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Discussion
The large, unstable shingle bank described on page 4 was deposited in 1947 when there was a complete redistribution of the shingle and a change in the course of the stream in the upper part of the main valley. A further deposit of shingle and heavy silting of the downstream areas occurred in September, 1948, and by the summer there was a scattered vegetation in which Polygonum hydropiper was prominent. The stream occupied a similar course in March, 1951, to that of September, 1948, but several changes had occurred in the intervening years. The main changes are summarised in Fig. 1. There was little change in the bank during the winter of 1951, and in mid-October the plant communities were mapped (Fig. 2). After a flood on the 26th October the changes in the shingle areas and the vegetation were recorded (Fig. 3).
The fresh deposit of shingle was colonized by Spergula arvensis (Fig. 11) and areas of silt and shingle were colonized by both Polygonum hydropiper and Spergula (Fig. 12). Polygonum, which is a much larger plant, grew very rapidly and appeared to be the dominant. In two months these two plants had formed a complete cover, and they are probably the true pioneers in the colonization of silt and shingle in this stream. Both are annual species which grow rapidly and easily from seed which germinates at any time of the year.
Most of the shingle of area 3 had been deposited in 1950, and by October, 1951, there was an open community including Rumex acetosella, Stellaria media, Plantago lanceolata and Hypochaeris radicata. The plants were broken and battered by the 1951 floods but gradually recovered, and by December, 1951, there was a dense, mixed herbaceous community in which flowering Chrysanthemum species were most prominent.
Most of area 5 had been undisturbed since 1948, and a closed turf cover had developed in which Holcus lanatus, Dactylis glomerata, Bellis perennis and Trifolium species were the most abundant; the only effect of the flood in this area was a slight silting and deposition of debris.
This suggests that on these banks three to four years may be sufficient time for the development of a closed community in which grasses predominate. From this one could postulate an increasing stability of the shingle with an increasing plant cover, in a direct time sequence.
There are, however, other factors to be considered:
| 1. |
The most important is the size and frequency of the floods. In the Horokiwi Stream the water supply is mainly surface run-off from the pasture and is quickly affected by the local rainfall. Allen (1951) found a very close relationship between the amount of rain and the rise in stream level Slight floods cause surface gravel movement, dislodging all vascular plants in the stream bed but having no effect on the shingle banks. Moderate floods cause a rise of up to 3 feet in stream level, usually resulting in slight marginal changes in the shingle banks in the unstable areas and building up the stable areas by silt deposition. Vegetation may be damaged but not completely destroyed. Whereas severe floods (over 3 feet) result in changes in the stream course and disturbance of the |
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shingle to a considerable depth. Plants are uprooted, especially on the islands, and bank erosion undermines the pasture. Even low-lying stable banks may be swept clear of vegetation, cut away and the shingle redeposited. Severe floods may completely change stable as well as unstable areas. |
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| 2. |
Mechanical settling of the shingle leads to consolidation of the banks and increases the resistance to flood waters. Thus a flood occurring while the shingle is loose and unstable will cause more disturbance than one of similar size after a long period of low levels during which the shingle has consolidated. |
| 3. |
In this stream the position of the shingle bank in the river is also important. In the upper part of the main valley there is a considerable depth of loose shingle in the stream bed, and during the last 10 years changes in the course of the stream and changes in the shingle banks have been frequent, but the lower part of the stream is relatively stable and floods cause only small changes. |
| 4. |
Another factor is the position of the stabilized part of the bank with a closed turf cover in relation to the direction of stream flow. The upstream ends and outer margins of the banks are most frequently disturbed and are often affected by small floods, while silting continues to build up the more stable, inner and downstream parts of the bank. |
| 5. |
In the case of moderate floods, a factor which may be significant is the time of the year at which the flood occurs; the herbaceous community is more open in winter and less resistant to floods, becoming denser in the summer with the growth of annuals and the flowering of perennials. |
| 6. |
There is a plentiful and continual supply of seed from the adjacent areas so colonization of the bare shingle is rapid. Slight disturbances uncover seed already buried in the shingle, and many of the perennial plants can grow from broken rootstocks or pieces of rhizome or stolon, so that the early stages of the succession may be omitted or may be of very short duration, except after heavy floods which cause complete destruction of the vegetation and redistribution of the shingle. |
The interaction of these factors results in a tendency for an area of shingle to stay at the particular stage of stabilization and plant succession that it has reached. A bank of loose shingle with a sparse plant cover will be continually disturbed by small floods, whereas a bank of consolidated shingle with a dense plant cover is affected only by severe floods. A closed turf recovers quickly after inundation and small deposits of silt.
The permanency of the beds or islands of shingle in midstream, however, may depend more on the frequency of major course changes than on the stability of the shingle and the density of the plant cover. These islands are more common in the middle reaches of the main water where the stream bed is unstable, they are frequently swept clear of vegetation and the shingle cut away and redeposited. Characteristic of these islands is a mixed herbaceous community in which flowering Digitalis and Chrysanthemum species are the most conspicuous plants in summer. The final stage of a closed grass cover is rarely developed on these islands.
Banks on which grasses are dominant, often to the complete exclusion of Polygonum, Digitalis and Chrysanthemum, are a more common feature in the relatively stable lower reaches of the stream. Their vegetation is very similar to that of the adjacent pasture and where accessible they are grazed by the cattle.
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Floods may cover these banks and recede without apparent change in the extent of the bank or the plant cover.