Tuatara: Volume 31, Issue 1, July 1991
Indirect Wind Effects on Crown Height of Miro (Prumnopitys Ferruginea)
Indirect Wind Effects on Crown Height of Miro (Prumnopitys Ferruginea)*
The vertical structure of miro/kamahi forest on the upper slopes of Mt Maunganui near Wellington is described. The heights reached by emergent miro crowns were determined by the height of the kamahi canopy, which is in turn determined by exposure to wind.
Key words: Ecology, miro crowns, wind, Prumnopitys ferruginea, Weinmannia racemosa.
Miro occurs as a lowland to montane tree in many forests in New Zealand, except in eastern and inland South Island areas of low effective rainfall. It rarely grows above 900 m altitude in the North Island and northern South Island, and grows up to 30 m tall as an emergent in broadleaf forests or as a subdominant in dense podocarp forests (Hinds and Reid, 1957).
The structure of the forest canopy was examined as part of a study of forest dynamics of miro on Mt Maunganui near Wellington. An explanation for the canopy structure and height is presented.
The Study Area
Mt Maunganui, 708 m, is a high point on the western ridge of the Akatarawa upland north of Wellington (Fig. 1). Access is via a foot track up a spur from the Maungakotukutuku Valley.
At the altitudes studied (450-580 m) the vegetation is emergent miro over kamahi (Weinmannia racemosa) forest with hinau (Elaeocarpus dentatus) and occasional Hall's totara (Podocarpus hallii) also in the canopy.
Basement rock is Triassic-Jurassic greywacke overlain by colluvium and loess (Suggate, 1978). Soils are classified as moderately clay illuvial, steepland Central Yellow Brown Earths, Makara Soils derived from greywacke, developed on steep slopes under mull-forming broadleaved-podocarp forest, and are moderately leached (N.Z. Soil Bureau, 1968). Soils in the study area are 12-40cm deep. Vegetation and soils are described in more detail by Hyland (1987).
The study area is fully exposed to prevailing northerly quarter winds. Annual rainfall is estimated to be at least 2500 mm. Winter snowfall is rare, light, and does not lie for more than two days.
Field work was carried out early in 1987. On the spur followed by the track up Mt Maunganui, pairs of temporary 20 × 5 m quadrats were established at three altitudes. They were placed along the contour on each side of the spur top at 450-470 m, 520 m and 580 m. Quadrats numbered 1, 2 and 3 were up the northeastern side, 4, 5 and 6 were up the southwestern side of the spur (Fig. 2).
Since the tallest trees were under 15 m, data for point height intercept (PHI) analysis were obtained from 100 points in each quadrat, along 4 lines (I m apart) of 25 points (0.8 m apart) each, following the methods of Park, 1973. Field data were analysed on an IBM 4341 computer at Victoria University using a PHI programme (Daget, Frost and McQueen, 1981). From the resulting printout data, graphs were drawn of the canopy height outline (profile) and vertical structure in each quadrat.
Results And Discussion
Canopy profiles drawn from top PHI intercepts of the six quadrats are shown in Fig. 3. All except quadrat 3 show a completely closed canopy. In this quadrat there was an open area around a broken-off miro stem 2.5 m high and a fallen kamahi.
Highest points on the profiles mark the top of the miro. In quadrats 2, 5 and 7 the lowest canopy point is in the horopito (Pseudowintera axillaris) — hupirau (Coprosma foetidissima) understory layer at 2-4 m. This also applies to quadrat 3 apart from the open area noted above. In quadrats 1 and 4 the understorey layer at 2-4 m is not as widely developed as in the other, higher altitude, quadrats. Most of the broadleaf canopy between about 5 and 9 m is made up of kamahi and hinau, with pokaka (Elaeocarpus hookerianus) replacing hinau in wetter parts of quadrat 6.
This layering by species in the understorey and canopy is clearly illustrated in the vertical profiles. Figs 4 and 5 show the vertical structure as profiles drawn of frequency of PHI foliar intercepts of the vegetation in each quadrat. Height intervals were 10 cm for the first 2 m, and 1 m thereafter. This made species separation below 2 m very difficult on the graphs, and for legibility species are separated only above 2 m on Figs 4 (northeastern quadrats) and 5 (northwestern quadrats).
In each quadrat miro crowns reach their fullest extent only in the upper part of and above the broadleaf canopy (Fig. 6). Beveridge (1974) has recorded that young miro develop a multi-stemmed bushy habit when grown in the open. In forest, miro trees have straight single stems with heavily branched crowns forming in full sunlight. Miro on Mt Maunganui reach barely half the maximum height of the species, although growing well below its maximum altitude. It therefore follows that the height of the broadleaf canopy determines the height at which miro forms crowns.
