Methods

1.Estimation of rainfall

A few official rain-gauges have contributed to the records presented on Map I, but most of the data are from improvised apparatus (Fig. 1) read at intervals of 4–6 weeks, either over the period September, 1952 to August, 1953, or April, 1953 to March, 1954. A spirit level was used to adjust each funnel in the field, and the exact area of its mouth was measured with a planimeter. In the beginning each can was primed with that quantity of water which it retained when inverted with the funnel removed. The reliability of these instruments was tested in two ways.

(a) Gauges were maintained alongside official ones at Lake Mahinerangi and at Musselburgh. The annual total recorded by the improvised gauge at Lake Mahinerangi was 31.54ins, while the official record was 31.18ins. At Musselburgh, the mean of two improvised gauges was 29.40ins and the official record 31.62ins. This larger discrepancy was undoubtedly due to the cans being placed on the surface of the lawn where they were subject to heating by the sun. At Lake Mahinerangi and in the field, cans were set in the ground and a cairn built about the base of the funnel. This protected them from heating and from freezing.

(b) In four localities two gauges were sited close together. The differences over the year were .01, .01, .65, and 1.30ins. Such differences are unimportant.

Rainfall figures entered upon Map I are suggested annual means Over the northern hills, where the records were taken between September, 1952, and August, 1953, they are the actual measurements made since all official stations in the district had recorded a rainfall over that period very close to their mean for the last ten years. But the corresponding means for Lake Mahinerangi and Taieri Airport, the two official stations nearest to Maungatua, were 1.2 times

the totals they recorded from April, 1953, to March, 1954, while the measurements upon Maungatua were being made. Maungatua readings have, therefore, all been multiplied by this factor before, being mapped.

Estimation of Evaporation.

Evaporation figures enclosed in square brackets were obtained from instruments of the pattern used by Barker (1953) and are at each station the average from a group of three or four since preliminary tests revealed considerable, somewhat inconsistent differences in their water consumption under uniform conditions. Measurements in this series are not directly comparable with those of the second series, enclosed in round brackets, which are for a summer period only (20/11/53—10/3/54) and are from cylindrical porous porcelam atmometers (Braun Blanquet 1932, p. 139) used singly and always placed in an entirely unprotected position. They were tested before and after service in the field and proved to give entirely consistent readings to which only small correction factors needed to be applied to make records from each instrument comparable with the rest. The porcelam cylinders were obtained from water-filter candles, and were 4.5 inches long and 1.9 inches in diameter. They were kept free of algae by occasional wiping with a cloth soaked in dilute mercuric chloride solution.

There is an open-pan evaporimeter at Lake Mahmerangi, and its records have given Precipitation/Evaporation ratios that fall below unity only from September until March The atmometers have thus been operated over most of the period in which evaporation is likely to be ecologically significant.

Temperature measurements

The thermometers used were mainly high-grade Zeal Horizontal Pattern minimum thermometers 0–120° F. which agreed closely in their readings and were used to calibrate the U-pattern maximum and minimum thermometers. They were placed either on the ground or on the south side of a cross-bar fastened to an angle-iron standard 3ft 6in above ground level. Compared with the thermometers at Musselburgh, mounted according to standard meteorological practice, they gave grass minima that were usually 1°–2° F. higher and air minima usually 1–3° F. lower than the official record, these differences depending, no doubt, upon the degree to which the bulbs were protected by their mountings.

Identification of log remains

Initially, representative log specimens were compared microscopically with known woods, papers by Garratt (1924), Phillips (1941) and Orman and Reid (1946) were studied, and the opinion of the Forestry Research Institute at Rotorua was sought on a series of samples It proved, with practice, easy to identify the four principal types in the field—i e, Nothofagus menziesii; Podocarpus hallii and Podocarpus totara; Dacrydium biforme and D. bidwilln; and Libocedrus bidwillii. But it was impracticable to separate the species in Podocarpus and Dacrydium since even microscopically this separation is difficult. Mr. H. R. Orman, of the Forest Research Institute, considered that two specimens which he studied closely were both P. hallii. D. bidwillii is only a shrub, so most Dacrydium log remains are probably D. biforme.