Claudelands Bush: Ecology of an Urban Kahikatea (Dacrycarpus Dacrydioides) Forest Remnant in Hamilton, New Zealand
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Tane 36: 131-155 (1997) CLAUDELANDS BUSH: ECOLOGY OF AN URBAN KAHIKATEA (DACRYCARPUS DACRYDIOIDES) FOREST REMNANT IN HAMILTON, NEW ZEALAND Patrick T. Whaley, Bruce D. Clarkson and Mark C. Smale Manaaki Whenua - Landcare Research, Private Bag 3127, Hamilton, New Zealand SUMMARY Structure and composition of Claudelands Bush, a 5ha urban remnant of alluvial forest in Hamilton, New Zealand, are described and the effects of fragmentation and isolation assessed. The forest comprises less than 2% of the largest expanse of forest present in the Hamilton Basin at the time of European settlement. Formerly a mixed Dacrycarpus dacrydioides-Prumnopitys taxifolia- Dacrydium cupressinum stand, logging c. 130 years ago has led to canopy dominance by an early-mature cohort of D. dacrydioides with Laurelia novae- zealandiae and Beilschmiedia tawa. Most present canopy dominants appear to be replacing themselves, though not necessarily in existing proportions. Replacement of D. dacrydioides is largely ineffectual and B. tawa is virtually absent from canopy gaps and near margins. One-third of the indigenous vascular flora of 122 species which survived grazing from the 1860s to the 1920s became extinct in the 26 years between 1954 and 1980, mostly ground layer ferns and understorey shrubs with small populations. Smothering by the adventive ground layer herb Tradescantia fluminensis, locally dominant for >40 years, and desiccation resulting from drainage of the locality earlier in the century appear to be primarily responsible. Nearly half the remaining 81 original species are in small numbers and some further losses are likely. Conversely, seven indigenous species not native to the district have established through planting or natural dispersal from nearby gardens. Sixty adventive species are now present, mostly ground layer herbs and understorey shrubs of horticultural origin. Although few are common, 17 are regarded nationally as aggressive 'problem weeds'. A variety of factors predispose the reserve to continuing invasion by such plants, and ongoing intervention will be necessary to ensure its survival as an intact, albeit structurally modified and floristically impoverished, relic of the natural vegetation of the district. Keywords: kahikatea, Dacrycarpus dacrydioides; alluvial forest; fragmentation; regeneration; Hamilton Basin; New Zealand. 131 Fig. 1. Location of Claudelands Bush in Hamilton, New Zealand. INTRODUCTION In much of the world conservation of regional biotas depends entirely on the retention and management of fragmented remnants of the original vegetation (Saunders et al. 1991). The same is true of closely settled parts of New Zealand such as the Hamilton Basin, where only vestiges of the pre-European landscape remain intact. Despite considerable interest elsewhere (e.g. Burgess & Sharpe 1981), few studies have been made of the effects of fragmentation and isolation on forest remnants here. Bagnall (1979) analysed human impacts on an urban forest remnant near Wellington, noting mortality of tawa (Beilschmiedia tawa)1 on newly-exposed edges and a total regeneration failure in the species apparently 'Botanical nomenclature follows Allan (1961), Moore & Edgar (1970), Connor & Edgar (1987), Webb et al. (1988), and Brownsey & Smith-Dodsworth (1989). 132 resulting from desiccation - a consequence of fragmentation. Young & Mitchell (1994) analysed the effects of edges on microclimate in some northern North Island forest remnants. This paper describes research conducted on the vegetation of Claudelands Bush2, a fragment of kahikatea (Dacrycarpus dacrydioides)-dominant forest and the only intact remnant of the largest tract of native forest present in the Hamilton Basin at the time of European settlement. The aim was to determine structure and composition, assess the effects of fragmentation and isolation, and predict future vegetation trends. THE STUDY SITE Claudelands Bush (5.2ha) is located in the Hamilton Ecological District on a floodplain bisected by the Waikato River (120784 NZMS 260 S14 Hamilton) (Fig. 1). The floodplain consists of low gravelly ridges (levees) separated by the flats that represent the former braided channels of the Waikato River (Singleton 1991). Alluvial rhyolitic sand, silt, and gravel, deposited by the Waikato River during formation of the floodplain, dammed tributary streams creating lakes and swamps; this was the environment in which lowland semi-swamp forest developed. Soils are Te Kowhai silt loams and clay loams (Bruce 1979), poorly drained gley soils derived from rhyolitic alluvium with a high proportion of fine material creating a compact subsoil with slow permeability (Singleton 1991). The closest