Differential facilitation by a nitrogen-fixing shrub during primary succession influences relative performance of canopy tree species

1 The facilitative and inhibitory effects of a nitrogen-fixing shrub, Carmi chaelia odorata, during primary succession were studied using both field me asurements in a New Zealand temperate montane valley, and manipulative glas shouse experiments on seedlings of the three dominant tree species, Griseli nia littoralis, Metrosideros umbellata and Weinmannia racemosa. 2 During a stand development chronosequence of < 100 years in which Carmich aelia colonized, dominated and senesced, there was significant development of soil organic horizons and a large build-up of soil nitrogen, especially in the organic horizon. Soil organic matter and nitrogen levels across the sequence were strongly correlated with the main DCA axis of vascular plant species composition, along which there was change in dominance from herbace ous to woody species. Vegetation increased in height and light levels decli ned with stand development. 3 Similar responses to shade that mimicked that in mature Carmichaelia stan ds suggested that inhibitory effects were likely to be uniform across the t hree tree species. 4 Nitrogen, either added via Carmichaelia litter or in solution, enhanced s hoot biomass and foliar nitrogen concentrations of all three tree species. Growth in soils of increasing development increased foliar nitrogen concent rations for Griselinia and Weinmannia, but not Metrosideros. 5 Overall, Metrosideros was the least responsive to potential facilitative effects of Carmichaelia, and Griselinia exhibited the highest degree of pla sticity of response. Future forest composition and spatial patterning of sp ecies in mixed stands here, as elsewhere, is likely to result from differen tial facilitative responses during early primary succession.