Plagianthus divaricatus at a High Elevation
[Read before the Wellington Philosophical Society, April 26, 1934; received by the Editor, April 27, 1934; issued separately, September, 1935.]
From the north saddle of Mount Matthews a long ridge runs down to Palliser Bay and covers many square miles of country. No name is given to this on the map, but a local name is the Big Hill. On the lower slopes of this Big Hill near the mouth of the Muku-Muku River is a spur with some very ordinary looking scrub on it. A closer examination of this scrub shows one most unusual constituent—Plagianthus divaricatus—which is a common plant of the saltmarshes in both islands. In this case the plant of the saltmarsh has become a member of a plant association on a dry, bare, windswept spur at nearly 600 feet elevation.
From the mouth of the Muku-Muku River an old river terrace runs inland for two miles or more at nearly the same grade as the stream. At the point where the spur meets the river-terrace the elevation is 325 feet, and the spur rises at a fairly stiff slope. This locality is about a quarter of a mile from the present mouth of the Muku-Muku River. Carmichaelia and Coprosma scrub commence near the bottom of the spur, and at 475 feet Plagianthus divaricatus appears as a member of the association, and is a fairly common, if intermittent, plant till a height of 575 feet is reached. Above this height Cassinia leptophylla becomes common and straggles a short distance further up the spur along with a few of the wind-resisting Coprosmas.
Carmichaelia odorata is the dominant plant of the association. The four Coprosmas. C. propinqua, C. rhamnoides, C. crassifolia, and C. rigida are common, as also are Cyathodes acerosa, Cassinia leptophylla, Helichrysum glomeratum, and the two Leptospermums, L. scoparium and L. ericoides.
Several juvenile trees are scantily present, but none are higher than the tallest shrub. They consist of Edwardsia microphylla, Hebe parviflora, Melicytus ramiflorus, Shawia paniculata, Pennantia corymbosa, Leucopogon fasciculatus, and Pseudopanax ferox. The vines present are the two Parsonsias P. heterophylla and P. capsularis and three Muehlenbeckias M. australis, M. complexa, and M. complexa var. microphylla. Two ferns were noted, Polystichum Richardi and Blechnum membranaceum, a few specimens of Uncinia leptostycha, Linum monogynum, Stellaria gracilenta, and Korthalsella salicornioides complete the list, with the addition of one plant of Hymenanthera crassifolia. This latter is a common plant at many stations round Palliser Bay beach, and, apart from P. divaricatus, is the only plant of the association that could be classed as a beach plant. The spur faces nearly south, and no doubt gets the full blast of the southerly gales that enter the bare, wide mouth of the Muku-Muku Stream. The Coprosmas, Cyathodes acerosa, and P. divaricatus are
all stunted and tangled, trimmed by the gale, and any straggly new shoots are promptly cut back to the line of least resistance. The gently sloping terrace just below the spur is grass meadow, and the place is well stocked with cattle and sheep, to say nothing of goats, pigs, rabbits, and hares. It is therefore no wonder that only inedible plants are left in the association. In fact, many acres round about have been burned off many times and exotic grasses and clover sown till most of it is bare grassland.
One possible, and I think probable, explanation of Plagianthus divaricatus occurring at this altitude is that when the river-bed was elevated to its present position a saltmarsh containing a patch of P. divaricatus was also raised to form part of the spur. It is probable that the present plants are not the plants originally raised along with the terrace, but their descendants. Possibly H. crassifolia shared in the upheaval. In any case, the solitary specimen of H. crassifolia on this spur is the only one I have found in the Matthews district, except on the sea-beach a few feet above sea-level.
Of the plants left in the association E. microphylla seems to suffer most from stock. Plant after plant can be found up to two inches thick in the stem, yet not more than one foot high, and the only way it can get above the reach of sheep, etc., is to grow through another bush to avoid getting eaten off. No doubt many former members of the association have been suppressed many years ago. Probably, also, many former constituents of the saltmarsh found it impossible to accommodate themselves to life on the bare, steep spur. All the more marvellous, therefore, is the change-over accomplished by Plagianthus divaricatus from the saltmarsh to the wind-swept spur at nearly 600 feet high. It seems able to bear up with the altered conditions and to flower and fruit prolifically, and can maintain its position as a member of the plant association there. There are many plants on the spur, as I counted fifteen specimens within sight at one place. It is not a question of a few stray plants, but a quantity of them, and, personally, I cannot understand their getting established there except as members of an upraised saltmarsh.