Floodplains along the braided gravel-bed Waimakariri River are discontinuous although generally extensive landforms composed predominantly of gravel bars capped with vertically accreted fines In order of importance, three mechanisms lead to floodplain formation River bed abandonment by lateral migration of the braid-train initiates the formation of the largest floodpiains, which usually occur downstream of tributary fans and valley bedrock spurs In the headwater reaches, localized river bed aggradation during high magnitude events leads to floodplain formation by producing a surface resistant to erosion by lesser events due to either the coarse nature of the deposit, or by its elevation over the river bed, or both The least important mechanism initiating floodplain development is localized river or channel incision A six stage model is proposed for the sequential development of floodplains on the Waimakariri River from: (i) active river bed; (ii) stabilizing bar; (iii) incipient floodplain; (iv) established floodplain; (v) mature floodplain; and to (vi) terrace Two mechanisms, lateral migration by the braid-train and reactivation of abandoned channels within floodplains, operating separately or in combination, are responsible for floodplain reworking and their relatively young age (<250 years) Clearly, braided rivers can construct substantial areas of well developed floodplain

Formation of braided river floodplains, Waimakariri River, New Zealand

Summary There are three major tussock grassland communities in the Mackenzie Country: (i) fescue-tussock grassland where Festuca novae-zeladiae is abundant, (ii) red- tussock grassland where Chionochloa rubra is the physiognomic dominant, and (iii) snow-tussock grassland where Chionochloa rigida is the physiognomic dominant. Socioligical analyses of these communities in which severa phases are recognised are given in tables. The history of the grasslands is traced from the destruction of the forests through tall-tussock stages to the current extensive short-tussock grassland of recent origin.

Tussock grassland communities in the Mackenzie Country, South Canterbury, New Zealand

Aim To determine the composition and biogeographical origins of the native and naturalized flora of braided shingle riverbeds in New Zealand and whether the proportion of naturalized species is greater than in similar habitats elsewhere in the world. To test whether regional species pools, landscape matrix configuration, and local riverbed environment are all equally important in determining community structure in these systems.



Location  The braided reaches of four catchments on each of the eastern and western sides of North Island and South Island, New Zealand.



Methods  Plotless records of all native and naturalized seed plant and fern species on disturbed mobile surfaces were made over the length of four rivers. Altitude, climate variables, riverbed attributes, and surrounding land-cover from maps were recorded for each site. For all species, the taxonomy, life form, origin and history, and presence within the province through which the rivers flowed, was obtained from published floras. Direct and indirect ordination and variance partitioning were employed to examine how native and naturalized species composition varies among and within rivers, and the degree to which this variation reflects climate, characteristic of riverbeds, and the surrounding land-cover. Regression was used to determine how much introduction date and native geographical range influence the frequency of naturalized species.



Results  The total riverbed flora of 289 species comprises 40% native species and 60% naturalized species, both dominated by Asteraceae and Poaceae. The relative contributions of other plant families differ, and the two groups comprise different life-form spectra. Native species occur across fewer rivers (mean 1.6 rivers) than naturalized species (mean 2.1 rivers). Species common in at least one river system tend to be widespread, occurring in at least three rivers. The rivers differ in their floras, with distinctions between North Island and South Island, and eastern and western rivers. The South Island rivers have more native species and a higher proportion of their regional native species pools than North Island rivers, whereas they have a lower proportion of their naturalized species pools. Introduction date and native geographical range are correlated with frequency of individual naturalized species. Geographical position and climate, riverbed variables of substrate size and seepage presence, and the type of adjacent land-cover, differ between rivers and are significantly related to species composition. Land-cover variables alone account for 46% of the explained variation in species composition, environmental variables alone 32% and components of the environment and land-cover that co-vary, 22%.



Main conclusions  New Zealand shingle riverbeds are among the riparian communities in the world most invaded by naturalized species but comparisons are difficult because studies of similar riverbeds are rare. New Zealand riverbeds are dominated by short-lived terrestrial species from Eurasia. Native species are most frequent in South Island rivers draining extensive high mountains, and their abundances are determined to a greater extent by riverbed processes than are those of the naturalized species, which are more abundant when the surrounding landscape is modified. The distribution of the naturalized flora is not yet at equilibrium with the environment. Each river has a distinctive flora determined by ambient environmental factors, aspects of the riverbed environment itself, adjacent land-cover reflecting the presence of native vegetation, the intensity of human modification or use, and invasion history.

Determinants of regional and local patterns in the floras of braided riverbeds in New Zealand

Summary: Vegetation on 5 km (c. 100 ha) of the braided bed of the Orongorongo River, Wellington, was sampled in March from 1973 to 1990. The riverbed has become aggraded since an earthquake in 1855. Surface water covered little of the riverbed; Callitriche stagnalis was the only common vascular aquatic plant. Most grasses and dicot herbs were adventive. The scabweed Raoulia tenuicaulis was the commonest dicot. The extent of plant cover was measured on 300 circular plots (radius 1.5 m); it ranged between years from 5% to 22%, depending on the severity of floods. The number of plots with little vegetation varied widely, independently of plant cover. Plots with a sandy substrate had most plant cover. Much wind-blown sand accumulated beneath established vegetation. Patterns of early succession, which relied on available seed, were largely dictated by floods and droughts. If not disturbed for 20-25 years, succession invariably led to a temporary dominance of kanuka (Kunzea ericoides), with or without manuka (Leptospermum scoparium).

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Plant succession on the braided bed of the orongorongo river, wellington, new zealand,

Communities dominated by species of cushion plants are widespread in the alpine and subalpine zones ofNew Zealand and Tasmania. In New Zealand this vegetation type contains six major associations. These are described as alpine herbmoor, cushion bog, high alpine cushion moor, alpinc cushion herbfield, and rock and river shingle cushion communities, their distributions being closely related to particular geomorphic environments. In Tasmania there are four major associations. These are described as AbrotancHa cushion heath, high mountain cushion heath, mosaic cushion heath, and Donatia cushion heath, their distributions being most closely related to altitude, precipitation, and rock type. Ofthe six New Zealand cushion associations, herbmoor and cushion bog have the highest proportion ofspecies also occurring in Tasmania. The differences between Tasmanian and New Zealand cushion associations largely result from differences in topography. Tasmania lacks the rock and river shingle environments, complet...

https://www.tandfonline.com/doi/pdf/10.1080/0028825X.1985.10434227

A comparison of the cushion plant communities of New Zealand and Tasmania

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

D. M. Calder

Papers

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