Art. XLVII.—Notes on the Growth of certain Native Trees in the Auckland Domain.
[Read before the Auckland Institute, 9th October, 1905.]
The rate of growth and age of our native trees have often formed a subject of inquiry, and observers have from time to time recorded particulars, principally relating to increase in height and girth of the trees mostly used for building purposes and in the industrial arts of the colony. In vol. xx of the “Transactions of the New Zealand Institute” appears the last of three articles by the late Mr. James Baber, who took much interest in the subject, and, from his early residence in the colony, and some practice in aboriculture, was well qualified to speak on it.
In Mr. Baber's notes alluded to, written in 1887, the height and girth of three kauris, planted in the Auckland Domain in 1865, are given, the average girth at 2 ft. from the ground being 2 ft. 2 in., and average height 25 ft. This represents a diameter of 8 ¼ in. only, and, as the trees had been established twenty-two years, it shows an average increase of ⅜ in. in diameter and about 1 ft. in height per annum, making allowance for dimensions at the time of planting. It is to be regretted that the trees so measured by Mr. Baber in 1887 were not in any way fixed for future identification. It is therefore impossible to trace
the increase of growth from that time in other than a general way.
As forty years have now elapsed since these trees were planted by Mr. Chalmers in 1865, it was thought to be a good time for further inspection, and I undertook, at the request of the Council, to measure and locate for future reference the various trees in the plantation. This work, on examination, proved to be one of more magnitude than was anticipated. The plantation extends in a nearly north and south direction almost an eighth of a mile, and occupies a width in the valley of 50ft. to 60ft., excepting at the southern end, where the trees are more scattered. A dense undergrowth (since cleared off) rendered the work of survey all the more troublesome. In all, the positions of ninety-six pines and two rewarewas were fixed by systematic survey, and their girths measured at 2ft. above ground. The heights of fifteen of these were accurately measured, all as existing on the 1st June, 1905.
The situation is in a comparatively sheltered valley in which is a small but permanent stream. The surface soil is dark and heavy, and is partly alluvium from the volcanic basins forming the upper levels of the domain. The subsoil is a hard yellow clay. The conditions, on the whole, are very similar to the more fertile forests, such as Waitakerei, where kauri and mixed bush are found in perfection. The trees, without exception, present a healthy appearance, and have been for forty years in the normal conditions of the “bush.”
On the plan herewith is recorded the position and species of each of these trees, and their girths in feet and inches. A series of totara pegs, numbered 1 to 9, are placed, from one or other of which the distance in feet and direction of every tree is shown.
The magnetic bearings and distances of the traverse pegs are given, and the whole of the observations have been plotted on strong mounted paper, enclosed in a tin case, and deposited in the Auckland Museum. This record should, in the future, and from time to time, enable any of these trees to be identified, their dimensions compared, and rate of growth determined.
The trees shown on the plan consist of—totara, 12; kauri, 18; rimu, 36; miro, 8; tanekaha, 15; kahikatea, 7; and rewarewa, 2. The only pine not represented is matai. The heights of the fifteen, as mesured, average—kauri, 41ft. 8in., girth 3ft. 1in.; rimu, 44ft., girth 2ft. 6in.; totara, 40ft. 8in., girth 3ft. 1in.; miro (one only), 32ft. 10in., girth 2ft. 4in.; kahikatea (one only), 43ft. 3in., girth 2ft. 11in.; rewarewa, 43ft. 2in., girth 3ft.
It will be observed that, allowing for the probable size when planted, the average rate of growth for the first forty years in
this location may be taken at about 1 ft. in height and 5/16in. in diameter per annum. The fifteen trees whose heights were measured are indicated on the plan by arrows, and the heights figured in feet and inches.
Some doubt has at times been expressed as to whether the rings or increments of growth as shown by the cross-sections of kauri and other New Zealand pines indicated annual growths, as they admittedly do in the deciduous trees and pines of other countries. It was deemed to be quite probable that two increments might be formed annually, especially in the case of kauri, in the subtropical north of Auckland. All analogical reasoning pointed to one very distinct increment of growth per annum, but something more conclusive was desirable. As the time of growth of the trees in question was definitely known, it was seen that an opportunity was here presented, probably unique, to determine the point. With this view, and in the objects of science, application was made last year to the then Mayor, the Hon. E. Mitchelson, for permission to cut one kauri, to be selected by Mr. W. Goldie, Superintendent of Parks. This was readily granted, and, when the time came, confirmed by His Worship A. M. Myers, Esq., the present Mayor. In selecting the tree to be cut, Mr. Goldie drew attention to the circumstance that at one place the trees were too much crowded, and suggested that one of them, a rimu, should also be cut down. The consent of His Worship the Mayor having been formally obtained, the two trees were cut on the 4th September last, and cross-sections obtained of each. The kauri was 3ft., and the rimu 3ft. 9in., in girth.
The sections of the kauri show much irregularity, not only in the thickness of rings, but in the size at different places in the same ring. In some places there are eight rings to ½in., in others three to 1in. On account of this irregularity and constriction, it is a matter of some difficulty to count the rings. The growth of the kauri has been exceedingly slow during the first four or five years, but there is no doubt that the section shows fortytwo rings at 1ft. from the ground, and from the eccentricity of the pith these are contained in 3 ⅜in. on one side and in 8 ¼in. on the other. The increase of height being roughly 1ft. per annum, one ring less will be found for every foot in height at which the section is cut.
The rimu section is much more regular and concentric. The tree had taken twelve years to reach a diameter of 2in. This was succeeded by a rapid rate of increase during the next ten years, amounting to ½in. diameter per annum. During the last twenty years the rate has been less but very regular. There are in all forty-two rings, and with the exception of the first
twelve they are very distinct, and the annual growth unmistakable. In several of the rings, especially in the ten larger, there are quite evident traces of subgrowths, two, three, or even four in one year.
Plate L shows the sections of both trees. The irregularity of growth and the particulars as above noted can be easily traced. The extreme constrictions of the inner 2in. of the rimu may in part be due to compression, but the transition to a rapid rate has been very sudden. An aged rimu commonly shows a pole of hard and resinous wood, about 4in. or 5in. in diameter, which parts easily from the main body of the trunk. In these cores it is very difficult to distinguish the rings.
The rate of growth having, it is believed, been thus demonstrated, attention was turned to the cross-section of kauri, 8ft. in diameter, presented to the Museum about six years ago by Messrs. Leyland and O'Brien. A sector of it was dressed and oiled, and the rings counted as accurately as possible. But the task is a difficult one. Within a small limit of error, however, there can be counted 455 annual growths, the rate varying from twenty-five per inch to seven or less. This tree had been cut in its prime, as evidenced by the extent and dimensions of healthy sapwood, which averages 6 ½in. all round, or 25 ¼ per cent. of the area of section. This noble kauri, then, must have been a vigorous spling on the slopes of Mangawai before the Wars of the Roses. And from the same data we must conclude that there are kauri-trees still in good condition whose ages date from the early centuries of our era.
Plate LI shows totara and a small rimu and kauri, and is from a photography by Mr. Joseph Martin.