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Density-dependent processes fluctuate over 50 years in an ecotone forest


Spatial patterns can inform us of forest recruitment, mortality, and tree interactions through time and disturbance. Specifically, successional trajectories of self-thinning and heterospecific 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 fluctuating patterns of density-dependent mortality and recruitment that persisted into late seral stages. The combined effects of conspecific and heterospecific density-dependent effects 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.

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We acknowledge Jerry Shaw for his work as a volunteer digitizing the 1967–1997 data. E. H. Hogg provided feedback on the manuscript and analysis. Chloe Christenson, Evan Fellrath, Marc La Flèche, Dana Hopfauf, Paul Metzler, and Joshua Wasyliw assisted with stem mapping in May 2017. Tucker Furniss assisted with spatial analysis. Lee Foote granted access to the GLR plot in the University of Alberta Botanical Gardens. Funding was provided by the Alberta Conservation Association Grants for Biodiversity to Joseph Birch, and a Natural Sciences and Engineering Research Council of Canada Discovery Grant to Justine Karst.


Funding was provided by Canadian Network for Research and Innovation in Machining Technology, Natural Sciences and Engineering Research Council of Canada (RGPIN 2017-03813), Alberta Conservation Association.

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JB, SS, JL, and JK designed the 2017 census. GLR founded the plot and led the 1967–1988 censuses. RP assisted with the digitization and data management of 1967–1997 data. JB led the 2017 census. JB analyzed the data with assistance from JL and JK. JB wrote the manuscript with JL, RP, and JK providing editorial support.

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Correspondence to Joseph D. Birch.

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The authors declare that they have no conflict of interest.

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Communicated by Marie-Josée Fortin.

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Birch, J.D., Lutz, J.A., Simard, S.W. et al. Density-dependent processes fluctuate over 50 years in an ecotone forest. Oecologia 191, 909–918 (2019).

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