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Palynology of Pohehe Swamp, northwestWairarapa, New Zealand: a study ofclimatic and vegetation changes duringthe last 41,000 yearsW.L. McLea aa Research School of Earth Sciences , Victoria University ofWellington , Box 600, Wellington , New ZealandPublished online: 12 Jan 2012.

To cite this article: W.L. McLea (1990) Palynology of Pohehe Swamp, northwest Wairarapa, NewZealand: a study of climatic and vegetation changes during the last 41,000 years, Journal of the RoyalSociety of New Zealand, 20:2, 205-220, DOI: 10.1080/03036758.1990.10426725

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© Journal of the Royal Society of New Zealand, Volume 20, Number 2, June 1990, pp 205-220

Palynology of Pohehe Swamp, northwest Wairarapa, New Zealand: a study of climatic and vegetation changes during the last 41,000 years

w.L. McLea*

In Pohehe Swamp, adjacent to the eastern edge of the Northern Tararua Range, sediments up to 4.5 m thick yielded abundant palynomorphs. They were present through most of the sequence but fewer at depths between 1.2 m and 1.6 m. Dating controls were provided by Aokautere Ash at 22.6 ±.0.23 ka BP, 1.9 m below the surface; a radiocarbon date of;:::. 36 ka BP on wood, 3.7 m below the surface; and the superposition of the swamp deposits on Eketahuna Gravels (c. 70 ka BP). The palynological sequence indicates four distinct floras in chronological sequence. (I) Between 41 ka and 24 ka BP, a regional Nothofagus menziesii (silver beech) forest with grassland in exposed areas, an estimated timber line at c. 500 m and a cool climate; (2) between 24 ka and 13 ka BP, grassland with shrubs in a cold climate; (3) between 13 ka and 10 ka BP a podocarp forest with a variable distribution of grass and shrubs during fairly rapid warming of climate; (4) Between 10 ka and 5 ka BP a well developed podocarp broad leaf forest growing in the climatic maximum. Nothofagus menziesii, now absent in the beech gap extending from the northern Tararua Range to the central Ruahine Range, started to decline in quantity at c. 24 ka BP and disappeared from the region around Pohehe swamp by c. 19 ka BP.

Keywords: Aokautere Ash, Holocene, Last Glaciation, grassland, Nothofagus menziesii (silver beech), podocmp broadleafforest, palynology, Tararua Range

INTRODUCTION Palynology has been used to deduce the Late Quaternary vegetation and climate of several

districts in the North Island of New Zealand (McIntyre, 1970; McGlone and Topping, 1977; McGlone et at., 1978; McQueen and Mildenhall, 1983; McGlone et at., 1984; McGlone, 1985b; Lees, 1986; McGlone et at., 1988; Bussell, 1988). This paper uses data from Pohehe Swamp to extend the coverage to include the northwest Wairarapa area for most of the last 41 ka BP.

Zotov et at. (1938) described the ecology and flora of the Tararua Range, and Franklin (1967) described the synecology of the indigenous forests and mapped the forest types of the Tararua Range; he mapped the pre-European forest as rimu / kamahi for flat land and highland softwoods / hardwoods for the steep land around the swamp. Most of the modem forest in the district near Pohehe swamp was cleared for pasture farming early in the century. The timber trees were Dacrydium cupressinum (rimu) and other podocarps with some Beilschmiedia tawa (tawa) (J.G. Hawkins, pers comm., 1986. Mr Hawkins, of 30 Kirton Street Masterton, formerly farmed in the area).

Franklin (1967) recorded that all Nothofagus taxa (southern beeches) are absent from the northern Tararua Range. They are also absent from the southern end of the Ruahine Range for a distance of about 100 km north from Pohehe Swamp (Elder, 1965). Franklin surmised that the beech gap formed at the end of the Last Glaciation. Evidence herein shows that the gap started to form at about 24 ka BP and was established by 19 ka BP, that is, before the

* Research School of Earth Sciences, Victoria University of Wellington, Box 600, Wellington, New Zealand.

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206 Journal of the Royal Society of New Zealand, Volume 20,1990

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Fig. I - Maps showing the location of the study area, New Zealand localities mentioned in the text and details of the study area. A-F are the sample sites.

