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, Carmichaelia odorata, during primary succession were studied using both field measurements in a New Zealand temperate montane valley, and manipulative glasshouse experiments on seedlings of the three dominant tree species, Griselinia littoralis, Metrosideros umbellata and Weinmannia racemosa. 2 During a stand development chronosequence of < 100 years in which Carmichaelia 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 herbaceous to woody species. Vegetation increased in height and light levels declined with stand development. 3 Similar responses to shade that mimicked that in mature Carmichaelia stands suggested that inhibitory effects were likely to be uniform across the three tree species. 4 Nitrogen, either added via Carmichaelia litter or in solution, enhanced shoot biomass and foliar nitrogen concentrations of all three tree species. Growth in soils of increasing development increased foliar nitrogen concentrations 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 plasticity of response. Future forest composition and spatial patterning of species in mixed stands here, as elsewhere, is likely to result from differential facilitative responses during early primary succession.