The high-alpine vegetation of Central Otago, New Zealand

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Abstract
Extensive areas of dwarfed, tundra-like vegetation, mostly beyond the upper limit of the low-alpine snow tussock grassland, are described from 10 mountain ranges in Central Otago. Environmental data from one representative range indicate that, despite a moderate latitude and altitude, summer temperatures, wind, and snow cover in this vegetation zone are comparable in severity with most high-alpine areas overseas. Four major habitat-physiognomic types are recognised: fellfield, herbfield, cushion, and snowbank. They differ both in physiognomy and flora, while their habitats are differentiated by lithology, altitude, exposure and/or duration of snow cover. Fellfield, being essentially lithologically controlled, is the most distinct type and dominates those ranges with either greywacke or weakly or non-foliated schists, as these tend to fracture into angular blocks. Partially active large stone nets may be present. On the remaining ranges, all with strongly foliated schist, fellfield occurs locally at the highest altitudes. The three other major types also dominate extensive areas, their relative distribution being controlled by macro-topography. On the windswept, cryoplaned, summit plateaux where soils are shallow, there is an extremely dwarfed cushion vegetation in which nine angio-sperm families are represented among the 22 cushion- and mat-forming species, and where lichens are important. Herbfield, of somewhat larger plants, occupies less severe sites on deeper soils usually near the lower limit of the high-alpine zone, while snowbank communities occur in numerous cirque basins and other depressions where snow accumulates and persists. Within herbfield and cushion vegetation, mosaics occur along gradients of increasing altitude and/or exposure. These have been described quantitatively. Ten zones have been recognised to cover the total variation of snowbank vegetation in the region. They are defined by duration of snow cover from quite minor effects to a predominance of cryptogams under near-permanent snow. Cushion, herbfield, and snowbank communities, unlike fellfield, may be associated in distinct patterns. This occurs where microtopography—soil hummocks, soil stripes, solifluction terraces, solifluction lobes — differentiates micro-environments sufficiently to subsume the tolerances of the species characteristic of each community. Soil hummocks and stripes are widespread on relatively sheltered sites, chiefly among cushion vegetation. Apart from local degradation by wind, these structures are maintained by an essentially intact vegetation, the pattern of which varies with their size, which in turn is related to wind exposure and soil moisture. They appear to be relic structures, probably derived through surficial frost action on largely wind-borne material under more severe conditions than prevail today. Their mode of origin is unknown but there is a close morphological similarity to the widespread miniature stone nets and stripes attributed to current frost action on bare areas. Seven zones are recognised along the vegetation gradient from the exposed top to the far lee of the locally numerous solifluction terraces. Their dependence on terrace height is discussed. Buried topsoils and locally collapsed over-steepened fronts indicate that certain of the terraces are still active. Vegetation patterns on solifluction lobes from certain snowbanks are also described. A floristic list includes 166 vascular plants, 26 bryophytes, and 104 lichens. Undescribed taxa in Abrotanella and Raoulia are included. Occurrence of the lichen Usnea (Neuropogon) antarctica is of phytogeographical significance.