Interactions among tree-line conifers: differential effects of pine on spruce and larch

1 Plant-plant interactions are increasingly considered as complex phenomena involving both negative and positive components. Within a community, the relative importance of these components is probably species-specific and may also vary among life-history stages and along environmental gradients. 2 We used the tree line of the north-eastern Calcareous Alps of Austria, composed of shrubby Pinus mugo and upright Picea abies and Larix decidua, as a simple system in which to investigate these interactions. We focused on the largely unknown effects of pines on spruce and larch, rather than on P. mugo, which is known to be competitively displaced by the two tree species. 3 We used regression models on observational data to analyse the responses of the trees to a gradient of pine cover in terms of recruitment, growth, fecundity and browsing damage, and to determine whether effects involved both competitive and facilitative components, if they depended on the life-history stage of the trees and if they were species-specific. 4 We detected a pronounced negative effect of pine cover on recruitment and growth of both spruce and larch, whereas seed production was unaffected. Larch turned out to be more sensitive to pine competition: its recruitment and growth are superior to that of spruce in open habitats but this advantage vanishes in dense pine thickets. 5 Contrary to expectations, the effects of pine cover on growth rates of spruce and larch did not depend on the life-history stage of the trees. 6 Pine cover is a major determinant of browsing damage for both spruce and larch, indicating that it does have a positive effect in providing shelter against herbivores. 7 The differential effects of pines on spruce and larch are likely to favour spruce at the expense of larch in realizing potential habitat expansion as a result of climate change. Disregarding the complex details of plant-plant interactions may thus result in unrealistic predictions of species responses to environmental changes.