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McLea - Palynology of Pohehe Swamp 207

maximum of the Last Stadial of the Last Glaciation. Pohehe Swamp, adjacent to the Tararua Range near Nireaha, northwestern Wairarapa, lies

at an altitude of c. 280 m, has an area of c. 1.6 ha (800 m x 200 m) and is bounded to the east by a ridge separating it from the Mangatainoka River and to the west by the lower slopes of the northern Tararua Range (Fig. I). Most of the drainage is to the north by the Mangaraupiu Stream which joins the Mangatainoka River 6 km to the north (Fig. I). One hundred metres to the south of the swamp, drainage is south and directly into the Mangatainoka River.

The swamp sediments are up to 4.5 m thick at site D, and 4. 3 m thick at site A (Fig. 1). They rest on gravels interpreted to be the top of the Eketahuna Terrace (Neef, 1984; Vella et al., 1988). At an average depth of 1.9 m they contain a more or less continuous layer, 0.1 m thick, of the rhyolitic tephra widely mapped in the Manawatu, Rangitikei and Northern Wairarapa districts as Aokautere Ash (Cowie, 1964; Milne, 1973; Kaewyana, 1980; Vella et af., 1988). Aokautere Ash is a member of the Oruanui Formation with a radiocarbon age of 22.6 ±.0.23 ka BP (Wilson et at., 1988). It is often referred to as Kawakawa Tephra Formation (Vucetich and Howorth, 1976). Aokautere Ash is a valuable stratigraphic marker bed, and I used it as the primary datum surface for estimating the ages of other layers in the swamp sediments.

New Zealand Meteorological Service (1973) data give the isohyet for the Pohehe Swamp district as 2,000 mm; for Eketahuna, 8 km to the west as 1,600 mm; for Manawatu Gorge 40 km to the north as 1,400 mm and for Woodville and Pahiatua as 1,000 mm (Fig. 1). If this pattern is typical of the last 40 ka, there would have been ample precipitation to support the forest types now typical of the Tararua Range near Pohehe Swamp (Franklin, 1967). Winds are strong and mainly from the westerly quarter (Tomlinson, 1976) and I assume that a similar pattern prevailed during the period covered by this paper.

METHODS Six sites A, B, C, D, E and F (Fig. I) were examined by spading banks and au gering buried

deposits. Sites A to E are on the swamp, and Site F is at the top of a road cutting on Mangaraupiu Road about one km south of site A. Palynomorph samples were collected from sites A, B, C and D. At Sites A and D, the upper 2 m was exposed in the side of a drain and it was possible to cut out blocks measuring 50 x 50 x 200 mm from the cleaned, open cut. Material from below the 2m mark, and from Band C, was collected by using a Dutch auger. The blocks were carried back to the laboratory in plastic bags. Blocks were usually taken at intervals of about 100 mm, but a closer spacing of 20 or 25 mm was used over barren or sparse intervals.

Of the four sites, A which was in a silty clay, gave an almost continuous record, but with some breaks above the tephra layer. Site B gave a record for the top 0.5 m only. Site C was in sandy clay and gave usable samples for 0.3 m above and below the tephra layer. Site D was closely comparable with Site A. Because the data from sites Band C were of limited value, they have not been presented in this paper, but are discussed in McLea (1988).

Slides for microscopic examination were made following the procedure of Moore and Webb (1978). Each block was trimmed and a subs ample of about 4 g was removed for processing. Care was taken to see that the subsample was taken from inside the block and had not come in contact with contaminants during collection. Subsamples were treated with 10% potassium hydroxide, sieved, washed, heated with 35% hydrofluoric acid for four hours and left overnight (some required an extra four hours to remove most of the silica), washed, acetolysed, washed and then mounted in glycerine jelly. After examination and counting they were deposited in the fossil collection of the Geology Department of Victoria University of Wellington. The NZ Fossil Record File's references are T25/fllO for site A, grid reference NZMS 260 Series T25/318636; T25/flll for site B, grid reference T25/318635; T25/fl12 for site C, grid reference T25/319636 and T25/fl13 for site D, grid reference T25/318636. The blocks taken from the site and from which the slides were made are lodged in the rock collection at Victoria University.

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Fig. 2 - Columns through the swamp sites A, B, C. D and E to show how they correlate with the loess site, F.

