- Impacts of a native and an invasive exotic shrub on flood-plain succession - 135 Journal of Vegetation Science 16: 135-142, 2005 © IAVS; Opulus Press Uppsala. 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 concentra- tions, light levels, plant community composition and the above- ground 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 oc- curred 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 succes- sional 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 domi- nates early succession and accumulates P whereas Coriaria dominates later succession and accumulates N. A key ecosys- tem effect of the invasive exotic Buddleja is alteration of soil N:P stoichiometry. Keywords: Buddleja davidii; Coriaria arborea; Diversity; Facilitation; N:P stoichiometry; Nutrient accumulation; Pri- mary succession; Soil nutrient. Nomenclature: Allan (1961) with amendments suggested by Connor & Edgar (1987), and Webb et al. (1988). Contrasting impacts of a native and an invasive exotic shrub on flood-plain succession Bellingham, Peter J. 1* ; Peltzer, Duane A. 1,2 & Walker, Lawrence R. 3 1 Landcare Research, P.O. Box 69, Lincoln 8152, New Zealand; 2 E-mail peltzerd@landcareresearch.co.nz; 3 Department of Biological Sciences, Box 454004, University of Nevada Las Vegas, 4505 Maryland Parkway, Las Vegas, NV 89154-4004, USA; E-mail walker@unlv.nevada.edu; * Corresponding author; Fax +64 33252418; E-mail bellinghamp@landcareresearch.co.nz Introduction Plant species can modify above- and below-ground resource availability (Wardle 2002) in ways that drive successional species replacements (Clements 1916; Tilman 1988). Invasive species with novel functions or impacts on resource availability can produce novel suc- cessional sequences (Vitousek 1990; D’Antonio et al. 1999). Woody plants with bacterial symbionts that fix N (‘N-fixing shrubs’) often drive primary succession through N-accretion and enhanced soil development (Van Cleve et al. 1971; Walker 1993; Chapin et al. 1994). However, N-fixation demands high levels of P that may, like N, be in low supply in primary succession (Vitousek 1999; Uliassi & Ruess 2002). Although P can limit primary productivity over the long term (Walker & Syers 1976; Richardson et al. 2004), the implications of alterations of N:P stoichiometry by plants during pri- mary succession are not known. Many plant species have the ability to accumulate N, P or other elements well beyond the concentrations found in other plants or in the soil (Epstein 1972; Grime 2001). To our knowledge, impacts of co-occurring N and P accumulators have not been examined in studies of primary succession. New combinations of species can alter successional and ecosystem dynamics in novel ways that need examination. We studied a native N- fixing shrub and an invasive shrub that does not fix N but accumulates both N and P in a New Zealand flood- plain system. We asked the following questions: what were the relative impacts of the two shrub species on (1) species richness and composition; (2) soil nutrient avail- ability and N:P stoichiometry and (3) how did the two species interact during early succession?