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New Zealand Journal of Forestry (2006) 51(3): 29–34
©New Zealand Institute of Forestry

Refereed Articles
Determining the effect of increasing vegetation competition through fertiliser use on the establishment of wildings in unimproved high country grassland

Nick Ledgard

The spread of introduced conifers is causing concern in many parts of New Zealand, particularly the unimproved grasslands of the South Island high country. Many local District Councils require prospective tree planters to apply for a Resource Consent, a major condition of which is full consideration of the risk of wilding spread occurring outside the planted area. Competition from other plants is known to influence the seed germination and establishment of wilding conifers. A trial was established in unimproved high country grassland at Mt Barker, near Lake Coleridge in the Rakaia catchment, central Canterbury. The aim was to determine the effect of vegetation competition enhanced by fertilisers on the emergence of nine introduced conifer species - Lodgepole or contorta pine (Pinus contorta), Scots pine (P. sylvestris), Corsican pine (P. nigra), dwarf mountain pine (P. mugo), radiata pine (P. radiata), ponderosa pine (P. ponderosa), Maritime pine (P. pinaster), European larch (Larix decidua) and Douglas-fir (Pseudotsuga menziesii). Fertiliser, in the form of diammonium phosphate, applied at the rate of 100kg N/ha, increased dry matter production from 255 g/m2 to 544 g/m2 two years after application. Over 4 years, the increased vegetation cover caused by the addition of fertiliser had a significant negative effect (p < 0.01) on the seedling emergence of all species. The average suppression was 42%, ranging from 28% (maritime pine) to 70% (contorta pine). There was a negative relationship between seed weight and amount of suppression. For the majority of conifers, most seedling emergence occurred in the first year and was largely completed by year 2, with little additional emergence in year 3, and especially year 4. In a number of locations where introduced conifers grow well, there will always be some spread risk, requiring a commitment to the removal of wilding trees. The Mt Barker trial indicates that fertilisers could be used to increase the competition from existing vegetation and significantly lower wilding numbers, and hence control costs.
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