The Origin of the Indigenous Grasslands of Southeastern South Island in Relation to Pre-Human Woody Ecosystems
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McGLONE: ORIGIN OF INDIGENOUS GRASSLANDS 1 REVIEW ARTICLE The origin of the indigenous grasslands of southeastern South Island in relation to pre-human woody ecosystems M. S. McGlone Landcare Research, P.O. Box 69, Lincoln 8152, New Zealand __________________________________________________________________________________________________________________________________ Abstract: Immediately before human settlement, dense tall podocarp-angiosperm forest dominated the moist Southland and southern coastal Otago districts. Open, discontinuous podocarp-angiosperm forest bordered the central Otago dry interior, extending along the north Otago coast. Grassland was mostly patchy within these woody ecosystems, occurring on limited areas of droughty or low-nutrient soils and wetlands, or temporarily after infrequent fire or other disturbance. Podocarpus hallii, Phyllocladus alpinus and Halocarpus bidwillii, small- leaved and asterad shrubs formed low forest and shrub associations in the semi arid interior, with Nothofagus menziesii prominent in the upper montane-subalpine zone. Substantial grasslands were confined to the alpine zone and dry terraces in intermontane basins. The arrival of the first Maori settlers at c. 800 BP led immediately to widespread burning and near-elimination of the fire-sensitive woody vegetation from all but the wettest districts. Non-Chionochloa grasses (probably species of Poa, Elymus and Festuca ) and, in particular, bracken were the first to spread after fire; later, with continued fire, the more slowly spreading Chionochloa tussock grasslands became common. A unique suite of dryland woody ecosystems has thus been replaced with fire-induced grasslands. Recreation of the pre-human vegetation cover from the surviving small remnants is problematical because of the anomalous fire-sensitivity of the indigenous drought-tolerant flora. In the current historically unprecedented fire-prone environment, perhaps the best that can be hoped for is preservation of the status quo. __________________________________________________________________________________________________________________________________ Keywords: Chionochloa; fire; grasslands; human settlement; pollen analysis; South Island. Introduction environmental and conservation attributes, and issues such as their conservation, threats from fire, weed At the time of European settlement in the first half of invasion and over-grazing, and their role in water the 19th century, grassland extended across the leeward supply and preservation of biodiversity are now the South Island, from the flanks of the western mountain focus of scientific attention. ranges to the southern and eastern coastlines (Fig. 1). Grasslands below tree line have an unusually Fernland and scrub were common associates, and problematic status arising in large part because they patches of forest occurred throughout, with some occupy land that was mostly occupied by forest and extensive tracts in coastal districts. It was realised early scrub before human settlement. Issues therefore arise that fire had destroyed the original forests (Buchanan, as to the long-term stability of these grasslands and 1868), but it was not until the publication of radiocarbon how that might affect their conservation, and to what dates from soil charcoal and subfossil wood (Molloy et extent the woody elements which remain within them al., 1963), that it was fully appreciated these fires should be encouraged. began at the time of Maori settlement. The ongoing assessment of the status and future These indigenous grasslands – whether pristine or prospects of the South Island grasslands requires an modified – have been the focus of ecological research historical perspective. There are now sufficient for many years (see reviews by O’Connor, 1986; Mark, palaeoecological sites and other sources of historical 1992). This interest in the grasslands was at first almost information from the southeastern sector of the South entirely economic, since from the 1850s they had Island to permit an overview of grassland history from provided the basis for the South Island pastoral industry. the alpine zone to the lowlands. In this paper I first Increasingly, however, they are being valued for their discuss the representation of grasses in the fossil record, New Zealand Journal of Ecology (2001) 25(1): 1-15 ©New Zealand Ecological Society 2NEW ZEALAND JOURNAL OF ECOLOGY, VOL. 25, NO. 1, 2001 Figure 1. Indigenous grasslands of the southern South Island at the time of European settlement (c. 1850 AD). Grassland distribution of physiognomic dominants, after Mark (1992) and Holloway (1959); forest boundaries after New Zealand Forest Service (1983). Localities indicated are pollen sites discussed in the text. and then review the plant historical literature and remains, mainly as phytoliths and pollen. Grass present new information concerning the indigenous phytoliths are distinctive and can be isolated from soils grasslands of the southeastern South Island, the and analysed. In New Zealand they can be divided into fernlands that accompanied their fire-induced spread, nine classes, permitting some separation of genera and the complex pre-fire associations they replaced. (Kondo et al., 1994). Initial work has been promising, but as yet only a few sequences have been analysed (e.g., Carter, 2000). Pollen analysis has been the primary Grasses in the New Zealand technique for charting grassland history both in New fossil record Zealand and elsewhere. Pollen grains are ubiquitous in the environment Grasses are poorly lignified, decay rapidly, and therefore and their ornamented outer walls preserve well in a generally leave few identifiable macrofossil remains. range of sediments. They can be extracted by means of However, they are well represented as microfossil simple chemical and physical techniques in large McGLONE: ORIGIN OF INDIGENOUS GRASSLANDS 3 Figure 2. Percentages of major pollen types in southeastern South Island for the period immediately before human impact. Data from published sites and unpublished material in the Landcare pollen data base. numbers from peats, lake sediments and many soils. Chionochloa and Cortaderia can be distinguished as a Grass pollen is highly distinctive at the family level. group more or less reliably from other New Zealand Although different grass genera have subtle variations grasses, by their larger diameter (>40 µm), thin walls in size, shape and wall structure and ornamentation, relative to their size, and fine, even, wall structural they are difficult to determine accurately and consistently elements (N. T. Moar, Landcare Research, Lincoln, during routine fossil pollen analysis. In general, pollen N.Z. unpubl.), and are here referred to as the analysts have not attempted to do so. However, ‘Chionochloa’ type. A diameter of 40 µm or greater is 4NEW ZEALAND JOURNAL OF ECOLOGY, VOL. 25, NO. 1, 2001 used as a cut-off point that will exclude most pollen 1988); a 60 µm diameter grain falls through air four grains from other grass genera while encompassing times as rapidly as one of 30 µm. Therefore, large most Chionochloa and Cortaderia, provided the grains, such as those of Chionochloa, should be morphological attributes of the grain are also deposited much closer to the parent plant on average characteristic. For the purposes of this paper, grass than smaller grains. The exotic agricultural or weed pollen has been reanalysed at selected sites. grass species which have now spread throughout many Grass pollen is abundantly produced and, being indigenous tussock grasslands produce abundant, adapted for wind transport, is usually well dispersed. mainly small pollen grains that vastly outnumber those New Zealand modern pollen rain studies have shown of many of the native grasses. that grasses are well represented in the pollen rain (Moar 1970; McGlone 1982, Pocknall, 1982; Macphail and McQueen, 1983; Horrocks and Ogden, 1994; The pattern of pre-human McGlone and Moar, 1997; McGlone and Meurk, 2000). vegetation Modern pollen samples from lowland areas dominated by grasses tend to have high percentages of grass pollen Eight hundred years of fire and 150 years of pasture (>40%). Above tree line pollen spectra are usually development have obscured the original vegetation dominated by pollen derived from forests downslope, patterns of the southeastern South Island. Over large and thus alpine grasslands have a relatively low pollen stretches of country no vegetation associations representation generally in the range 5-30% (Moar, corresponding to those of the pre-human situation 1970; Pocknall, 1982; Horrocks and Ogden, 1994). remain. Fossil information is therefore essential to gain This is largely due to the relatively small pollen source an insight into the original patterns. In Fig. 2, pollen area of the uplands versus the surrounding lowlands, percentages for major types in the period immediately and lower pollen production in cool subalpine and before human settlement are mapped for all known alpine climates. sites from the southern South Island. Some illustrative A recent modern pollen study showed that in pollen diagrams are presented here: in Fig. 3, sites from surface samples from some inland southeastern South coastal southeastern districts; in Fig. 4, sites from the Island grasslands dominated by Chionochloa, interior; and in Fig. 5, high altitude sites from the Old Chionochloa type pollen was usually much less than Man and Lammerlaw Ranges. 10% of the pollen sum, the grass pollen rain being Some pollen