Shrub encroachment in grassland environments is observed in many regions worldwide. However, in New Zealand, there is no consensus on the trend and magnitude of this phenomenon, and we lack empirical data to determine what environmental variables may promote shrub invasion. Here, we present a comprehensive study evaluating shrub cover change in a tussock water catchment in eastern Otago, New Zealand.
We produced the first national-scale quantitative classification of non-forest vegetation types, including shrubland, based on vegetation plot data from the National Vegetation Survey Databank. Semi-supervised clustering with the fuzzy classification algorithm Noise Clustering was used to incorporate these new data into a pre-existing quantitative classification of New Zealand’s woody vegetation.
The low level of plant nutrients in exposed high-altitude subsoils, and the effects of soil frost and needle ice on plants attempting to colonise these subsoils combine to make natural revegetation very difficult. Artificial revegetation trials established in 1965 at three sites in the Canterbury mountains tested the effect of a fertiliser mixture which supplied a wide range of nutrients, and compared ten herbaceous species as providers of an initial protective cover, and of a cover that would persist.
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.
Monitoring the effect of management in rangelands is an integral part of the process of adaptive management. An understanding of how individual species react to management has two major benefits. Firstly, monitoring, can be simplified by avoiding species which are reacting mostly to other influences, and secondly the abundance of species can be interpreted in a meaningful way for assessing the influence of previous management.
A plant sociological survey of tall-tussock grasslands in the Mackenzie country was repeated after an interval of 26-28 years. Changes in physiognomy of the grasslands which have been inferred from earlier studies have been found to be continuing on many sites. A noteworthy feature of most sites has been a reduction in number of indigenous species found. An increase in abundance of Hieracium pilosella or H. praealtum has occurred at most sites.
Species-rich moth faunas at two sites in a montane tussock grassland at Cass show major declines in the abundance of many common species between 1961-63 and 1987-89, furthering a 50- to 70-year trend. The recent faunal record (202 species) is quantified by a 3-point light-trapping methodology based on independence of serial samples, minimised sample variability and a posteriori data standardisation. An historical record of vegetation change is also presented, pointing to a major decline in endemic herb species with the advances of an adventive grass, Agrostis capillaris.
Our evaluation of pre-settlement Holocene (10 000–1000 BP) fire, using radiocarbon-dated charcoals and pollen and charcoal spectra in pollen diagrams, concludes that fires were infrequent and patchy in the eastern South Island of New Zealand. Charcoal radiocarbon dates point to three broad phases of fire frequency: infrequent patchy fires from 10 000 to 2600 BP; a slightly increased frequency between 2600 and 1000 BP; and an unprecedented increase of fires after 1000 BP, which peaked between 800 and 500 BP.