Plant ecology of subalpine shingle river-beds in canterbury, new zealand

TLDR: It is reasonable to suggest that the colonization by plants now taking place on the shingle river-beds, bears some resemblance to the process of events which took place in the development of the present tussock grassland vegetation.

Abstract: The northern and central portions of the mountain backbone of the South Island of New Zealand are composed, for the most part, of a hard, much-faulted sandstone or greywacke which is easily disintegrated by the action of weather. A good deal of this country was glaciated during the Pleistocene Period (Gage 1958) and the gross features of landscape are attributable to glacial erosion. This glacial period resulted in over-deepening of the valleys which have subsequently been filled, in some cases to a depth of 1000 ft (305 m) with rock fragments of various sizes washed down from the mountains during the present cycle of erosion. Characteristics of the present-day landscape are the well-developed scree slopes, the unique vegetation of which has been described by Fisher (1952); the huge, delta-shaped tussock-covered shingle fans which have spilled out over the valley floors and the wide, aggrading, shingle river-beds, each with a small volume of water out of all proportion to the width of the shingle (Phot. 1). These shingle river-beds provide a most unstable habitat, being subject to frequent flooding. However, even when in flood, rarely is all the shingle covered at one time, the water is confined to temporary beds which are gradually built up by deposition until the banks break and a new channel is cut. In this way a large proportion of the shingle is churned over and the established plants destroyed, while 'islands' of untouiched shingle remain. An understanding of the vegetational changes associated with the stabilization of the shingle can be gained by a study of these shingle islands and the land bordering the rivers. Rarely is the opportunity provided for the study of plant succession on a newly developed geological formation. As much of the subalpine and lowland areas of Canterbury originated from alluvial shingle deposits, it is reasonable to suggest that the colonization by plants now taking place on the shingle river-beds, bears some resemblance to the process of events which took place in the development of the present tussock grassland vegetation. This report is based mainly on observations made on the Cass river-bed, a locality within 1 mile of the University of Canterbury Mountain Biological Station, Cass; some 70 miles (1 13 km) westwards of Christchurch, at an altitude of 1800 ft (550 m). This river-bed is similar to many found to the east of the Main Divide, although Cockayne & Foweraker (1916) consider that it is atypical in that it is far more stable than the shingle river-bed habitat in general. If this were in fact true, the result would be to allow a greater development of the various stages of succession and it is believed that the study of such a river-bed will illustrate this succession much more clearly than the study of one where some of the stages may be poorly represented or completely absent. There are several distinct plant communities found in the Cass district, and it appears that these are in a state of flux induced by a relatively recent climatic change (Holloway 1954). The re-adjustment of the vegetation has been further complicated by the