Location

The study was carried out in the southern Rimutaka Range forests some 17 km S.E. of Wellington. Two study areas were selected along the middle stretch of the Orongorongo River where it runs N.E. to S.W. between steep ridges rising to 941 m in the east and 706 m in the west (Fig. 1).

The main study area (area A) also used by Crawley (1973) was on the east bank of the Orongorongo River between two tributaries - Brown's and Green's streams. Area A at 100–160 m a.s.l. extended over approx. 14 ha. of lowland podocarp-broadleaf forest mostly on gently sloping fan material and along the base of steeper slopes behind. The second study area (area B) covering approx. 12 ha. was in a broadly similar situation 3 km downstream of area A near Wootton's Stream at 70–120 m a.s.l. (Fig. 1).

Weather

Annual rainfall over 19 years in area A averaged 2420 mm, ranging from 1363 mm to 3662 mm. Rainfall is fairly evenly distributed over the year but with a maximum in winter and a minimum in summer. The mean winter and summer temperatures over 1969-71 averaged 6.7°C and 15.5°C respectively. The mean winter minimum was 2.8°C, frosts being unusual and generally light. The maximum summer temperature over 1969-71 was 32.1°C with a mean daily maximum of 19.4°C. (D.J. Campbell pers. comm.; Ecology Division, D.S.I.R. 1980).

Continuous weather records for 1966-75 were not available from the study areas so a measure of local weather trends over this period was obtained from rainfall and temperature data at Kelburn, Wellington (17.5 km N.W., 126 m a.s.l.) and on rainfall data from the Orongorongo Weir (7.5 km upstream of area A at ca. 240 m a.s.l.). Annual and seasonal weather trends are considered in the Discussion section of this paper (Tables 10 and 11).

Vegetation

The vegetation of area A outlined by Crawley (1973) is described more fully by Fitzgerald (1976). Comparative data from area B is based on a survey in January 1977 by A.E. Fitzgerald and J.A. Foggo, supplemented by personal observations (over 1970-75) and by information in unpublished Ecology Division reports (Waddington et al. 1980; R.E. Brockie, P. Cowan, page 89

Fig. 1 Location map of the two study areas situated within the DSIR forest research area in the Rimutaka range, east of Wellington at the south end of the North Island.

A.E. Fitzgerald, pers. comm.). Vegetation surveys in each area were based on sample plots within the main possum-trapping area (4.4 ha); in area A Fitzgerald (1976) sampled 13.5% of this trapping area, but in area B only 3.3% of the trapping area was covered.

In area A the vegetation comprised lowland podocarp-broadleaf forest on the lower slopes and beech Nothofagus spp. on the higher, steeper slopes and on ridges. On the central 4.4 ha trapping area the northern rata Meterosideros robusta and rimu Dacrydium cupressinum are dominant emergent species (<30 m tall); some miro Podocarpus ferrugineus and matai P. spicatus exceed 20 m. The canopy, between 6 m and 20 m, comprises principally hinau Elaeocarpus dentatus, pukatea Laurelia novaezelandiae, mahoe Melicytus ramiflorus, pigeonwood Hedycarya arborea, rewarewa Knightia excelsa, kamahi Weinmannia racemosa, pate Schefflera digitata, horopito Pseudowintera axillaris, and tree ferns (Cyathea cunninghami, C. dealbata, C. medullaris, C. smithii and Dicksonia squarrosa). The multi-layered subcanopy comprises regenerating trees of most of the canopy species together with Coprosma spp., putaputaweta Carpodetus serratus, hangehange Geniostoma ligustrifolium, kawakawa Macropiper excelsum, mapau Myrsine australis and heketara Olearia rani. Epiphytes are abundant, especially kowharawhara Astelia solandri, kokaha Collospermum hastatum, and puka Griselinia lucida, while lianes such as supplejack Ripogonum scandens, ratas (Meterosideros diffusa, M. fulgens and M. perforata), and kiekie Freycinetia banksii are also common. About 60% of the forest floor is open, the rest covered mostly with ferns, seedlings and kiekie (Fitzgerald 1976).

The vegetation in area B was also lowland podocarp-broadleaf forest, but there were differences in the structure and composition of vegetation between the two areas. On the central 4.4 ha trapping grid in area B the heights of emergent trees and the forest canopy were lower, no trees >20 m tall being recorded there compared with 7 species in area A (A.E. Fitzgerald, P. Cowan, R.E. Brockie, pers. comm.). Of larger trees (<10 cm d.b.h.) only pukatea and rewarewa occurred at heights >15 m. Totara Podocarpus totara, mahoe, pigeonwood and hinau comprised 93% of the vegetation 10–15 m high, while mahoe, pigeonwood and kawakawa represented 79% of trees in the 5–10 m height class, other species including kaikomaka Pennantia corymbosa, kanuka Leptospermum ericoides, tawa Beilschmedia tawa, putaputaweta, totara, matai and miro. Kawakawa, pigeonwood and mahoe comprised the 2–5 m height class.

Epiphytes and lianes were conspicuous throughout the area B survey area and included puka and kokaha as epiphytes and Meterosideros spp., supplejack, kaihua Parsonsia heterophylla and kohia Passiflora tetranda as lianes. About 50% of the ground was vegetated comprising 30% seedlings, 51% ferns, 14% grasses, 3% nettle Urtica ferox and 2% mosses.

Northern rata occurred in area B until 1970 when the last tree died, presumably as a result of possum browsing (Meads 1976 and pers. comm.). Horopito and kamahi were also present in area A but not in area B, while hinau, heketara, mapau, pate, lemonwood Pittosporum eugenioides, nikau Rhopalostylis sapida, karamu (Coprosma lucida and C. robusta) were more scarce in area B. Species present in area B or more common there included totara, kanuka and nettle. Amongst lianes supplejack was more widespread in area A, while kaihua and kohia were only recorded in area B (A.E. Fitzgerald, P. Cowan, R.E. Brockie, pers. comm.; pers. obs.).

There was relatively more grazing and browsing by sheep and cattle in area B compared with area A, especially in 1970-71 when extensive trampling of vegetation occurred.