A test of the indirect facilitation model in a temperate hardwood forest of the northern French Alps

1 We tested the hypothesis that the more frequent occurrence of tree seedlings below the adult trees than in canopy openings might be explained by indirect facilitation. In a temperate hardwood forest, we compared the performance of five target tree seedlings (Picea abies, Abies alba, Fagus sylvatica, Acer pseudoplatanus and Quercus petraea), transplanted with or without a herbaceous competitor (Molinia caerulea), either within the forest or into experimentally created gaps. 2 We quantified changes in understorey biomass, light penetration and available forms of soil nitrogen during three growing seasons. 3 Photosynthetic photon flux density and total biomass of Molinia were significantly higher in the gap treatment than within the forest. Total available nitrogen was higher in the gaps in the absence of Molinia, but higher in the forest in the presence of Molinia. 4 Quercus survival was very low within the forest because of fungal infection, whereas survival was very high for the four other tree species in all combinations of the two treatments. 5 Although the competitive effect of Molinia on the growth of the tree seedlings was much greater in the gap treatment, seedling growth was lower within the forest. We conclude that the tree canopy imposed strong light competition, and that this direct negative influence was much greater than any indirect positive effect of increased availability of nutrients to tree seedlings, due to reduced nutrient uptake by Molinia. 6 Target species responses to treatments were similar, despite strong differences in nitrogen requirements between species. This may be due to the overwhelming negative influence of the tree canopy in our experiment.