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P.R. ESPIE and C.C. BOSWELL
AgResearch, Invermay Agricultural Centre, Private Bag 50034, Mosgiel, New Zealand.
Hawkweeds, species of Hieracium, are one of the most serious issues facing hill and high country pastoral farming. The traditional land-use, low-input extensive pastoralism, appears to be unsustainable due to Hieracium excluding forage species. Four species already dominate extensive areas in the South Island, and have the potential to become a major problem in the North Island.
Although the extent of the invasion is widely documented, the rate of invasion and the factors responsible for Hieracium expansion remain unclear. Two alternative hypotheses have been suggested to explain Hieracium invasion. The first considers it as typical of an aggressive invasive species ideally suited to grazed grasslands, the second attribute its expansion to degradation from previous pastoral management, principally burning and grazing. A way to test between the hypotheses is to determine if Hieracium species actively displace resident vegetation rather than simply exploiting an unused niche. To do this, we examined vegetation change over 30 years at three sites near Tara Hills Research Station, Omarama. The sites were located along a climate gradient, representative of many eastern South Island short tussock grasslands.
Mouse-ear hawkweed (Hieracium pilosella) increased from almost nothing in 1967 to 46.7% ground cover in 1997 on the dry semi-arid grassland flats at Tara Hills (Figure 1). It similarly increased to 36.8% on a higher, wetter, hill block and to 40% in wetter grassland on the Ahuriri flats. King devil hawkweed (H. praealtum) increased to 2.2%, 7%, and 22% cover at each of these sites, respectively.
During this period, the previous dominant species, Fescue tussock (Festuca novae-zelandiae), decreased from 25% to 1.5% - 3% in the three grasslands. The adventive grass, sweet vernal (Anthoxanthum odoratum), also decreased from 25% to 4%, or less, in the grasslands.
Vegetation patterns differed between sites. H. pilosella formed discrete patches surrounded by a zone of bare soil at the semi-arid site, but not at the wetter sites. This suggests competition for soil moisture is important in dry grasslands, and that Hieracium may have some ecological advantage that allows it to displace resident tussocks and grasses where moisture may be limiting. But this does not explain why it also displaced the species in the wetter grasslands.
So we looked at soil chemistry under and between the patches. Soils directly under both Hieracium species had significantly lower pH and higher soluble aluminium concentrations than under other grassland species. Soil acidity and Al levels were intermediate in adjacent bare soil and lowest under the resident grasses A. odoratum and Agrostis capillaris.
We suggest that by increasing acidity, which also increases the soluble Al in the surrounding soil, Hieracium species obtain a further competitive advantage by restricting root growth and hence nutrient and moisture uptake by less Al-tolerant species.
These results support the explanation that the competitiveness of Hieracium species is a major reason for their successful expansion in New Zealand.
Fig 1. Expansion in Hieracium pilosella cover, Red Flat, Tara Hills 1967-97
This paper has been developed for this site and cleared for reproduction here by AgResearch. Your comments are welcomed and should be forwarded to Colin Boswell colin.boswell@agresearch.co.nz; or Peter Espie peter.espie@agresearch.co.nz
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