Most predictive models forecast significant upward displacement of forest species due to increases in temperatures, but not all the species respond in the same way to changes in climate. In temperate or mountain systems, biotic competitive interactions drive species distributions, and responses to climate change will ultimately depend upon productive and demographic processes such as growth, recruitment and mortality. We parameterized and used an individual-based, spatially explicit model of forest dynamics (SORTIE-ND) to investigate the role of species-specific differences in juvenile performance induced by climate change (juvenile growth and recruitment ability) in the dynamics of mixed forests located in the montane-subalpine ecotone of the Pyrenees. We assessed this role for two types of forests composed of three species with differing light requirements and sensitivity to climate change: (1) a mixed forest with two shade-intolerant pines (Pinus uncinata and Pinus sylvestris) and (2) a mixed forest composed by a shade-intolerant pine and a shade-tolerant fir (Abies alba). Our results show that for species with similar light requirements (i.e., both pines), small differences in sapling growth response to climate change can lead to significant differences in future species composition (an increase in P. sylvestris growth of 10% leads to an increase in its abundance from 42% to 50.3%). Conversely, in pine-fir forests, shade-tolerance results more decisive than climate-induced changes in growth in driving the future forest composition. The authors are particularly grateful to M. Beaudet, M.J. Papaik, L. Murphy, and C.D. Canham for their help during parameterization of the model and for technical support. We acknowledge W.F.J. Parsons (CEF) for helpful comments and English corrections on an earlier version of the manuscript. Financial support for this study was provided by the Spanish Ministry of Science and Innovation through the projects Consolider-Ingenio Montes (CSD2008-00040), DINAMIX (AGL2009-13270-C02) and RESILFOR (AGL2012-40039-C02-01), and by the European Commission through the Marie Curie IRSES project “NEWFORESTS”. LC was supported through a Ramon y Cajal contract (RYC-2009-04985), while the Spanish Ministry of Education provided AA with support through a predoctoral grant (FPU Programme – AP2007-01663).