Soil Changes under Mouse-Ear Hawkweed (Hieracium pilosella)

The rate of spread of mouse-ear hawkweed (Hieracium pilosella) patches, the effect of hawkweed on soil properties, and the nutrient content of hawkweed biomass was investigated on grazed unfertilised land on Glencairn Station (altitude 440 m, mean annual rainfall 500-600 mm) in the Mackenzie basin, southern South Island, New Zealand. Pallic soils (Typic Ustochrepts) under hawkweed patches and under surrounding herbfield were analysed for exchangeable cations, organic C and total N. Total nutrients in hawkweed and herbfield biomass were measured. Previous results showing lower topsoil pH and more organic C under hawkweed than under herbfield were confirmed. Soils under hawkweed had higher values of exchangeable Ca and Mg than soils under herbfield. The hawkweed patches had 85% more biomass (herbage plus roots) per unit area than the surrounding herbfield, and hawkweed biomass contained about three times the Ca and K of the herbfield biomass. In each of two successive years hawkweed patches, which had initial diameters of 1-1.2 m, increased their diameter c. 13 cm by expanding into the ''halo'' of bare soil surrounding them. Mean patch area increase was 25% per year. The halo soil contains less organic C and total N and has lower total exchangeable cations values compared to soils under hawkweed and herbfield. The halo, which is underlain by hawkweed roots, appears to be a zone in which nutrients are depleted to the benefit of the hawkweed patches. As patches grow in size, the advantage that the inferred nutrient depletion in the halo confers on hawkweed patches will become limited to patch margins.