© 2006 The Authors DOI: 10.1111/j.1466-8238.2006.00277.x 90 Journal compilation © 2006 Blackwell Publishing Ltd www.blackwellpublishing.com/geb Global Ecology and Biogeography, (Global Ecol. Biogeogr.) (2007) 16, 90–102 RESEARCH PAPER Blackwell Publishing Ltd Ecological factors explaining the location of the boundary between the mixedwood and coniferous bioclimatic zones in the boreal biome of eastern North America Yassine Messaoud 1 *, Yves Bergeron 1,2 and Alain Leduc 2 ABSTRACT Aim Climate is often regarded as the primary control determining the location of an ecotone between two vegetation zones. However, other ecological factors may also be important, especially when the northern limit of the dominant species of a vegetation zone extends further than the limit of the zone itself. This study aimed to identify the ecological variables explaining the transition between two zones within the boreal biome in Quebec (eastern Canada): the southern mixedwood forests dominated by balsam fir (Abies balsamea) and white birch (Betula papyrifera), and the northern coniferous forests dominated by black spruce (Picea mariana). Location Quebec (eastern Canada). Methods Data from 5023 sampling plots from the ecological inventory of the Québec Ministry of Natural Resources distributed throughout the two bioclimatic zones were used in logistic regressions to determine the relationships between the presence or absence of balsam fir stands and different abiotic and biotic variables, at both stand and landscape scales. Results The presence of balsam fir stands was negatively related to the thick organic horizons, coarse xeric deposits and low positions on the slope, whereas stands were favoured by high elevations, steep slopes and moderate drainage. These results defined the suitable conditions for the development of balsam fir stands. In the coniferous zone these suitable conditions were less abundant. Furthermore, the saturation level of suitable sites was lower, as well as the incidence of balsam fir stands in unsuitable sites (overflow). Balsam fir stands were mostly located near lakes and rivers. All significant variables at both the stand and landscape scales explained between 34 and 42% of the location of the potential northern distribution limit of the mixedwood zone. Main conclusions Our results suggest the important role of historical factors related to post-glacial vegetation and past disturbances in determining the relative abundance of balsam fir in both zones of the boreal biome. Keywords Boreal biome, coniferous forest, eastern North America, ecological factors, logistic regression, mixedwood forest, northern distribution limit, scale. *Correspondence: Yassine Messaoud, Chaire industrielle CRSNG-UQAT-UQAM en aménagement forestier durable, Université du Québec en Abitibi-Témiscamingue, C.P. 700, Rouyn-Noranda, Québec, J9X 5E4, Canada. E-mail: yassine.messaoud@uqat.ca 1 Chaire industrielle CRSNG-UQAT-UQAM en aménagement forestier durable, Université du Québec en Abitibi-Témiscamingue, C.P. 700, Rouyn-Noranda, Québec, J9X 5E4, Canada and 2 Groupe de Recherche en Écologie Forestière inter-universitaire and Département des sciences biologiques, Université du Québec à Montréal, C.P. 8888, Succ. Centre-Ville, Montréal, Québec, H3C 3P8, Canada INTRODUCTION In North America, the southern limit of the continuous boreal biome, dominated by coniferous species such as black spruce (Picea mariana (Mill.) B.S.P.), white spruce (Picea glauca (Moench) Voss) and balsam fir (Abies balsamea (L.) Mill.), decreases in latitude from Alaska eastwards (Fowells, 1965; Burns & Honkala, 1990) and reaches its lowest latitude (c. 48° N) in eastern Canada, between eastern Ontario and western Quebec. Except for pockets at high elevations, this is where the boreal biome reaches its most southerly limit world-wide (Pouliot, 1998). In eastern Canada, this boreal biome is subdivided into two zones; to the south, between c. 48° and c. 49° N, lies the mixedwood, a forest dominated by balsam fir and white birch (Betula papyrifera Marsh.), with white spruce also abundant (Bérard, 1996). Some species reach their northern distribution