Abstract
Many studies have highlighted the relationship between nutrient fluctuations and enrichment in the process of plant invasion. However, invasion may be also associated with conditions of plant stress, either nutrient depletion or toxicity, in the environment. In this study, we investigate the possible role of nutrient stress in the invasion of Hieracium lepidulum (Stenstroem) Omang, in South Island, New Zealand. We do this by comparing several performance attributes, and their plasticity, for H. lepidulum and a number of co-occurring species, across a series of nutrient depletion and toxicity tests. H. lepidulum had intermediate yields, high root:shoot ratios and high tissue nutrient contents at control. nutrient concentrations. H. lepidulum differed in eclaphic tolerance from all but Chionochloa flovescens var. brevis, in being insensitive to nutrient dilutions other than nitrogen. The significance of performance in terms of eclaphic tolerance and adaptations are discussed. Intermediate yields at control nutrient levels suggest that H. lepidulum should not be competitive compared to high yielding species including Agrostis stolonifera and Poo cita, but should be competitive with lower yielding Coprosma rugosa and C. flavescens var. brevis. Conversely, significant yield decreases under nitrogen (imitation stress suggests that H. lepidulum will not likely occupy very nutrient poor sites. H. lepidulum, along with C. flavescens var. brevis, were found to be tolerant of ammonium as an alternative source of nitrogen, while other species were not. These data suggest that H. lepidulum and C. flavescens var. brevis would be relatively tolerant of the stresses associated with acidic soils compared to the other species, but not to stresses associated with absolute shortage of nitrogen. Combined results point to the likely occurrence of H. tepidutum at sites of intermediate fertility. The possible roles of ammonium stress. and disturbance reliance in further defining H. lepidulurn ecology is discussed. (c) 2005 Gesellschaft fur Okologie. Published by Elsevier GmbH. All rights reserved.