Palynomorphs found on each slide were counted using a standard Zeiss RA comparison microscope at a magnification of x400. Pollen grains and spores were identified by comparison with slides in the reference collection held in the Geology Department, VUW and in consultation with palynologists at Geological Survey, Lower Hutt, who gave me access to their reference collection. Counts of palynomorphs on pairs of slides representing thirty eight levels at site A and twenty six at site D were made.

GEOLOGY Stratigraphy

The swamp deposits at sites A to E cannot directly be correlated with Late Quaternary alluvial gravel and loess deposits in the surrounding districts (Kaewyana, 1980; Vella et al., 1988). They are designated informally as the Pohehe Swamp formation, nominal type locality site A (Fig. 1), grid reference, NZMS260 T25/318636.

I recognised three lithostratigraphic divisions within the formation. The top and middle divisions can be distinguished by colour differences (Oyama and Takehara, 1967) and the Aokautere Ash separates the middle and lowest divisions. The three divisions are designated as the lower, middle and upper members of the Pohehe Swamp formation. Although the three members cannot be correlated directly to the alluvial and loessic beds, the Aokautere Ash provides one primary correlation horizon of uniform age throughout the swamp and adjacent loessic beds.

At site A the sequence from the bottom, as shown in Fig. 2 is: Lower member (4.3-2.0 m)

Very dark reddish brown (2.5YR 2/2) silty clay at the base grading upwards to lighter hued greyish red (lOR 6/2) silty clay at the sharp contact with Aokautere Ash. The member rests on a gravel surface inferred to be Eketahuna Gravels. Across the swamp the member thickens to 4.5 m at site C, where the colours are lighter in hue and the texture is sandier. At site E the member laps over Pliocene Atea Sandstone (Neef, 1984). About 20 m west of site E, probing with a steel rod showed that there are gravels at a depth of about 1.5 m below the tephra layer and I infer them to be Eketahuna

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MeLea - Palynology of Pohehe Swamp 209

Gravels. At site D the member is 2.5 m thick and overlies inferred Eketahuna Gravels. Aokautere Ash Member (2.0-1.9 m)

Sharp boundary 0.06 m light grey (lOYR 8/2) pumiceous sand Diffuse boundary 0.01 m light grey (lOYR 7/l) pumiceous sand 0.02 m dull brownish yellow (lOYR 5/3) pumiceous sand Diffuse boundary 0.01 m pale yellow (5Y 8/3) P4miceous sand Sharp boundary I

Middle member (1.9-0.55 m) Silty clay grading vertically into and out of thin lenses of sandier clay. It is coloured brownish grey (7.5YR 6/1) at the contact with the Aokautere Ash at the base and grades to light grey (2.5Y 7/l) at the top. The colour change from the top of the lower member to the middle member is slight. The middle member is much less carbonaceous than the members above and below it. (At site D, the eastern edge of a gravel wedge is intercalated near the top, Fig. 2.)

Upper member (Q.55 m - surface) Very dark reddish brown (2.5YR 2/2 and lOR 4/2) silty clay with a contact that is gradational over about 20 mm with the middle member below. Its top is the ground surface. The upper half contains abundant pieces of wood, probably the roots and other debris of vegetation cleared by the first European farmers of the district.

Analyses of subsamples from nine representative levels at site A (McLea, 1988) showed variations in the amount of organic carbon from <0.05% to 16.6%. The overall pattern was a general decline in the amount of organic carbon from the base of the lower member (13%) to the top of the middle member «0.5%) and then a high level, 13%-16%, in the upper member.

At 3.7 m from the surface (1.7 m below the Aokautere Ash layer) a piece of Phyllocladus wood, well preserved and lying in a horizontal position, was found. Probing with a steel rod suggested that it was at least 0.6 m long and with a diameter of about 0.1 m. A piece cut off with a Jowett auger gave a radiocarbon date of~. 36 ka BP (R No 11,419: NZ No 7,305). Accepting 36 ka BP as a minimum age, the minimum average sedimentation rate to the time of the tephra fall was 0.13 m per ka (1.7 m in 13.4 ka). The age of the sediment 0.6 m deeper and resting on the gravel base is c. 5 ka older, i.e. about 41 ka BP.

