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The Vegetation of New Zealand

5. Artificial communities

page 370

5. Artificial communities

Under this head come all those communities — some designed to last for a considerable time, and others of brief duration — which are produced directly by man in his ordinary agricultural, forestry (so far as direct planting, sowing and thinning goes) and horticultural operations. Here only two classes of communities are dealt with, but they are of particular interest representing, as they do, agricultural proceedings unknown in Europe.

Displacement of rain-forest and swamp-forest by burning and replacement by artificial pasture without ploughing the ground.

Some 4050000 hectares of forest, the greater part in North Island, have been converted into grassland. For many years from 400 to 800 sq. km. were dealt with in each year but, at the present time, only a comparatively small area is available and the bush-buru days of New Zealand's pastoral development have almost passed away. The process of conversion of forest to grassland is of extreme phytogeographical interest since in one year's time a formation;, apparently attuned to a special habitat, is replaced by another ordinarily supposed to depend upon altogether different conditions, which more-over remains permanent so long, at any rate, as it is kept fully grazed. The processes involved consist of the following phases: — 1. Felling the forest. 2. Burning the fallen timber. 3. Sowing the seed. 4. Stocking the ground. 5. Burning the old and fallen logs. 6. Stumping the ground.

The forest is felled in winter and early spring. First, the undergrowth is cut and allowed to lie where it falls in order to provide the actual kindling-wood for the succeeding fire. Next, all the trees having a smaller diam. than 90 cm. are cut down, the others being left untouched, but the procedure differs in different localities and larger trees are felled on level ground than on steep slopes. Burning usually takes place after Christmas but the date depends entirely upon the state of the weather, since on a successful "burn" depends the future success of the subsequent operations. The following conditions are essential: — 1. the fallen trees &c. must be dry enough; 2. the weather must keep fine during the burn; 3. the wind must be favourable both in direction and intensity. The lighting of the fires takes place along as long a line as possible at right angles to the wind. Rapidity of burning is essential, so that on a large "block" as many as forty men may be required. Almost before the ashes have stopped smoking, and certainly before they are cold, the seed is sown.

The sowers sow by hand, carrying the seed1 in bags, which are ordinary

1 1) The seed is packed on horses to the ground from the nearest road in bags containing 35 kg., all the different seeds having been previously mixed together, and they are placed in position over the burnt area.

page 371sacks cut half-way down, a flap being thus formed in front, while a hole is cut in the back through which the man thrusts his head. The seed is scattered right and left, both hands being used, and, at each step, a handful of seed is thrown. The sowers form a diagonal line, so that one man slightly overlaps the work of another. The sight of the line of sowers crossing a log-strewn area where walking unburdened is no easy matter for a novice, each carrying a heavy bag and scattering the seed without cessation is not easily forgotten.

The amount sown per hectare varies from 20 to 30 kg. The following are the chief species sown: — Dactylis glomerata, Lolium perenne, L. italicum, Phleum pratense, Alopecurus pratensis, Poa pratensis, Cynosurus cristatus, Agrostis alba, Trifolium pratense, T. repens, T. hybridum. Other grasses (Festuca spp.) and clovers are occasionally used but the bulk of the seed consists of the first two species in the list1. A certain amount of rape, mustard and soft turnip is included in the mixture, so as to provide food for such stock as are turned on to the land within a few months of sowing.

Within 12 months from felling the forest the land is fully stocked and the trampling of the animals consolidates the ground and greatly assists in forming a sward. Where hilly, sheep are generally pastured, but the richer bottomlands are used for cattle.

By slow degrees, in process of time, the unburned logs decay or are burnt, the standing trees fall, and, if the ground is to be cropped, the stumps are extracted. At present, every stage of the conversion of forest into meadow is to be seen, but there are many areas where no vestige remains of the original plant-covering.

If the grass does not entirely cover the ground, certain indigenous shrubs may become abundant, especially Aristotelia serrata. Also, plants not present in the original vegetation may appear, especially Leptospermum scoparium and, near the coast, Cassinia leptophylla. Pteridium heath may also enter in, and were it not for abundant "stocking" would become permanent.

Replacement of Pteridium heath by artificial grassland ("Fern crushing").
As seen in Part II, there are large areas of Pteridium heath, much of which is certainly not primitive. Many areas of such heath have been converted into artificial grassland by a somewhat similar process as that just described. The heath ("fern" of the settlers) was burnt in the early autumn and from 15 to 30 kg. per hectare of grass and clover seeds, similar to those used on bush-burns, sown by hand. Between autumn and early spring little, if any, growth of Pteridium occurs, so time is afforded

1 1) In many localities Lolium perenne does not persist, so that eventually Dactylis and Trifolium repens dominate, and such form the basis of many meadows on soils which are "fertile", but over wide areas where lower "fertility" conditions were present the dominant species of grasses consist of such as were not originally sown but species of Danthonia (indigenous) or Agrostis (exotic).

page 372for the germination and establishment of sown grasses and clovers. But in the spring new leaves of Pteridium are rapidly developed and, if left to grow, the whole area during the one growing-season would be dominated by the fern and Pteridium heath hold sway, the young grass and clover being killed. However, as soon as the young leaves commence to develop and before their circinate tips have unrolled, large numbers of either dry sheep or dry cattle, depending upon which was the more economic, are grazed on the area and, to use the expressive term of the farmer, the fern is "crushed out". The stock are kept on the ground until lack of feed leads to considerable loss in body weight at which time they are removed. The area is then spelled until the fern begins to put forth fresh leaves when the crushing is repeated. According to the topography of the area, and the number of the stock available for crushing, in a variable time the fern becomes so weakened that the artificial grassland obtains the mastery and can be continuously grazed with that number of animals that, without falling away in condition, can make use of the feed produced. Not in every case, by any means, is success attained after a single burning of the fern, but, in certain instances, several resowing of grass seeds become necessary and in many cases this leads to the incoming of Leptospermum shrubland.

The underlying principle of fern-crushing is one that also is regularly adopted on bush-burn areas which are developing into induced Pteridium heath. In most cases, the application of crushing is successful, but in certain localities heavy crushing of such heath is followed by the establishment of circular sheets or deep mats of Paesia scaberula, and if the crushing is continued, the Paesia patches will coalesce and unpalatable species become dominant.