Art. 37.—On the Behaviour of certain New Zealand Arboreal Plants when gradually buried by River-shingle.

By C. E. ChristensenHead Masseur, Tourist Department, Rotorua.

Sphagnum-moss Plants.

The plants inhabiting the sphagnum-moss bogs have to grow faster than the moss or else be smothered: a slow-growing plant would very soon be covered and killed.

Nothofagus cliffortioides (Hook, f.) Oerst. can withstand a fair amount of burial, some of the trees that were examined being covered to the tips of the lower branches and still looking quite healthy.

Several plants of Senecio Bidwillii Hook, f. under observation were unable to grow as rapidly as the sphagnum moss, and in course of time were completely buried and killed.

Leptospermum scoparium, Dracophyllum uniflora, Discaria toumatou, Olearia cymbifolia, Gaultheria rupestris, Veronica buxifolia, Veronica glaucophylla, V. salicornioides, and numerous other shrubs and plants inhabiting the moss-bog manage to live and thrive through possessing the power of either putting forth adventitious roots or of being able to grow rapidly enough to keep well above the surface of the moss.

Shingle-buried Plants.

The shingle being moist and well aerated enables certain of the buried plants to put forth adventitious lateral roots, while others not possessing this property eventually die.

Numerous observations taken show that the moisture reaches to within 4 in. of the surface, and that there is a difference on sunny days of 20° between the surface-temperature and that of 6 in. below. For instance, on the 14th March, 1920, at 3 p.m. the thermometer registered 95° F. on the surface of the shingle and 75° F. 6 in. below.

Although the plant may be buried to a depth of 6 ft., the root-system proper does not seem to be at all injured. There are many examples of plants after a burial of some years having had the shingle removed and the adventitious roots withered, still flourishing as if nothing unusual had happened.

In very few instances have Nothofagus cliffortioides managed to survive the burial. Plate 51, .fig. 1, shows one healthy specimen that still thrives in spite of having its trunk and lower branches deeply buried. Plate 51, fig. 2, shows one that did not survive. One dead tree had stones lodged in its branches some 8 ft. above the present level of the shingle.

Leptospermum scoparium and L. ericoides, where growing in the path of rapidly-moving shingle, are occasionally so badly injured that they die. (See Plate 52, fig. 3.) The action of the shingle is sometimes so severe that all the bark is worn off the trunk.

The following are a few examples of adventitious rooting:—

Rubus australis Forst. f.—The vine of this plant had only recently been uncovered, and during its period of burial it had developed many adventitious roots and fibrous rootlets, some of which were 12 ft. in length. An inspection of this plant eighteen months afterwards showed the uncovered roots to be withered, and to have practically disappeared.

Fuschia excorticata Linn, f.—The branches of this tree had been partially buried, and had put forth numerous roots from 1 ft. to 3 ft. in length.

Griselinia littoralis Raoul.—This tree was growing at the edge of some forest, and years previous to this observation had been buried deeply in shingle. Owing to a flood the shingle had been washed away and the trunk exposed. At a height of 4 ft. 6 in. above the present ground-level

adventitious lateral roots had been put forth; some of these were 12 in. long and ¾ in. diameter. From 1 ft. to 3 ft. above the surface other roots were growing, their extremities deeply buried in the shingle. These latter roots were very stout, measuring up to 3 in. in diameter. (See Plate 52, figs. 1, 2.)

Leptospermum ericoides A. Rich.—Height about 12 ft.; diameter 4 in. Trunk uncovered for 2 ft., showing long fibrous adventitious roots up to 9 ft. in length.

L. ericoides A. Rich.—Height approximately 10 ft.; diameter of trunk up to 4 in. Stem had been buried to a depth of 6ft., and put forth numerous adventitious roots and fibrous rootlets. Branches that had been buried had also developed roots. (See Plate 53, fig. 1.)

L. ericoides A. Rich.—Height approximately 14 ft.; diameter 6 in. Trunk had been buried to a depth of 2 ft. above present level of the shingle, but now exposed and showing a mass of adventitious roots and fibrous rootlets. (See Plate 53, fig. 2.)

Coprosma propinqua A. Cunn.—This shrub can withstand much burial and damage to bark by the moving shingle, and behaves in a similar manner to Leptospermum.

Remarks.

The comparison made between the burial of certain plants by sand, moss, or shingle shows the close connection referred to in the commencement of this paper. The conditions necessary for the growth of adventitious roots are present in all three environments, distinct as they are from one another. Moisture and aeration are plentifully supplied, and, as shown, some plants that have adapted themselves to one environment are equally at home in the others. Advantage could be taken of the special properties possessed by these plants in using them to reclaim shingly wastes.

List of Species mentioned in these Notes.

• Cassinia fulvida.

• Coprosma acerosa.

• Coprosma propinqua.

• Discaria toumatou.

• Dracophyllum uniflora.

• Fuchsia excorticata.

• Gaultheria rupestris.

• Leptospermum scoparium.

• Leptospermum ericoides.

• Olearia cymbifolia.

• Pimelea arenaria.

• Nothofagus cliffortioides.

• Senecio Bidwillii.

• Sambucus nigra.

• Salix alba.

• Rubus australis.

• Griselinia littoralis.

• Veronica buxifolis.

• Veronica glaucophylla.

• Veronica salicornioides.