Correlation Site F, in loess previously examined by Kaewyana (1980), is at a cutting on Mangaraupiu

Road, one km south of site A (Fig. 1). There the sequence that I determined was, from the surface down (Fig. 2),

0-1.7 m Hukanui loess 1.7-1.9 m Aokautere Ash 1.9-2.6 m Hukanui loess 2.6-3.1 m Pukewhai loess Eketahuna Gravels

The Hukanui loess is light yellow (2.5Y 7/3) clay loam consistent with the western high rainfall facies of Hukanui loess described by Kaewyana (1980). It is thicker (2.4 m) than the modal thickness for the district (1.5 m), but not abnormally so. The Aokautere Ash is at the modal depth in the loess (Milne, 1973; Kaewyana, 1980; Vella et at., 1988). The Pukewhai loess is dull orange (7.5YR 6/4) silty clay and its thickness is modal (Kaewyana, 1980).

Kaewyana reported 2.07 m of Hukanui loess with Aokautere Ash at the modal depth, 0.3 m ofPukewhai loess and 0.8 m of Eketahuna loess, resting on gravels which he considered to represent the Greenhills Terrace which is older than the Eketahuna Terrace. However, he noted the absence of the Makakahi Tephric Palaeosol (Vella et al., 1988) between what he called Pukewhai and Eketahuna loesses. The tephric palaeosol (originally called by Kaewyana,

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214 Journal of the Royal Society of New Zealand, Volume 20,1990

Cliff Tephric Palaeosol) is a thick (usually 0.2 m to 0.3 m) chocolate brown layer, correlated with the ± 0.60 ka BP middle Tongariro tephras (Milne, 1973). It is an easily recognised marker, usually present in the cover bed sequences overlying Greenhills and older terraces in northwestern Wairarapa.

I examined the loess section along the cutting at several different points but could not confirm Kaewyana's observations. I conclude that his identification of Eketahuna loess was a mistake, and that all the loess below Hukanui loess is Pukewhai loess, which means that the gravel below it is Eketahuna Gravels.

PALYNOLOGY Two pollen diagrams (Fig. 3 and 4) were made from the palynomorph counts for sites A

and D. The pollen sum used for each diagram was the total of all taxa counted. (Pollen of Pinus, although sometimes present, was was excluded from the count because it was considered to be a laboratory contaminant; processing was done in July, August and September, the time when Pinus releases its pollen.) Taxa are presented in groups.

Beech Nothofagus menziesii and N.fusca type.

Subalpine Halocarpus bidwillii and biformis type, Phyllocladus.

Podocarp-broadleaf Dacrydium cupressinum, Prumnopitys ferruginea, P. taxifolia, Podocarpus totara,

Elaeocarpus, Metrosideros, Nestegis, Weinmannia, Dacrycarpus dacrydioides, Laurelia novae-zelandiae, Syzygium maire.

Understorey Ascarina, Aristotelia, Cordyline, Griselinia, Neomyrtus, Pittosporum, Plagianthus,

Pseudowintera, Pseudopanax, Schefjlera, Hoheria and others.

Shrubs Coprosma, Pimelea, Coriaria, Epacridaceae, Hebe, Leptospermum, Muehlenbeckia,

Myrsine, Compositae, Urtica and others.

Herbs and grasses Freycinetia, Collospermum, Acaena, Gentiana, Geranium, Haloragis, Umbelliferae,

Gramineae, Astelia, Liliaceae, Wahlenbergia, Dactylanthus, Rubus, Lophomyrtus, Tupeia, Epilobium, Ranunculus, Rumex and others.

The taxa in the groups above were considered to represent the regional pollen rain. I defined the region as the country bounded by the Tararua Range to the west, the headwaters of the Mangatainoka River to the south of Pohehe Swamp, Eketahuna to the east and Pahiatua to the north of Po he he Swamp (Fig. I). The remaining taxa, listed below, were considered to belong to the local area of the swamp itself, or its immediate surroundings.

Swamp Gleichenia, Restionaceae, Gunnera, Chenopodium, Cyperaceae, Potamogeton, Phormium,

Callitriche and other swamp flora.

Ferns Dicksonia squarosa, D. fibrosa, Cyathea smithii, C. dealbata, Lycopodium fluviatile, L.

billardieri, Blechnum, monolete fern, trilete fern, Pteridium and other ferns.

