust herbs and climbers common in open inland situations may form dense communities: toetoe (Cortaderia fulvida), Phormium tenax, species of leafless broom (Carmichaelia), matagouri (Discaria toumatou), species of Cassinia, manuka (Leptospermum scoparium), the introduced shrubby Lupinus arboreus and others. Given continuing stability and sufficient time a progression to forest can take place.

In the hollows between dunes, damp or even swampy conditions often prevail. The first plants to establish in such moist and relatively sheltered situations are such small herbs as Gunnera arenaria, Ranunculus acaulis and the sedges Isolepis nodosus and Carex pumila. The taller jointed rush (Leptocarpus similis) succeeds these, followed sometimes by manuka. The more extensive swampy interdune areas may eventually support swamp forest.