Contrasting impacts of a native and an invasive exotic shrub on flood-plain succession

Abstract

Question: How do Coriaria arborea, an N-fixing native shrub, and Buddleja davidii, a non-N-fixing exotic shrub, affect N:P stoichiometry in plants and soils during early stages of primary succession on a flood-plain? Location: Kowhai River Valley, northeast South Island, New Zealand. Methods: We measured soil and foliar nutrient concentrations, light levels, plant community composition and the aboveground biomass of Coriaria and Buddleja in four successional stages: open, young, vigorous and mature. Results: Coriaria occurred at low density but dominated above-ground biomass by the vigorous stage. Buddleja occurred at 5.3 ± 1.0 stems/m 2 in the young stage and reached a maximum biomass of 520-535 g.m –2 during the young and vigorous stages. Mineral soil N increased with above-ground Coriaria biomass (r 2 = 0.45), but did not vary with Buddleja biomass. In contrast, soil P increased with Buddleja biomass (r 2 = 0.35), but not with Coriaria biomass. In early successional stages, 70-80% of the species present were exotic, but this declined to about 15% by the mature stage. Exotic plant species richness declined with increasing Coriaria biomass, but no other measures of diversity varied with either Coriaria or Buddleja biomass. Conclusion: These results demonstrate that Buddleja dominates early succession and accumulates P whereas Coriaria dominates later succession and accumulates N. A key ecosystem effect of the invasive exotic Buddleja is alteration of soil N:P stoichiometry.