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McLea - Palynology of Pohehe Swamp 215

DISCUSSION

Chronological zonation I recognise four zones, from oldest to youngest 1,2,3, and 4, and define the boundaries by

the changes in the ratio of tree to grass pollen on the diagram for site A, Fig. 3. The relationship between zones, depth, sediment and ages is summarised in Table I.

Zone 1 Zone 1 represents the time from 25 to at least 41 ka BP. It is the lowest 1.9 m of the swamp

sediment and the greater part of the lower Pohehe member. In it two important tree pollens, Nothofagus menziesii (silver beech) and N fusca type together make up 30-40% of the regional pollen through most of the zone. Nothofagus menziesii is typical of wetter montane and subalpine forests, and is recognised as being one of the hardiest New Zealand beeches. In the pollen record it is usually under-represented (Macphail and McQueen, 1983), so that its abundance (20-30%) here suggests that it was the dominant forest tree of the region during Zone 1 time. Conversely, Nfusca type is usually over-represented, and includes four beech taxa, N fusca, N truncata, N solandri var. solandri, and N solandri var. cliffortioides. Present throughout the zone are small but significant quantities of the pollen of Dacrydium cupressinum, Prumnopitys ferruginea, P. taxifolia, Podocarpus totara, Dacrycarpus dacrydioides, Elaeocarpus and Weinmannia. Wardle (1984) recognised a modem forest type with a similar composition in the central and southern Tararua Range, describing it as "mid slope silver beech - red beech forest". The forest of Zone 1 time was probably of the same type.

Pollen of the smaller trees Aristotelia, Griselinia, Pittosporum, Pseudopanax and some Coprosma represent the under-storey. Throughout the zone the pollen of the subalpine trees, Halocarpus biformislbidwillii type, Phyllocladus, the shrubs Hebe, Myrsine, Pimelea, Epacridaceae and Gramineae were found along with herbs including Acaena, Geranium, Haloragis and Umbelliferae. Subalpine herbs and shrubs form an association usually found above the timber line, but also live in exposed places in forest stands.

The proportions of the taxa from depths of 4.00 m to 2.30 m are more or less constant (Fig. 3 and 4) suggesting that climatic fluctuations were minor during the time represented. In the lowest 0.2 m, however, the quantity of tree pollen is low, which may indicate that the climate was cool when pollen first started to collect.

In the Bay of Plenty region during the same time as that of Zone 1 the mean annual temperature is estimated to have been 3°C. lower than at present (McGlone et al. 1984). The

Table 1 - Relationship of zones, depth, sediments, approximate dates, vegetation, timber line, climate and age at Site A.

Zone Depth Sediment Date Vegetation Timber Climate Age (m) (ka BP) line (m)

4 0.01 -0.05 Upper Pohehe 5-10 Conifer 1,000 Warmer than Mid broadleaf present post forest glacial

3 0.6 -1.15 Middle Pohehe 10-13 Conifer 1,000 Cooler than Early broadleaf present post forest glacial

2 l.l5 - 1.9 Middle Pohehe 13 -22.6 Grassland 250 Cold Glacial 1.9-2.0 Aokautere Ash 22.6 2.0-2.2 Lower Pohehe 22.6-24 maximum 2.3 -4.2 Lower Pohehe 24-41 Silver beech 500 Cool Moerangi

forest

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216 J oumal of the Royal Society of New Zealand, Volume 20, 1990

present timber line for the beeches on the Tararua Range is c. 1,000 m (Franklin, 1967). The generally accepted lapse rate in temperature is 1°C per 170 m of altitude gained (Willett (1950). A lowering of 3°C. would therefore bring the timber line in Zone 1 time down to about c. 500 m.

The Zone 1 swamp supported bog vegetation varying in composition from time to time, but including Gleichenia (a bog fern), species of Restionaceae, probably Empodisma and some Cyperaceae. On the edge there was growing a mixture of swamp-tolerant taxa including Coprosma, Leptospermum, Phyllocladus and the podocarp tree Dacrycarpus dacrydioides (kahikatea).

Small amounts of Potamogeton were recorded at most levels of both sites, showing that there was some open water. Projection of to day's topography back to Zone 1 time suggests a swamp area 200-400 m wide by perhaps 2-3 km long, with the long axis oriented N-S. Such an area and shape would be more than wide and long enough to collect the regional pollen (Macphail and McQueen, 1983).

