Are competitive effects of native species on an invader mediated by water availability?

Question: Climate change processes could influence the dynamics of biotic interactions such as plant competition, especially in response to disturbance phenomena such as invasional processes. Are competitive effects of native species on an invadermediated bywater availability? Location: Glasshouse facility, New SouthWales, Australia. Methods: We constructed competitive hierarchies for a representative suite of species from coastal dune communities that have been invaded by the Asteraceae shrub, bitou (Chrysanthemoides monilifera subsp. rotundata). We used a comparative phytometer approach, where the invader species was grown with or without a suite of native species in glasshouse trials. This was used to construct competition hierarchies under two water stress conditions: non-droughted and droughted. The treatments were designed to simulate current and potential futurewater availability respectively. Results: We found that the invader experienced fewer competitive effects from some native species under water stress, particularly with regard to belowground biomass effects. Native species were often poor competitors with the invader, despite their adaptation to periodic water stress in native coastal environments. Of the native species with significant competitive effects on the invader, functionally similar shrub species were the most effective competitors, as expressed in below-ground biomass. The relative position of species in the hierarchy was consistent across water treatments based on below-ground bitou biomass, but was contingent on water treatment when based on above-ground bitou biomass. Conclusions: The competitive effects of native species on an invader are affected by water stress. While the direction of response to water stress is species- specific, many species have small competitive effects on the invader under droughted conditions. This could allow an increase in invader dominance with climate change.