Journal of the Royal Society of New Zealand abstracts

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Southern hemisphere patterned mires, with emphasis on southern New Zealand

A. F. Mark¹, P. N. Johnson², K. J. M. Dickinson³ and M. S. McGlone⁴

Extensive patterned mires in valley heads and on glacial benches in southern New Zealand: (45deg. 22[[minute]]S; 1250-1400 m a.s.l.), and valley floors in Tierra del Fuego (54deg. 53[[minute]]S; 60-100 m a.s.l.) have developed under cool, moist climatic conditions favourable for peat accumulation.

In southern New Zealand's tussock grassland landscape, aapa mire pools (often with peat islands) and ridges, are elongated (pools: mean 24.5 +/- 2.4 m x 7.5 +/- 1.0 m) and aligned 90deg. to gentle slopes. Peat depths (0.3-2.5 m) are greatest on downslope margins of pools and sometimes associated with basement profile concavities. Pools up to 1.3 m deep have peat bases, are generally not fed by nor linked to surface streams, and have steep downslope, with more gentle upslope, margins. Pools may be incipient (occupying small peat depressions), fluctuating or permanent. In steeper headwater sections, surface and underground channels link small stream pools. On glacial benches, pools (to 1 ha) occupy basement rock depressions with up to 1.5 m of underlying peat.

Vegetation displays a repeating basic pattern. On pool rims aquatic mosses (Acrocladium sarmentosum and/or Sphagnum falcatulum) cover steep pool edges; S. squarrosum inner faces of rims; S. cristatum, Carex gaudichaudiana and C. lachenalii on rims and declines. The tall, vertical peat pedestals support a similar vegetation pattern. Carex gaudichaudiana occupies shore margins prone to intermittent flooding with peat-forming Sphagnum mosses absent. Islands are produced from elongated peninsulas or sinuate mainland shores. These persist if peat formation exceeds water level rise. Snow tussocks of Chionochloa macra grow on localised elevated areas unrelated to subsurface topography.

In Tierra del Fuego, raised (4-6 m) rain-fed aapa mires, bordered by intermittent lagg streams, develop amongst partially cleared Nothofagus pumilio forest. Elongated (3-60 m length), steep-sided and island (circular: c. 2 m diameter)- studded pools, form generally parallel crescentic lines with regular to sinuate margins associated with incipient islands. Occasional islands are either decadent, or submerged and moribund. Sphagnum magellanicum and Empetrum rubrum dominate above water level, amongst pure stands of Marsippospermum grandiflorum on higher ground. Sphagnum fimbriatum characterises a narrow zone at water line.

Vegetation/peat development reinforces pool-vegetation boundaries. Until drained, vegetation/firm peat development is prevented in pools >=20 cm deep. Following localised subterranean drainage, non-wetland species may colonise.

Underlying topography, differential peat formation, hydrological and drainage properties, plus vegetation development all influence patterning in the mires. Biotic factors are emphasised over physical processes in the origin and maintenance of the patterned wetland systems but are not mutually exclusive.

Keywords: aapa mire; dynamics; flora; peatland; Tierra del Fuego

(c) Journal of The Royal Society of New Zealand,

Volume 25, Number 1, March 1995, pp 23-54

PDF file of entire paper: medium quality (5416K); (scanned from paper original: notes about this process)

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