The climatic conditions at Pohehe Swamp in Zone 1 time were probably similar to those in other parts of New Zealand. Between 25-60? ka BP there was an interstadial, the Moerangi, which can be identified in Taranaki, Hawkes Bay, Bay of Plenty and Tongariro regions (McGlone, 1985b). Nothofagus grew where the climate was suitable, as in Taranaki and Bay of Plenty, but where the climate was dry, grassland was prevalent. Northwest Wairarapa can now be added to the list of areas supporting Nothofagus forest during the Moerangi Interstadial.

Zone 2 I consider that Zone 2 represents the period from c. 24 ka BP till c. l3 ka BP, during which

time the climate became much colder than before, and the vegetation in the area changed from predominantly forest to grassland with shrubs. The period is represented by the sediments between 2.2 m and 1.15 m and includes the Aokautere Ash.

My reason for placing the Zone 2 boundary at 0.2 m below the Aokautere Ash is that at site A this level marks the beginning of the decline in Nothofagus menziesii pollen, which continues until N. menziesii disappears further up in the zone (Fig. 3). Accompanying this decline there is an increase of Gramineae. I interpret the two changes to reflect a cooling of climate. I calculate a date of 24.2 ka BP for the start of the change from the sedimentation rate of 0.13 m per ka for 0.2 m of sediment. Such a date is consistent with the cooling inferred from the oceanic record of oxygen isotope ratios (Martinson et a/., 1987). The palynofloras above and immediately below the Aokautere Ash Member show that cold climate vegetation in the form of grassland was well established by 22.6 ka BP.

Associated with the grasses are variable proportions of Compositae, probably Senecio and Olearia, plus Halocarpus, Phyl/ocladus and Coprosma, all of which would have been growing in scattered clumps amongst the grasses. Herbs are also present, particularly Umbelliferae, Acaena, Haloragis and Liliaceae. In the swamp itself, there was an increase in the proportion of Cyperaceae at the expense of Gleichenia and Restionaceae (Fig. 3).

Soons (1979) considered that during the Last Stadial in Canterbury, three degrees of latitude south of Po he he Swamp, the mean annual temperature was no more than 4.5°C lower than today. If so, the Zone 2 timber line in Canterbury was probably at 250 m ASL, the present day altitude of the flood plain of the Mangatainoka River to the east of Pohehe Swamp (Fig. 1 ).

Two Last Stadial palynological sites, at Taita (McQueen and Mildenhall, 1983) and near Paraparaumu (McIntyre, 1970) both at 40 m ASL, provide information about Last Stadial flora. At Taita (Fig. 1) there was subalpine forest dominated by Nothofagus menzies ii, Phyllocladus, Compositae and Gramineae; at Paraparaumu, there were Phyllocladus, Halocarpus, Gramineae, Compo sitae and a few Nothofagusfusca type trees. Pohehe Swamp was 240 m higher and the scarcity of tree pollen there suggests that it was above the tree line and any forest enclaves.

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McLea - Palynology of Pohehe Swamp 217

Zone 3 Zone 3 represents the time from 13 ka to 10 ka BP. At site A it is 0.5 m thick (between

depths 1.15 m and 0.65 m), encompassing the upper part of the middle Pohehe member. A rapid change from grassland to forest marks the boundary between Zone 2 and Zone 3 (Fig. 3 and Fig. 4).

At site A, Dacrydium cupressinum and Prumnopitysferruginea (miro) pollen, about 2% of each, along with traces of two other podocarps, P. taxifolia (matai) and Podocarpus totara (totara), appear at a depth of 1.15 m, in an assemblage in which grass pollen is decreasing and shrub pollen is increasing (Fig. 3). Above 1.15 m, the change from the grassland of Zone 2 is rapid. By 0.95 m Dacrydium cupressinum has increased to about 20% along with about 4% Prumnopitysferruginea and 3% P. taxifolia. The latter is considered to be a pioneer species that can grow on new soils in areas oflow rainfall (McKelvey, 1963; Wardle, 1979) and in a normal successional sequence it is eventually replaced by Dacrydium cupressinum (Morton et at., 1984). Dacrydium cupressinum can also recolonise grassland in a high rainfall area following a glacial period (Moar and Suggate, 1979) and in the wetter conditions prevailing then at Pohehe Swamp I think that it was the pioneer tree taxon. Lack of competition is likely to have been a deciding factor encouraging the first pioneers, because if my interpretation of Zone 2 is correct, then there would have been a large area of grassland open for colonisation by forest.

