Short-term soil nutrient and plant community responses to the eradication of a nitrogen fixing tree, Lupinus arboreus

Abstract

Invasive plant eradication aims to restore an original state by eliminating the plant and its alteration of ecosystem structure and function. The method by which an invasive plant is eradicated is known to influence restoration success; however, success may be impeded by a legacy effect. We sought to examine how the soil nutrient pool and plant community assembly responded to the eradication of an invasive nitrogen-fixing legume, Lupinus arboreus, using three different techniques. We applied herbicide to six mature plants and with a handsaw removed the above ground biomass of 12 mature plants; six of these were left in situ to decompose and six were removed from site. Soil nutrient concentrations were measured, and plant community composition was monitored, in the treatment and reference plots over the following 13 months. Eradication resulted in a significant increase in extractable ammonium-N and potassium, in contrast nitrate-N declined. The plant community changed as a result of felling L. arboreus, and the removal of all biomass initially resulted in an increase in native plant cover. By the end of the experimental period, however, L. arboreus seedlings were increasing in abundance and native cover was declining. The application of herbicide resulted in the slow decay of the mature plant and as such delayed an increase in exotic species cover and inhibited L. arboreus germination. Nutrient concentrations in the eradication plots were not significantly different from the reference plots at the end of the experimental period. The impact of L. arboreus on the soil nutrient pool remains after eradication, a legacy effect which will hinder restoration of the original structure and function in the long term.