Vol.:(0123456789) 1 3 Oecologia https://doi.org/10.1007/s00442-019-04534-6 COMMUNITY ECOLOGY – ORIGINAL RESEARCH Density‑dependent processes fuctuate over 50 years in an ecotone forest Joseph D. Birch 1  · James A. Lutz 2  · Suzanne W. Simard 3  · Rick Pelletier 1  · George H. LaRoi 4  · Justine Karst 1 Received: 12 April 2019 / Accepted: 10 October 2019 © Springer-Verlag GmbH Germany, part of Springer Nature 2019 Abstract Spatial patterns can inform us of forest recruitment, mortality, and tree interactions through time and disturbance. Specif- cally, successional trajectories of self-thinning and heterospecifc negative density dependence can be interpreted from the spatial arrangement of forest stems. We conducted a 50-year spatial analysis of a forest undergoing succession at the ecotone of the southwestern Canadian boreal forest. The forest progressed from early to late sere and experienced repeated severe droughts, forest tent caterpillar outbreaks (Malacosoma disstria), as well as the outbreak of bark beetles. Cumulatively, the forest lost 70% of stems due to natural succession and a combination of disturbance events. Here, we describe spatial patterns displaying signals of successional self-thinning, responses to disturbance, and changes in patterns of density dependence across 50 years. Forest succession and disturbance events resulted in fuctuating patterns of density-dependent mortality and recruitment that persisted into late seral stages. The combined efects of conspecifc and heterospecifc density-dependent efects on mortality and recruitment resulted in near-spatial equilibrium over the study period. However, the strength and direction of these demographic and spatial processes varied in response with time and disturbance severity. The outbreak of forest tent caterpillar, pronounced drought, and bark beetles combined to reduce stand aggregation and promote a spatial equilibrium. Density-dependent processes of competition and facilitation changed in strength and direction with succession of the plot and in combination with disturbance. Together these results reinforce the importance of successional stage and disturbance to spatial patterns. Keywords Aspen · Density dependence · Ecotone · Spruce Introduction Spatial patterns within forests arise from underlying pro- cesses that shape the spatial distribution of stems (Cale et al. 1989). Given the longevity of trees, spatial patterns are often our only insight into density-dependent processes such as competition and facilitation in forests (e.g., Lutz et al. 2014, Després et al. 2017, and Furniss et al. 2017). Long-term studies are critical for predicting forest responses to future climates and disturbance regimes, yet few exist, particularly those that track spatial patterns. Spatial patterns of aggre- gation, dispersion, or randomness can vary through time between species, living and dead stems, and can be used to infer underlying processes shaping forest development (Das et al. 2011; Aakala et al. 2012; Fraver et al. 2014; Gendreau- Berthiaume et al. 2016). For example, density-dependent mortality is expected to result in a forest with stems that are increasingly uniform through time (Kenkel 1988; Moeur 1993). Climate-driven changes in demographic rates can Communicated by Marie-Josée Fortin. Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00442-019-04534-6) contains supplementary material, which is available to authorized users. * Joseph D. Birch jcooper@ualberta.ca 1 Department of Renewable Resources, University of Alberta, Edmonton, AB, Canada 2 Department of Wildland Resources, Utah State University, Logan, UT, USA 3 Department of Forest and Conservation Sciences, University of British Columbia, Vancouver, BC, Canada 4 Edmonton, AB, Canada