In the eastern foothills ofthe Ruahine Range near Manawatu Gorge (Fig. 1), Prumnopitys taxifolia was present in small numbers at 12.9 ±.0.2 ka BP (Lees, 1986) and can be interpreted to represent the change from grassland to podocarp forest at the end of the Last Stadial. From the evidence at Pohehe Swamp I think that when Prumnopitys taxifolia forest was developing at Manawatu Gorge, forest in which Dacrydium cupressinum was the main podocarp tree was also developing at Pohehe Swamp. Using the inferred sediment accumulation rate of 0.13 m per ka (see above) the 0.75 m between the tephra and 1.15 m depth represents 6 ka. This reasoning sets the date for the arrival of Dacrydium cupressinum at Pohehe Swamp as 16.2 ka BP, which is much earlier than the Manawatu Gorge radiocarbon date (Lees, 1986). If the Manawatu date of c. 13 ka BP represents the start of rapid warming, it is more in agreement with the oxygen isotope ratio evidence in ocean water (Martinson et at., 1987) and therefore I have taken 13 ka BP as the date of the change from the Last Stadial to Recent at Pohehe Swamp.

Erratic fluctuations in the relative proportions of tree and grass pollen recur through Zone 3 (Fig. 3), as expected from previous reports that the amelioration of climate was punctuated by temporary reversals (McGlone and Topping 1977; Burrows, 1979; McGlone 1985a). Zone 3 therefore represents the early post-glacial interval (McGlone, 1983) at Pohehe Swamp, when the average climate was a little cooler than today with occasional cold periods sufficient to cause temporary reversals of reafforestation.

Zone 4 Zone 4 time is estimated to be from 10 ka to 5 ka BP, when the vegetation was podocarp

broadleaf forest. It is represented by sediments of the upper Pohehe member (0.5 m to the surface). The change to the middle member below is marked by a 20 mm transition layer of silty clay darkening from light grey (22.5Y 7/1) below to greyish red (lOR 4/2) above and increasing in organic carbon from <0.5% to over 16% (McLea, 1988). Zone 4 sediments are more homogeneous than those of Zone 3, and the abrupt change may indicate that there was a hiatus in sedimentation between Zones 3 and 4.

Dacrydium cupressinum is the most abundant pollen type (between 22% and 40%) and is associated with Prumnipitys taxifolia, P.ferruginea, Podocarpus totara, Elaeocarpus (hinau), Weinmannia (kamahi), Nestegis (maire), Metrosideros (rata) and by inference Beilschmiedia tawa. In the swamp or on its edges were Dacrycarpus dacrydioides and Laurelia novae-zelandiae (pukatea). The relatively large number of tree fern spores (Cyathea and Dicksonia) suggest that forest grew up to the swamp edge. -

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The understorey was typical of podocarp broadleaf forest, with Aristotelia, Griselinia, Neomyrtus, Pittosporum, Pseudopanax, and Plagianthus. Significant in the assemblage is the pollen of Ascarina lucida, which is considered to indicate a climate warmer than at present (McGlone and Moar, 1977) and to be typical of the mid post-glacial interval (McGlone, 1983). Its frequent occurrence in the zone, averaging about 2% of regional pollen, contrasting with its almost complete absence from the area today (Zotov et al., 1938; McGlone and Moar, 1977; J. G. Hawkins 1986 pers. comm.; Lees, 1986), indicate a climate warmer then than now. Because there is no change in the palynoflora, sediment at the top of the section is presumed to represent the end of the middle post-glacial interval of McGlone (1983). There appears to be no sediment containing palynomorphs from the last 5 ka BP.

The beech gap between the northern Tararua and southern Ruahine Ranges Pohehe Swamp is at the southern end of a beech gap which starts near the headwaters of

the Mangatainoka River and extends northwards into the Ruahine Range for about 100 km (Wardle, 1984). Pollen evidence from site A of Po he he Swamp shows that beeches started to decline in the area at the time represented by sediments at a depth of 2.2 m, and had disappeared by the time the sediments at 1.65 m had been laid down. (A few beech pollen grains were counted in the layers above but are presumed to have come from outside the region.)

