1048 Biotic interactions affect the elevational ranges of high-latitude plant species Peter C. le Roux, Risto Virtanen, Risto K. Heikkinen and Miska Luoto P. C. le Roux (peter.leroux@helsinki.fi) and M. Luoto, Dept of Geosciences and Geography, Univ. of Helsinki, FI-00014 Helsinki, Finland. – R. Virtanen, Dept of Biology, Univ. of Oulu, FI-90014 Oulu, Finland. – R. K. Heikkinen, Finnish Environment Inst. (SYKE), Natural Environment Centre, FI-00251 Helsinki, Finland. Ecological theory suggests that positive plant–plant interactions can extend species distributions into areas that would otherwise be unfavourable. However, few studies have tested this hypothesis, and none have explicitly examined the associated prediction that inter-specific interactions between plants may broaden species altitudinal distributions. Here we test this prediction, using fine-scale species distribution data for 156 bryophytes, lichens and vascular plants spanning a 900 m elevational gradient in north-western Finland and Norway, analysed with a niche modelling approach. Species altitudinal ranges of all three groups of plants were more accurately predicted when including the cover of any of the 24 most wide-spread and abundant species (‘dominants’) than when using abiotic variables alone, emphasizing the importance of including relevant biotic predictors in species distribution models. Half of the models showed that species had very low probabilities of occurrence under high cover of dominants, suggesting a strong negative impact of dominant species. Similarly, for species that are predicted to occur irrespective of dominant species cover, 62% of mod- els showed narrower species altitudinal distributions when occurring under high dominant cover, with contractions of species’ lower and upper elevational limits being common. Nonetheless, high cover of dominant species was associated with upslope range extension in 43 species, and a net range expansion in nearly 10% of all models. Species distributional responses to dominants were only weakly related to species traits, with larger range contractions associated with arctic- alpine dominants. herefore, dominant species appear to exert a strong influence on the elevational distribution of other species in high latitude environments. Biotic interactions are known to affect the performance of individual plants and their fine-scale distributions, both posi- tively and negatively. While negative interactions like com- petition and herbivory have been shown to exclude plant species from habitats that would otherwise be favourable (Gaston 2003, Crain et al. 2004, Lau et al. 2008, Sexton et al. 2009), there has been little empirical support for the theory that local impacts of facilitative interactions may scale up to expand species realized niches (Choler et al. 2001, Stachowicz 2001, Bruno et al. 2003). Some indications of the influence of positive interactions on species distributions have been dem- onstrated by, for example, studies showing that the absence of mutualists (pollinators, seed dispersers) can exclude plant species from parts of their potential range (Stanton-Geddes and Anderson 2011). Furthermore, the inclusion of positive biotic interactions creates more accurate distribution mod- els for some species when using niche modeling techniques (Heikkinen et al. 2007, Pellissier et al. 2010). However, few studies have explicitly examined how inter-specific interactions affect species distribution along environmental gradients. Altitudinal gradients are particularly suited to such examinations as they comprise steep environmental gradients over short spatial distances, maximizing variation in some variables (e.g. temperature and productivity) while simulta- neously minimizing other sources of variation (e.g. historical and biogeographic differences; Körner 2003). Indeed, altitudinal gradients are frequently examined to study both the determinants of species distributional limits (Graves and Taylor 1988, Lee et al. 2009, Ettinger et al. 2011) and variation in the nature of inter-specific biotic interactions (Callaway et al. 2002, Badano et al. 2007, le Roux and McGeoch 2010). Because many species upper elevational boundaries appear to be set by extreme abiotic conditions (Gaston 2003, Normand et al. 2009), and since biotic inter- actions are generally thought to be facilitative under severe conditions (Brooker et al. 2008, although see Maestre et al. 2009), it has been proposed that positive plant–plant interactions will allow species to establish and survive at higher altitudes than would be possible in the absence of those interactions (Choler et al. 2001, Stachowicz 2001, see also empirical evidence in Badano et al. 2007). However, no studies have yet been explicitly designed to test this hypothesis along broad elevational gradients for multiple species and/or different plant groups. In contrast to the uncertainty about the impacts of facilitative interactions, numerous studies have confirmed Ecography 35: 1048–1056, 2012 doi: 10.1111/j.1600-0587.2012.07534.x © 2012 he Authors. Ecography © 2012 Nordic Society Oikos Subject Editor: Francisco Pugnaire. Accepted 27 January 2012