meteorological station is at Ruakura 2km to the east. Mean annual rainfall is 1200mm with a maximum in winter and minimum in summer. Air temperature averages 13°C through the year, with a summer average of 18°C and a winter average of 9°C (Singleton 1991). Claudelands Bush is subject to the prevailing westerly winds and in late summer a combination of low rainfall, low water table, and high temperatures can cause wilting in the vegetation. Only common native and introduced birds are present (Appendix 1). HISTORY When Hamilton was founded in 1864, Claudelands Bush was part of the 800 acre tract of native forest (Burstall & Sale 1984) known as Claude's Bush and was included in a 400 acre allotment granted to Colonel William Moule, Commander of the Fourth Waikato Regiment (Gibbons 1977). Milling began in 1864 and most large kahikatea and matai (Prumnopitys taxifolia) and all large rimu (Dacrydium cupressinum) were felled. Some large kahikatea were left 2Although known officially as 'Jubilee Park', the name 'Claudelands Bush' is widely used by local residents. 133 because they were unsuitable for pitsawing and splitting, or too difficult to remove. Domestic cattle (Bos taurus) had access to the forest between 1864 and 1927, causing soil compaction and damaging vegetation (Gudex 1955). Pioneering settler Francis Claude bought Moule's allotment in 1876 and later subdivided it (Gibbons 1977). In 1886 Hukanui (Peachgrove) Road was formed on the southeastern edge of what is now the reserve and Boundary Road on the northwestern edge in 1902/1903 (Gudex 1955), both subsequently cleared for residential housing. In 1928 the Agricultural and Pastoral Association gifted Claudelands Bush to the Borough of Hamilton. The reserve is trapezoidal in shape, two of its four boundaries having been shaped by road formation (Fig. 1). The eastern and southwestern edges are more-or-less natural boundaries, the latter between semi-swamp forest and former cabbage tree (Cordyline australis)-manuka (Leptospermum scoparium)-fla.x (Phormium tenax) shrubland now occupied by the Claudelands Showgrounds. Flax was planted in 1923 as a windbreak along the southwestern edge of the reserve and a number of other species including wineberry (Aristotelia serrata), kohuhu (Pittosporum tenuifolium), and lacebark (Hoheria populnea) in 1951 beyond the flax (Gudex 1955). Some injudicious planting of indigenous species not native to the district took place in subsequent years but was not documented. Possums (Trichosurus vulpecula), liberated on Sanatorium Hill 23km southeast of Hamilton in 1883 (Pracy 1962), were first noted at Claudelands in 1952 (Gudex 1955) but little of their impact has been recorded. Already locally dominant in 1954 (Gudex 1955), adventive wandering Jew (Tradescantia fluminensis) covered most of the forest floor near the perimeter in 1980 (Boase 1985) and a major clearing exercise was subsequently undertaken. More recently, a shade-cloth barrier has been erected on the southwestern side of the reserve to try to reduce the effects of the prevailing wind, the track network has been reduced and remaining ones fenced, and an elevated boardwalk constructed. PREVIOUS RESEARCH In 1933 the pioneering Waikato botanist Michael Gudex surveyed Claudelands Bush for his National Diploma of Horticulture thesis. He also recorded recollections by surviving original (1864-) European residents of the 'natural' vegetation of the district. Later he listed the native species present and their abundance (Gudex 1955) and noted a number of changes which had occurred since his earlier survey: • the forest floor had become considerably drier • large kahikatea were becoming stag-headed because of crown dieback • there were fewer epiphytic asteliads (Astelia spp., Collospermum spp.) but 134 more ground asteliads • ferns and tree ferns had increased except where wandering Jew was dominant • lianes were far less numerous, largely due to vandalism • sedges (Uncinia spp., Car ex spp.) and bush rice grass (Microlaena avenacea) had increased • possums were now present. Significantly, no losses in the flora present in 1933 were noted. Adventive species other than wandering Jew were not recorded, though they were undoubtedly present. Between 1980 and 1984 Murray Boase surveyed the remnant and found that after eliminating doubtful species records and those categorised incorrectly as native rather than adventive, 52 species and hybrids had apparently been lost since 1954, mostly ferns and dicotyledonous shrubs (Boase 1985). He also presented a list of 'more conspicuous' adventive species. METHODS A vegetation sampling programme was conducted at various times between 1974 and 1994 as follows. Structure and composition of the vegetation The reserve was divided into a grid of thirty-nine 50m x 25m rectangles, with a 10m x 10m plot located in the western corner of each.