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(b.) Exotic-Induced Communities.
1. *Ulex europaeus Thicket of the Riverbed.
This association now occupies very large areas on the riverbed of the Rangitata, and completely overshadows the remnants of the indigenous Discaria toumatou thicket. It must, however, be emphasized that this development occurs only on the main river-bed where water-supply from beneath is considerable. On the small shingle-fans where the mountain streams debouch on to the terraces, and on the smaller stream-beds incised in these terraces the conditions are much more xerophytic. The layer of shingle before water is reached is much thicker, and the volume of water very much less. Here * Ulex europaeus is quite subordinate in amount, and occurs but rarely in the initial stages. Discaria toumatou thicket or mixed Coprosma shrubland are still the marked physiognomic features of these situations.
There is said to have been no Ulex on the Rangitata riverbed prior to 1873, about which time the plant was freely used for hedges. The account here given refers to the present state of affairs. On the recently vacated bed, for the most part covered with large boulders, odd plants of * Ulex europaeus and * Cytisus scoparius have established themselves. Between these are occasional small plants of any of * Holcus lanatus, * Agrostis vulgaris, Poa Lindsayi, * Aira caryophyllea, * Festuca bromoides, Muehlenbeckia axillaris, *Trifolium repens, Epilobium microphyllum, E. pedunculare var., * Hypochaeris radicata, Gnaphalium luteoalbum, Raoulia australis, R. tenuicaulis, and more rarely several other species. When floods occur patches of silt form round the Ulex, and on these patches * Agrostis vulgaris, * Rumex Acetosella, * Trifolium repens, Acaena microphylla, A. inermis form a turf. The small mats of Muehlenbeckia and Raoulia also collect silt and extend their borders, forming a seed-bed for various species, including Poa caespitosa. Usually the Ulex grows rapidly, seeds profusely, and forms dense patches that coalesce to form thickets under which the other plants become decadent. Successive floods increase the depth of the silt and completely bury the boulders. At this stage * Holcus lanatus and * Anthoxanthum odoratum are almost the only survivors below the Ulex.
Such thickets are often burnt and the sandy surface exposed. Ulex regenerates from the base and may be cropped by rabbits and sheep to a dense cushion form. The bare sand becomes occupied by * Holcus lanatus, * Rumex Acetosella, Oxalis corniculata (often very abun-
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dant) and a brown moss. Here too Raoulia lutescens, uncommon on the original riverbed, is often quite frequent. In places Triodia exigua, * Poa, pratensis, * Agrostis vulgaris, * Trifolium repens greatly assist in forming a turf. Such an area if stocked with sheep and the rabbits kept in subjection becomes an exotic-induced grassland that is invaded by Poa caespitosa and Festuca novae-zealandiae which produce a grassland very similar to the modified low tussock-grassland of the terraces. If the Ulex is not kept down it soon re-establishes a thicket.
2. * Ulex europaeus Thicket of the Hillside.
Great thickets of Ulex form a striking feature of certain hill-slopes on the Rangitata face. A similar thicket of * Cytisus scoparius occurs on one slope in the montane tussock-grassland.
Some fifty years ago a * Ulex europaeus hedge of the “bank and ditch” type was constructed from one gully forest to another across a “saddle” at 500 m. altitude. The hedge was on a sunny slope facing the east and ran through the Blechnum, capense belt and the lower tall tussock-grassland. From this hedge have developed the thickets now found, which are still extending their margins. Burning and cutting have been attempted from time to time, but never in a wholesale manner, and sufficient Ulex has always been left to reseed the burnt areas under more favourable conditions than before, while regeneration from the burnt stumps has been abundant. Over a considerable portion of the infested area the original associations have been obliterated. Towards the margins the course of the struggle may still be followed. The seedling Ulex grows rapidly, shading the herbaceous plants, which become drawn up and etiolated, and gradually succumb. Danthonia flavescens, Phormium Colensoi, Aciphylla Colensoi, and various shrubs, persist longer, but are killed out by the burning so that finally there is an almost pure Ulex association. In places, however, * Holcus lanatus and * Anthoxanthum odoratum are plentiful, with distant-noded culms stretching up through the bushes.
Where burning has been unusually severe on dry, sunny slopes Pteridium esculentum, may occur plentifully and even dominate the Ulex.
A noteworthy feature is the ready downward migration, of the * Ulex, and the much slower upward movement. Above an altitude of some 550 m. Ulex loses its aggressive powers and is represented by odd stunted bushes, and above 600 m. is hardly to be found at all.