Factors determining shrub abundance on uncultivable hill country

Development of mānuka (Leptospermum scoparium) and gorse (Ulex europaeus) is a widespread problem on pastoral hill country and is associated with reduced fertiliser inputs and grazing pressures. Using mānuka as a source for specialty products offers a potentially new, profitable enterprise alongside livestock and forestry. However, there is a lack of quantitative information on the optimum soil nutrient status and associated fertiliser programme to encourage presence and growth of this species. In a survey across 324 grazed sites, mānuka had greatest presence on steep slopes (>25°) and soils with Olsen P <10 μg/ml. Mānuka presence was three times greater on steep than medium slopes (13–25°) and 12 times greater than on low slopes (<13° ). Annual fertiliser inputs did not appear to change this outcome provided Olsen P remained low. Mānuka was virtually absent where Olsen P approached 50 μg/ml. At other grazed sites, mānuka and gorse were most likely to occur where Olsen P was <15 μg/ml. This study provided insights to underpin a set of nutrient guidelines for mānuka under grazing conditions to maximise its establishment and growth. It remains to be determined what the optimum Olsen P level is for this plant when managed in regimes without grazing and treading pressure.