Journal of Vegetation Science 26 (2015) 360–372 Ecological controls on post-fire vegetation assembly at multiple spatial scales in eastern North American boreal forests Juliette Boiffin, Isabelle Aubin & Alison D. Munson Keywords Black spruce; Boreal forest; Fire severity; Functional traits; MEM; PCNM; Quebec; Understorey Abbreviations COM = consumed organic matter; CWM = community weighted mean; MEM = Moran eigen vector maps; PAM = partition around medoids; PCNM = principal coordinate analysis of neighbour matrices; RDA = redundancy analysis. Nomenclature Flore Laurentienne (Marie-Victorin et al. 2002) Received 16 May 2014 Accepted 29 September 2014 Co-ordinating Editor: Stephen Roxburgh Boiffin, J. (corresponding author, juliette. boiffin@gmail.com) 1 , Aubin, I. (isabelle.aubin@nrcan-rncan.gc.ca) 2 & Munson, A.D. (alison.munson@sbf. ulaval.ca) 1,2 1 Centre d’ etude de la for^ et, D epartement des sciences du bois et de la for^ et, Facult e de foresterie, de g eographie et de g eomatique 2405, rue de la Terrasse, Pavillon Abitibi-Price, Universit e Laval, Qu ebec, QC G1V0A6, Canada. 2 Natural Resources Canada, Canadian Forest Service, Great Lakes Forestry Centre, 1219 Queen St. East, Sault Ste. Marie, ON P6A 2E5, Canada. Abstract Question: In fire-prone boreal forests, to what extent does fire severity influ- ence understorey plant community assembly? What are the abiotic and biotic factors controlling understorey community regeneration at regional, landscape and local scales? Location: Black spruce-dominated boreal forest in eastern Canada. Methods: A taxonomic and a trait-based approach were combined to evaluate the relative influence of habitat characteristics, burn severity and biotic pro- cesses on understorey regeneration at multiple spatial scales. Sampling of under- storey vegetation cover was carried out in 133 recently burned plots located in 14 different fires across a 600-km transect. The spatial hierarchy for sampling consisted of five fire regime zones (regional scale), two to four fires within each zone (landscape) and two to four toposequences within each fire (local). The environmental control of fire severity and habitat characteristics on understorey regeneration was assessed using a canonical redundancy analysis (RDA) and variation partitioning. We investigated environmental and biotic filters of spe- cies traits at different scales by modelling trait–spatial assemblages with Moran eigen vector maps (MEMs). Results: In spite of the large variability of environmental conditions covered by our sampling design, low depths of burn were measured in the large majority of the studied sites. Incomplete consumption of the forest floor is frequently observed in eastern boreal forests characterized by long fire cycles. In situ biologi- cal legacies persisted through the low-severity fires, which conserved the pre- fire relationships between plant communities and their environment. As a result, fire severity was neither the unique nor the dominant control on post-fire regeneration. Habitat characteristics explained a three times higher proportion of variation in species composition. Biotic controls on trait assemblage increased at the two finer scales. Conclusions: Severity alone cannot predict understorey vegetation assembly at different scales in the low-severity fire regime characteristic of the eastern North American boreal forest. Introduction In boreal forest, understorey vegetation is a key ecosystem compartment that exerts strong control on both above- ground and below-ground ecosystem processes (Messier et al. 1998; Nilsson & Wardle 2005; Kolari et al. 2006). The heterogeneous and often dense vegetation layer modulates light transmission, soil temperature and evapotranspiration, which in turn affect nutrient cycling, germination, growth and survival of young plant cohorts (Oechel & Van Cleve 1986; Messier et al. 1998; Aubin et al. 2000; Nilsson et al. 2000). In boreal forests of North America, wildfire is one of the main natural disturbance agents that structure the vegeta- tion mosaic at the landscape scale, and that initiate and ter- minate succession at the stand scale. In this biome, fire Journal of Vegetation Science 360 Doi: 10.1111/jvs.12245 © 2014 International Association for Vegetation Science