Domed bogs (Sporadanthus)

[Sporadanthus traversii] domed bog at Wharekauri, Chatham Island (Bev Clarkson)

Domed bogs (Sporadanthus) are raised bogs dominated by Sporadanthus ferrugineus or S. traversii, members of the family Restionaceae. The restiad species are the primary peat formers (as opposed to Sphagnum mosses) and the bogs typically develop extensive convex domes raised above the local topography. A limited suite of species are tolerant of the waterlogged, low-nutrient and acidic conditions. Gleichenia dicarpa is a common associate of both Sporadanthus bog types, whereas Empodisma minus (also a restiad), Leptospermum scoparium, and Epacris pauciflora grow only in the S. ferrugineus type, and Dracophyllum scoparium and Olearia semidentata are confined to the S. traversii type.

Synonyms

Raised bogs

Notable flora and fauna

Threatened and rare plants include the nationally critical swamp helmet orchid (Anzybas carsei) and swamp greenhood orchid (Pterostylis micromega); nationally endangered yellow bladderwort (Utricularia australis) and Carse’s schoenus (Schoenus carsei); and the nationally vulnerable bog clubmoss (Lycopodiella serpentina) and Ladys tresses (Spiranthes novae-zelandiae). Species in decline include swamp green hood (Pterostylis paludosa), Dianella haematica, stout watermilfoil (Myriophyllum robustum), and swamp matipo (Myrsine coxii). Relict species include bladderwort (Utricularia delicatula) and bamboo rush (Sporadanthus ferrugineus). A large number of wetland plants are naturally uncommon, including Anzybas rotundifolius, Chatham Island spear grass (Aciphylla traversii), red-bearded orchid (Calochilus robertsonii), Calochilus paludosus, Chathams sedge (Carex chathamica), Chatham Island gentian (Gentiana chathamica), Chatham Island aster (Olearia semidentata), Thelymitra formosa and swamp leek orchid (Prasophyllum hectorii) and a bryophyte (Goebelobryum unguiculatum).

Notable fauna include the fernbird (Bowdleria punctata), black mudfish (Neochanna diversus), and ‘Fred the Thread’ moth (Houdinia flexissima).

Threat status

Endangered (Holdaway et al. 2012)

Threats

The northern North Island bogs have been extensively drained and converted to pasture, so very little of the original resource remains. This is mostly legally protected although peat mining occurs on a small portion of privately owned land, and adjacent drains have ongoing impacts on regional water tables. Accidental fire is another major threat. The Chatham Island bogs are more intact but are increasingly under threat of clearance and pasture development, fire, drainage, stock grazing, and nutrient enrichment via fertiliser drift. Weed problems include the invasion of margins by grey willow (Salix cinerea) in the Waikato, and strawberry myrtle (Ugni molinae) on the Chatham Islands. Here too, the peats have been investigated for their high wax content and they face the threat of mining.

Where do they occur?

Domed bogs with Sporadanthus ferrugineus were once common in the northern North Island, but they are now confined to three sites in the Waikato region. In contrast, S. traversii bogs are endemic to and widespread on Chatham Island.