What is agreed about the Hieracium problem:
Hawkweeds are a very large group of northern hemisphere herbs in the daisy family (Asteracea). Of the many thousands of hawkweeds, nine species and one hybid are present in New Zealand (Webb et al 1988). They were probably introduced as contaminants of crop or pasture seed between 1850 - 1870 and their weed potential was first noted in the 1920s (Allen 1920). They remained relatively rare until after World War II (Connor 1964) but since the 1950s they have spread extensively and are now regarded as major weeds in New Zealand's rangelands, particularly in the South Island (Hunter et al 1992). Two species, mouse-ear (Hieracium pilosella) and king devil (H. praealtum), are now among the most abundant tussock grassland species in the moderate to low rainfall areas of the South Island high country. They are considered weeds because of their exclusion of other forage and indigenous species. Further increase in distribution and abundance seems likely.
Hieracium Workshop Consensus: Since the recognition of the impact of Hieracium on pastoral production and nature conservation systems, a series of workshops, involving scientific, management and policy personnel, have been convened to examine the problem (McMillan 1991, Hunter et al. 1992, Tekapo Workshop (unpublished 1999). The outcomes of workshops were a shared understanding of the main ecological factors identified through the above process and the management implications that arise from this. These factors included:
- differences among Hieracium species in distribution;
- differences between some Hieracium species in their interaction with grazing;
- the general pattern of historical influence of pastoralism on tussock grassland structure and their susceptibility to Hieracium invasion;
- the stage of invasion of Hieracium.
There is general agreement that Hieracium management is not a problem in areas where pastoral development can economically be undertaken and maintained. Existing fertiliser and pasture renovation technologies can be successfully applied to control Hieracium, particularly in areas of low relief. On steeper low to mid altitude hill blocks the success of oversowing and topdressing is less certain, dependant on climatic factors and the stage of Hieracium invasion. Good suppression is possible in favourable seasons for areas with low initial Hieracium densities. Control through competition with pasture species becomes progressively more uncertain with increased risk of poor establishment seasons and with increasing Hieracium cover. Farmer experience suggests there are threshold cover levels beyond which pastoral development is ineffectual, though these have yet to be determined for the various zones within the high country.
There is also consensus that the greatest management problems arise in the unimproved tussock grasslands. Here the structurally intact tall tussock grassland appear the most resistant to Hieracium invasion and the degraded short tussock grasslands most at risk. Biological control through the recent release of fungal pathogens and insect predation currently appear the best long-term options for limiting Hieracium in these rangelands.
The latest Hieracium workshop (Tekapo, 29 March 1999) reviewed current research results and management practises. Recent studies have improved the understanding of some of the ecological processes responsible for the success of Hieracium species. Small scale studies of patch dynamics show H. pilosella obtains competitive advantage through nutrient and water uptake from zones surrounding patches and by significantly altering soil chemistry, organic matter levels and microbial activity under its patches (Boswell & Espie 1998, Sagar et al. In Press). Grazing was shown to differentially affect Hieracium species with different growth habits, but provides no control for the prostrate mat -forming species. Hieracium invasion occurred almost universally across a wide range of sites throughout the high country over the last two decades. Sites with a high percentage of tussock cover had a slower increase of Hieracium frequency than those with less canopy cover. The greatest rate of increase occurred in the mid-altitude zone, which may correlate with physiological studies of water-use efficiency that demonstrated Hieracium species did not greatly restrict water loss but continued to maximise photosynthesis at the cost of greater transpiration loss. In a further study, Hieracium seed establishment was significantly higher in certain microsites (e.g. moss) and lower in others (e.g. litter; Rose et al 1999) The management strategies emerging for further discussion were:
- Minimise disturbance in intact tall tussock grasslands to limit Hieracium invasion;
- Strategic grazing to reduce seed spread and the abundance of upright growing species of Hieracium in short tussock grasslands;
- Release biological control agents to limit Hieracium in unimproved tussock grasslands.