Some New Zealand indigenous-induced weeds and indigenous-induced modified and mixed plant-communities

Abstract

SUMMARY The paper deals, though far from fully, with the effect of settlement in New Zealand in regard to the primitive vegetation, and it may also be considered a supplement to Part III. of ‘The Vegetation of New Zealand’, ed. 2. Taken, as a whole, the paper is an explicit denial of the mischievous biological belief, even yet common, that in the ‘struggle between the indigenous and exotic elements of the present flora the latter element is all-powerful.’ According to the most recent estimate, the flora of New Zealand consists of 1850 species, 427 groups of species-hybrids, a great many varietal hybrids, and 650 exotic species. In respect to the vegetation, the following terms used in the paper are defined:–primitive, modified, induced, and the last embraces indigenous-induced, exotic-induced, mixed, and artificial. Competition between the indigenous and the exotic elements is far less severe than is usually considered, each element consisting mainly of different classes of life-forms, and each element for its greater part restricted to a different group of edaphic habitats, the indigenous to those of undisturbed soil, and the exotics to those of greatly disturbed soil. Altitude also plays an important role, the exotics being essentially species of the lowland belt and decreasing both in numbers and power of attack with increase of altitude. The struggle between indigene and exotic is very rarely an equal one, for the latter is generally greatly aided by the introduced grazing and browsing animals, nor must it be forgotten that such were wanting in primitive New Zealand. The exotics have entered into no part of the primitive vegetation—rock and aquatic communities excepted. Where the balance of nature in the primitive vegetation has been upset by the direct or indirect actions of man, a number of indigenous species have broken their bounds and become weeds equal in that undesirable status to any that have been introduced, Pteridium esculentum and Leptospermum Scoparium being indeed worse. This matter of observing indigenes becoming weeds is of particular scientific interest, for the origin of the weed-host of Europe must surely be wrapped in obscurity. Amongst forest-weeds an outstanding class is those which cut off a large part of light from the ground and inhibit, or make difficult, the establishment of seedlings which can develop into trees. Tree-ferns and semi-tree-ferns belong to the above class, and it is shown how certain of these increase greatly by vegetative means, a fact hitherto unpublished. In addition to indigenous-induced weeds there are many indigenous-induced communities, a number of which were unknown in primitive New Zealand, and a selection of such cases forms an important part of the paper. Particularly interesting is the example where a forest, not greatly different from that of the neighbourhood, is being established on heaps of gravel, stones, and silt (gold-mining tailings) in competition with exotics. A few cases are discussed of indigenous species entering into and eradicating powerful exotic-induced communities, e.g. that dominated by gorse and broom being invaded by rain-forest species, including tree-ferns. Another unexpected example—but now quite commonplace—is artificial pasture of European grasses and clovers being transformed into one dominated by an indigenous grass (Danthonia pilosa), such a community being unknown in primitive New Zealand where D. pilosa must have been not particularly common. Still another example is the invasion of an induced-mixed, heavily-grazed pasture by Aciphylla Colensoi, once a highly-important member of montane low tussock-grassland, but now greatly reduced in numbers by fire.