ORIGINAL ARTICLE Climate change and the future distributions of aquatic macrophytes across boreal catchments Janne Alahuhta 1,2 *, Jani Heino 3 and Miska Luoto 4 INTRODUCTION Ecological systems are typically hierarchically structured, and local-scale patterns are often constrained by large-scale processes. Processes operating at the regional scale may, for example, override local-scale factors in determining species distributions (Levin, 1992; Cottenie, 2005). Hence, identifying the importance of large-scale processes is also a prerequisite for understanding variation in species distributions at local scales. Large-scale studies of species distributions have been focused 1 Department of Geography, University of Oulu, PO Box 3000, FI-90014 University of Oulu, Finland, 2 Finnish Environment Institute, Freshwater Centre, River Basin Management Unit, PO Box 35, FI-40114 University of Jyva ¨skyla ¨, Finland, 3 Finnish Environment Institute, Natural Environment Centre, Ecosystem Change Unit, PO Box 413, University of Oulu, FI-90014 Oulu, Finland, 4 Department of Geosciences and Geography, University of Helsinki, PO Box 64, FI-00014 University of Helsinki, Finland *Correspondence: Janne Alahuhta, Department of Geography, University of Oulu, PO Box 3000, FIN-90014 University of Oulu, Finland. E-mail: janne.alahuhta@oulu.fi ABSTRACT Aim Aquatic–terrestrial ecotones are vulnerable to climate change, and degradation of the emergent aquatic macrophyte zone would have severe ecological consequences for freshwater, wetland and terrestrial ecosystems. Our aim was to uncover future changes in boreal emergent aquatic macrophyte zones by modelling the occurrence and percentage cover of emergent aquatic vegetation under different climate scenarios in Finland by the 2050s. Location Finland, northern Europe. Methods Data derived from different GIS sources were used to estimate future emergent aquatic macrophyte distributions in all catchments in Finland (848 in total). We used generalized additive models (GAM) with a full stepwise selection algorithm and Akaike information criterion to explore the main environmental determinates (climate and geomorphology) of emergent aquatic macrophyte distributions, which were derived from the national subclass of CORINE land- cover classification. The accuracy of the distribution models (GAMs) was cross- validated, using percentage of explained deviance and the area under the curve derived from the receiver-operating characteristic plots. Results Our results indicated that emergent aquatic macrophytes will expand their distributions northwards from the current catchments and percentage cover will increase in all of the catchments in all climate scenarios. Growing degree-days was the primary determinant affecting distributions of emergent aquatic macrophytes. Inclusion of geomorphological variables clearly improved model performance in both model exercises compared with pure climate variables. Main conclusions Emergent aquatic macrophyte distributions will expand due to climate change. Many emergent aquatic plant species have already expanded their distributions during the past decades, and this process will continue in the years 2051–80. Emergent aquatic macrophytes pose an increasing overgrowth risk for sensitive macrophyte species in boreal freshwater ecosystems, which should be acknowledged in management and conservation actions. We conclude that predictions based on GIS data can provide useful ‘first-filter’ estimates of changes in aquatic–terrestrial ecotones. Keywords Aquatic–terrestrial ecotone, bioclimatic envelope models, climate change, emergent aquatic macrophytes, Finland, generalized additive models, GIS, global warming, littoral helophytes, species distribution. Journal of Biogeography (J. Biogeogr.) (2011) 38, 383–393 ª 2010 Blackwell Publishing Ltd www.blackwellpublishing.com/jbi 383 doi:10.1111/j.1365-2699.2010.02412.x