There are other observations of the formation of spreading crowns by emergent podocarps once above the broadleaf vegetation. In the Orongorongo Valley Bell (1981) recorded miro and matai (Prumnopitys taxifolia) crowns exceeding 20 m above a broadleaf canopy at 6-20 m. In the eastern part of the Akatarawa upland Weeber (1983) recorded miro of 20 m and more above kamahi 12-18 m tall. Lloyd (1960) recorded that when rimu (Dacrydium cupressinum) goes through an overhead cover, height growth slows and the uppermost branches develop into the broadly rounded and multiple-headed crown typical of emnergent rimu.
Kamahi generally grows to 25 m or more at altitudes higher than the study area (Allan, 1961). However it rarely reaches more than 9 m in the study area, which is attributed to wind. Hyland, MacDonald, Weaver and Oates (1987) have shown that kamahi growth is considerably reduced by exposure to strong winds. On Mt Maunganui the tops of kamahi crowns frequently have a windshorn appearance. Miro crowns are not windshorn, but mature trees are susceptible to uprooting, and sometimes stem-snapping 2-3 m above the ground.
The upper slopes of the western ridge of the Akatarawa upland, including Mt Maunganui, receive the full force of northerly winds coming across the sea. At Paraparaumu airport, at 7 m on the coast, 55% of surface winds per year are from the northerly quarter (N.Z. Meteorological Service). It seems likely that the study area would receive at least that amount of northerly winds, and that they would be stronger than at sea level where the coast may be partly sheltered by Kapiti Island. Winds up to storm force from the north or northwest usually occur before the passage of a cold front across the country.
A similarly exposed situation occurs above 450 m on the Kaitake Ranges, northwest of Mt Taranaki (Mt Egmont). Here Clarkson (1986) has recorded low miro emergents over a low, often windshorn, kamahi canopy.page 19
It is apparent that the height of miro emergents in broadleaf-podocarp forest is determined by the height of the broadleaf canopy. On Mt Maunganui the kamahi canopy height is determined by exposure to wind, so that the wind has an indirect effect on miro height.
This work was part of a B.Sc. Honours project carried out while the author held a University Grants Committee Senior Scholarship at Victoria University in 1987. Thanks are due to Ross McQueen, for supervision and advice, Bronwyn Hyland, for assistance in the field, Geoff Rogers, for computing assistance, Paul Baker, Forester, Wellington Regional Council, for permission to use the study area and the Wellington Botanical Society for assistance with travel costs for field work.
Allan. H.H. 1961. Flora of New Zealand. Vol. I. Government Printer, Wellington.
Bell. B.D. 1989. Breeding and condition of possums Trichosurus vulpecula in the Orongorongo Valley, near Wellington, New Zealand, 1966-1975. In B.D. Bell (Ed) Proceedings of the first symposium on marsupials in New Zealand. Zoological Publications fromn Victoria University of Wellington 74: 87-139.
Beverdge, A.E. 1974. Miro and matai. New Zealand's Nature Heritage 2(18): 494-498.
Clarkson, B.D. 1986. Vegetation of Egmont National Park, New Zealand. National Parks Scientific Series No. 5. Science Information Publishing Centre. DSIR, Wellington.
Daget, Ph., Frost, A. and McQueen, D.R. 1981. Point Height Intercept (PHI) programme. Unpublished. Victoria University of Wellington.
Hinds. H.V. and Reid, J.S. 1957. Forest Trees and Timbers of New Zealand. New Zealand Forest Service Bulletin No. 12. N.Z. Forest Service, Wellington.
Hyland, F. 1987. Forest dynamics of miro (Prumnopitys ferruginea (D.Don.) de Laubenfels) on Mt Maunganui, South West Tararuas. Unpublished B.Sc. Honours project, Victoria University of Wellington.
Hyland, F.M., MacDonald, I., Weaver, S.A. and Oates, M.R. 1987. Secondary succession in Windy Gully, Mt Rimutaka. Tuatara 29: 34-43.
Lloyd, R.C. 1960. Growth study of regenerated kauri and podocarps in Russell Forest. New Zealand Journal of Forestry 8: 355-361.
N.Z. Metereological Service. Various years. Meteorological Observations. N.Z. Metereological Service Miscellaneous Publications 109. Ministry of Transport, Wellington.
N.Z. Soil Bureau. 1968. Soils of New Zealand. Part I. N.Z Soil Bureau Bulletin 26(1).
Park, G.N. 1973. Point height intercept analysis. New Zealand Journal of Botany 11: 103-114.
Suggate, R.P. (Ed.) 1978. The Geology of New Zealand. Vol. 2. Government Printer, Wellington.
Weeber, Y.B. 1983. The ecology of isolated Nothofagus fusca trees in podocarp/broadleaf forest in the Akatarawa River West. Unpublished B.Sc. Honours project, Victoria University of Wellington.page 20
* Present address: MAF Technology, Horticultural Research Centre, Private Bag, Levin.