Direct and indirect effects of shade on four forest tree seedlings in the French Alps

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 nitro-en 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.