Factors determining shrub abundance on uncultivable hill country

Abstract

Development of mÄnuka (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 mÄnuka 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, mÄnuka had greatest presence on steep slopes (>25o) and soils with Olsen P <10 μg/ml. MÄnuka presence was three times greater on steep than medium slopes (13- 25o) and 12 times greater than on low slopes (<13o). Annual fertiliser inputs did not appear to change this outcome provided Olsen P remained low. MÄnuka was virtually absent where Olsen P approached 50 μg/ml. At other grazed sites, mÄnuka 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 mÄnuka 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. Keywords: mÄnuka, gorse, soil fertility, hill country