Observations on the Vegetation of Screes in Canterbury, New Zealand

Abstract

In the drier mountains of the eastern and north-eastern regions of the South Island of New Zealand, there occurs very commonly a type of scree inhabited by a community of plants rarely found in any other situation. Although belonging to several different families, these plants have many characters, in common which distinguish them from plants of other alpine associations. This paper deals mainly with observations made on a large scree near the Canterbury University College Mountain Biological Station at Cass, 70 miles inland on the midland railway from Christchurch, New Zealand. This particular scree forms the northern slope, from 3000 to 6000 ft. above sea-level, of Mount Bailey, the northernmost peak of the Craigieburn Range not far from the main ranges of the Southern Alps (P1. 13, phot. 1). During the summer in this region, fine days are common, and while the brilliant sunlight of the high clear atmosphere raises the stony surfaces of the screes to a temperature such that they are unbearable to touch, blustering hot dry winds frequently sweep across the slopes from the west. In winter, snow may cover the ground for several weeks at a time. The annual rainfall of the district is 50 in., distributed fairly evenly throughout the year. The vegetation of the region consists of a tussock grassland association dominated by Poa caespitosa and Festuca novae-zealandiae, modified in local areas by variations in conditions. In sheltered situations small stands of beech (Nothofagus cliffortioides) replace the tussocks, while a shrubland association (Cassinia fulvida, Discaria toumatou, Leptospermum scoparium, etc.) occurs in many exposed places. At higher altitudes (above 3500 ft.) the short-tussock association (Poa-Festuca) gives way to a narrow belt of the tall-tussock, Danthonia raoulii, which forms the uppermost border of the grassland vegetation. Other high-mountain plants are found on rocks and in other stable situations above this altitude, usually including species of Hebe and Celmisia. Around the edges of the screes several small-leaved shrubs are the first unspecialized plants to enter the partially stabilized debris, e.g. Podocarpus nivalis and various members of the Epacridaceae, including species of Dracophyllum. Vegetation of screes outside New Zealand appears to be of two kinds. On very unstable soils certain specialized plants sometimes occur, which can survive movements of the surrounding stones. On slightly less mobile soils, plants are recruited from the surrounding stable soil associations of heath or rock. Such vegetation is often in a state of flux, since large areas may be buried by scree movements (Schroeter, 1926; Harshberger, 1929; Tansley, 1939). Very few papers dealing with the ecology of unstable screes appear to have been published, and those dealing with screes in Britain, Europe and America are mainly concerned with the successive phases of vegetation as consolidation takes place