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The Vegetation of New Zealand
3. Exotic-induced communities
3. Exotic-induced communities.
a. Tree associations.
At Waitati, near Dunedin (SO.) the burning of dense Leptospermum shrubland — really an indigenous-induced community — allowed the seeds from a mature tree of Eucalyptus radiata to germinate and there is now Eucalyptus forest, thanks to the destruction of the Leptospermum having provided the essential light-requirement for the development of the seedling eucalypts, and their far more rapid growth than those of L. scoparium. In Kawarau Gorge (NO.), Eucalyptus globulus has formed dense groups on rocky ground, the seeds having come from neighbouring trees, originally planted by gold-diggers in the "sixties" of last century.
Salix fragilis and S. babylonica, to a lesser extent, planted in the first instance on banks of rivers, thanks to the rooting-power of their broken twigs, line the margins of many streams.
Albizzia lophantha is self-established in many parts of the North Auckland district. Acacia dealbata, both on dunes and the northern gumlands, page 359in places, spread from parent trees by means of creeping, underground stems and forms dense groves. So, too, Robinia Pseud-acacia makes close thickets in the neigbourhood of old mission stations in several localities in North Island.
Near Lake Wanaka (NO.) windborne seed of Cupressus macrocarpa and Pinus radiata, trapped by bushes of Discaria toumatou, germinates freely and many trees have been so established in the presence of stock and rabbits thanks to their natural spiny protector.
b. Shrub communities.
Leguminous shrubs.
Ulex europaeus1, Cytisus scoparius, Lupinus arboreus and, to a much lesser degree, Cytisus candicans, invade open communities replacing and displacing the primitive vegetation. The seeds germinate in the neighbourhood of the parent shrub, and, in the first instance, the invasion was from shrubs purposely planted. At the present time, there are vast impenetrable thickets of pure (usually) Ulex and Cytisus on stony river-bed and hillsides where forest has been burnt, which when in blossom are a glorious spectacle; both species, too, are abundant on lowland and lower-montane tussock-grassland and fixed dune. At an altitude of 700 m. in South Island, the above are no longer aggressive.
Lupinus arboreus2 is confined to dry sandy or stony stations forming close thickets, 1.8 m. or more high, on fixed or semi-stable dunes; it also occurs to some extent on river-bed.
Rosaceous shrubs.
Rosa Eglanteria, R. canina3Rubus fruticosus in a wide sense and R. laciniatus form individually, in many localities, extensive thickets which differ from those of the Leguminosae inasmuch as they owe their distribution to birds4 while climate restricts Rubus to wet and Rosa to dry areas.
Rubus thicket is especially aggressive in forest-clearings. Though occurring abundantly in many places, it attains the greatest luxuriance in the East Cape, Western, North-western, Egmont-Wanganui and Chatham districts.
1 1) This was early on introduced as a hedge-plant and is still extensively used for that purpose. Where left uncut, the hedge exceeds 3.6 m. in height and seed is shed in profusion. Burning has no effect in eradicating the plants; seedlings also are produced in millions and grow with great rapidity.
2 This shrub was purposely planted, or sown, on dunes, in the first instance, in order to check drifting sand. This it is unable to do, but it forbids all sand-movement on the ground it occupies.
3 Of local occurrence.
4 Some forms of Rubus fruticosus spread vegetatively by means of natural "layering".
Australian shrubs
1. Hakea acicularis2, Epacris purpurascens, E. microphylla and E. pulchetta form extensive colonies in Leptospermum shrubland (NA., SA.) the first-named being wide-spread, but the three epacrids being confined to one locality. The Hakea spreads after Leptospermum scoparium is burnt and probably the other shrubs first made their appearance after fire.
c. Communities of herbaceous and semi-woody plants.
Thymus vulgaris (garden thyme) association.
This well-known culinary plant occurs sporadically in several parts of semi-arid Central Otago, but on old mining tailings at Ophir several hectares are occupied by a close growth of this species, the flowers of which vary from crimson to white in many intermediate shades.
Some herbaceous plant communities.
Centranthus ruber occasionally forms a distinct and beautiful association with its abundant red, white and pink blossoms. The substratum preferred appears to be rocky slopes. One area under my close observation for some years is gradually extending its limits.
Where forest has been burned in areas subject to a considerable rainfall Digitalis purpurea forms colonies which extend for several kilometres at a time over hillsides. It also occurs in profusion on old mining tailings (SO.), rocky ground (NA.), and in the Taieri Gorge (SO.) along with Leptospermum scoparium until the average limit of the south-westerly downpour is reached, when the community halts, all on a sudden, and the induced steppe country begins.
1 1) It is obvious that Australian trees and shrubs, if sufficiently hardy, are much better suited to New Zealand conditions than was supposed. The belief that Australian plants would not thrive was partly based on the statement that Bidwill was in the habit of habitually scattering Australian seeds during his travels through the country. But we have no evidence as to the nature of the seeds, their age, the soil or locality where scattered or the time of year; indeed the experience of Bidwill, even if a fact, proves nothing.
2 2) Hakea saligna forms close thickets on sour, boggy soil near Collingwood (NW.), but F. G. Gibbs has informed me that they have originated from seed purposely sown, but probably seedlings are being established from seed shed from this artificial shrubland. Where H. acicularis equals the Leptospermum in quantity, or is not so abundant, the community will be merely a modified one.