The impact of exotic weed competition on a rare New Zealand outcrop herb, Pachycladon cheesemanii (Brassicaceae)

Nearly one quarter of New Zealand’s unique vascular plant flora is threatened, and weed invasion is implicated in the decline of more than half of these threatened species. However, there is little experimental evidence showing that invasive weeds have a direct impact on threatened native plants. This study experimentally tested the hypothesis that competition with invasive weeds threatens the rare outcrop plant Pachycladon cheesemanii (Brassicaceae). Pachycladon cheesemanii is a threatened South Island, New Zealand endemic with a distribution nearly confined to rock outcrops. It has disappeared from historical record sites throughout its range. The effects of weed competition and habitat on P. cheesemanii establishment, growth and survival were investigated by sowing seed into replicated plots subject to three treatments: weed removal, soil disturbed and unweeded control, in three habitat types: forested and open rock outcrops and open tussock grassland. The experiments were carried out at three locations: Mt Somers (Canterbury), Wye Creek and Diamond Lake (Otago). Within weedy rock outcrop habitat, weed removal significantly increased the rate of P. cheesemanii germination, and appeared to increase seedling growth rates, implying that weeds can negatively impact populations. Relative to rock outcrop habitat, P. cheesemanii germination was very low in adjacent open grassland habitat regardless of weeding treatment. Demographic monitoring of four natural populations of P. cheesemanii revealed that seed production is highly variable among populations and may be limited by browse and mechanical damage to inflorescences. Pachycladon cheesemanii does produce a persistent seed bank but most seed is found close to parent populations. Our results suggest that competition with invading weeds threatens current P. cheesemanii populations, that plant establishment can be enhanced by weed removal, and that considerable potential exists for artificially expanding populations by sowing seed into appropriate weed-free habitat.