The date for the start of the decline, at the boundary of Zones 1 and 2, has already been fixed at C. 24 ka BP by sediment accumulation rate (Table 1). The date for the disappearance of beeches at 1.65 m can be estimated from the sediment accumulation rate during the interval between the tephra and the Zones 2 and 3 boundary. A total of 0.75 m of sediment was deposited then, at an average rate of C. 0.08 m per ka. At such a rate, 0.25 m would be deposited in C. 3.1 ka, so the beeches probably disappeared at C. 19.5 ka BP. That means they were gone by 18 ka BP, which is considered to have been the coldest time of the Last Stadial

I •. (MartInson et al., 1987). There is a similar beech gap in Westland, between the Arnold River and Paringa 200 km

to the south (Fig. I; Wardle, 1984). It is attributed to the failure of beeches to recolonise the land after the retreat of the glaciers at the end of the Last Glaciation (Wardle, 1979). Ice advance and retreat cannot be proposed as a reason for the beech gap at Pohehe Swamp, because there were no large glaciers in the region. Small valley glaciers formed on the highest part of the central Tararua Range (Fig. 1) in the Last Stadial (Stevens, 1974) but they were unlikely to have affected the Pohehe Swamp region.

I conclude that the cause of the decline and final disappearance of the beeches was a lowering of mean annual temperature, probably accompanied by a decrease in mean annual rainfall. The reason that beeches did not recolonise when the climate ameliorated at the end of the Last Stadial is not apparent from the pollen record.

Summary of dates for the site 1. The ground surface represents the present day. 2. Slightly below the surface (about 10 mm) the age is inferred to be 5 ka BP from

palynology. The time from 5 ka BP to Present is not represented because no sediment has been deposited since then.

3. At 0.50 m the age is inferred to be no older than 10 ka BP from palynology. 4. At 1.15 m the age is estimated to be 13 ka BP from comparison with the date of 12.9 ka

BP for reafforestation at Manawatu Gorge (Lees, 1986). 5. The Aokautere Ash Member at 1.90 m has an age of 22.6 ka BP (Wilson et at., 1988). 6. At 3.70 m, or 1.6 m below the Aokautere Ash, the age is at least 36 ka BP, according

to radiocarbon dating of a piece of Phyllocladus wood. 7. At the contact with the Eketahuna Gravels at the bottom, the age is estimated to be

~0.41 ka BP from the estimated sedimentation rate of 0.08 m per ka. Since there is no evidence of Makakahi Tephric Palaeosol in the column, the maximum age cannot be

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MeLea - Palynology of Pohehe Swamp 219

greater than 60 ka BP (Milne, 1973).

CONCLUSIONS Pohehe Swamp gives an almost continuous record of vegetation changes from e. 41,000

years BP until e. 5,000 years BP. The changes in vegetation reflect changes in climate related to global glaciation and deglaciation episodes. The climatic changes were primarily shifts in mean temperature, but also include variations in rainfall. The dating of climatic changes is controlled by Aokautere Ash (22.6 ka BP) and a a radiocarbon date of ~36 ka BP within the swamp sediment sequence. The inferred climate changes provide palaeo-environmental data for interpreting terrace and coverbed sequences in the northwest Wairarapa depression, and as well a prehistoric perspective of the present vegetation.

ACKNOWLEDGEMENTS This study fonned the major part of the work for an M.Sc. degree at Victoria University, Wellington and I wish to thank my supervisors Professor P. Vella and Dr B. J. Pillans for their help during that period. Professor Vella then advised me on preparation for publication. Dr J. F. Harper designed the programme to print the pollen diagrams and Mr E. F. Hardy drafted the figures. Special thanks are due to Mr J. G. Hawkins, Masterton, who first drew attention to the site and its possible significance and helped with fieldwork; his local knowledge was appreciated. The manuscript benefited from the comments and suggestions of Mr D. C. Mildenhall who willingly shared his wisdom and expertise with me and suggested several significant improvements. Finally my thanks to the Journal Editor for her advice on layout and presentation.

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Received 27 October 1988; accepted 24 November 1989.

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