Competitive abilities of three indigenous New Zealand plant species in relation to the introduced plant Hieracium pilosella

The competitive abilities of three montane indigenous New Zealand plant spe cies (Acaena buchananii, Festuca novae-zelandiae, and Raoulia australis) wh en growing with the locally invasive, introduced Hieracium pilosella were c ompared in an outdoor pot experiment. Competitive ability was divided into the competitive effect, or the ability to deplete resources, and the compet itive response, or the ability to tolerate low resource levels. The plants were grown in pots with or without Hieracium, in shade or full sunlight, an d with high or low soil fertility. The competitive response rankings showed consistent hierarchies in the different treatments with Festuca being less suppressed than Acaena and Raoulia. Festuca performed especially well in l ow soil fertility and in shaded treatments, while the other two species wer e strongly suppressed by Hieracium even in those conditions. However, all t hree species did relatively better (less badly) in the low fertility and sh aded treatments than in the more resource-rich treatments when interacting with Hieracium. The effect on Hieracium biomass of the indigenous species w as generally small and the rankings of competitive effect showed no agreeme nt between the species in the different environmental treatments. We sugges t that competitive rankings based on the competitive response component is likely to be a more sensitive measure of competitive ability for these indi genous, slow-growing plants. The results also indicates that these two comp onents of a plant's competitive ability were negatively correlated and thus reflect trade-offs in dealing with competitive situations. Finally, in com petition with Hieracium, low-growing indigenous species are likely to perfo rm best when nutrients and light are maintained at low levels.