A number of authors have documented a higher occurrence of tree seedlings below the canopy of adult trees than in openings, particularly in mesic conditions, where increases in resources in openings stimulate the growth of competing shade‐intolerant forbs. These patterns may be explained by indirect facilitation. Indirect facilitation has been mainly explored using models, and too few experimental studies have been conducted to understand the conditions under which it is likely to occur. We test here the indirect facilitation model in natural openings of subalpine forests and explore the relationship that may exist between species response to indirect interactions and life‐history traits of target seedlings. Two evergreen conifers (Picea abies and Abies alba) and two deciduous broad‐leaved angiosperms (Fagus sylvatica and Acer pseudoplatanus) that dominate the mixed mesic forests of the northern external French Alps were planted in a tall forb community invading natural forest openings. Seedlings were grown for three growing seasons, with and without competing forbs, and with and without a 50% shade cloth, simulating the relatively open canopy of the surrounding forests. The direct effects of shade were negative for all four species, but the conifers were much more negatively affected by shade than the angiosperms. Shade strongly reduced aboveground biomass of competing forbs, which improved the availability of nitrogen for the tree seedlings. However, because the indirect positive effect of competition release was outweighed by the direct negative effect of light reduction, the net effect was negative for all four species. Species' responses to the indirect effects of shade were correlated with species traits; additional competition was the highest for the most conservative and shade‐intolerant species, Picea, and the lowest for the most nutrient‐demanding and shade‐tolerant species, Acer. We conclude that species traits may determine how a species responds to indirect interactions, but that further studies are needed to explore the real potential of indirect facilitation to determine patterns of tree seedling distribution.

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Questions: We addressed two poorly understood aspects of plant response to climate change: the impact of extreme climatic events and the mediating role of biotic interactions, through a study of heatwave effects on tree seedling survival rates and ability of the tree canopy to alter seedling responses. Location: Mountain belt of the northern French Alps (Maurienne Valley). Methods: The survival rates of two seedling cohorts from four tree species (Abies alba, Acer pseudoplatanus, Fraxinus excelsior and Picea abies) were measured during both the 2003 European heatwave and an average summer (2004) in deciduous broadleaf mountain forests. Seedlings were transplanted into two soil moisture conditions, and in experimental gaps or under the tree canopy. Results: The heatwave strongly decreased tree seedling survival rates, while there was an important species‐specific mediating role of biotic interactions. In the wettest conditions, the tree canopy strongly increased survival of Abies, buffering the negative impact of the heatwave. In contrast, in the driest conditions, the tree canopy decreased survival of Picea and Acer, amplifying the negative impact of the heatwave. We found evidence of increasing soil water stress in the understorey of the driest community, but further studies including vapour pressure deficit measurements are needed to elucidate the driving mechanism of facilitation. Conclusions: The high species specificity of the mediating role of biotic interactions and its variation along stress gradients leads to questions on our ability to predict large‐scale responses of species to climate changes.

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Summary 1We 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.