How dynamic are understory communities and the processes structuring them in mature conifer forests? B. GENDREAU-BERTHIAUME, 1,  S. E. MACDONALD, 1 J. J. STADT , 2 AND R. J. HNATIUK 3 1 Department of Renewable Resources, University of Alberta, 751 General Services Building, Edmonton, Alberta T6G 2H1 Canada 2 Forest Management Branch, Forestry and Emergency Response Division, Alberta Environment and Sustainable Resource Development, 9920-108 Street, Edmonton, Alberta T5K 2M4 Canada 3 Retired, Canberra, ACT Australia Citation: Gendreau-Berthiaume, B., S. E. Macdonald, J. J. Stadt, and R. J. Hnatiuk. 2015. How dynamic are understory communities and the processes structuring them in mature conifer forests? Ecosphere 6(2):27. http://dx.doi.org/10.1890/ ES14-00308.1 Abstract. Even though understory plant communities play important roles in northern forest ecosystems, our understanding of their long-term successional dynamics and the associated driving processes is still limited. This study used spatially explicit data from five, long-term, permanent plots in mature lodgepole pine (Pinus contorta) stands in the Canadian Rocky Mountains to: (1) assess whether understory communities showed homogenization over time either within or between plots and if compositional changes observed were supportive of either the initial or relay floristics model; and (2) evaluate the relative importance of the environment vs. dispersal limitations in structuring these understory communities and how this changes over time. We found between-plot convergence and within- plot homogenization over time, largely due to increasing cover of mosses and later successional species. These changes were mainly in relative abundance, thus supporting the initial floristic model. However, the appearance of late successional species and the disappearance of early successional species over the study period suggested that the relay floristic model was also important. Most of the variation in understory species composition and richness was explained by spatial variables that were independent of environmental factors, suggesting that dispersal limitation was important in these communities. However, environmental variables explained an important fraction of the variation in species richness and composition, especially in more xeric environments; from this we infer that deterministic processes still played an important role in structuring these communities. In most plots, our results also showed increasing importance of dispersal limitations following canopy closure and we attribute this to both the homogenisation of the understory environment and increased abundance of later successional species which were more dispersal limited. Key words: Canadian Rocky Mountains; complete initial floristics; conifer forest; deterministic processes; dispersal limitations; long-term permanent plots; Pinus contorta; relay floristics; successional dynamics; understory communities. Received 4 September 2014; revised 6 November 2014; accepted 20 November 2014; final version received 29 December 2014; published 25 February 2015. Corresponding Editor: D. P. C. Peters. Copyright: Ó 2015 Gendreau-Berthiaume et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. http://creativecommons.org/licenses/by/3.0/  E-mail: gendreau@ualberta.ca INTRODUCTION The structure and species composition (tree and understory species) of forest ecosystems change over time as a function of ageing and minor disturbances (Brulisauer et al. 1996, Clark et al. 2003, Hart and Chen 2006). Understanding the dynamics of understory plant communities is v www.esajournals.org 1 February 2015 v Volume 6(2) v Article 27