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.
KeywordsAspen Density dependence Ecotone Spruce
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.
Author contribution statement
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.
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.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
- DeRose RJ, Long JN (2012) Factors influencing the spatial and temporal dynamics of Engelmann spruce mortality during a spruce beetle outbreak on the Markagunt Plateau, Utah. For Sci 58(1):1–14Google Scholar
- Environment and Climate Change Canada (2017) Historical climate data: climate stations 3012205 and 3012216—concatenated, 1961 to 2017 [Data set]. https://climate.weather.gc.ca/historical_data/search_historic_data_e.html. Accessed 2 Feb 2017
- Flora of North America Editorial Committee (eds) (1993) Flora of North America North of Mexico, vol 19. New York, OxfordGoogle Scholar
- Kenkel N, Foster C, Caners R, Lastra R, Walker D (2006) Spatial and temporal patterns of white spruce recruitment in two boreal mixedwood stands. Duck Mountains, ManitobaGoogle Scholar
- Linares JC, Camarero JJ, Carreira JA (2010) Competition modulates the adaptation capacity of forests to climatic stress: insights from recent growth decline and death in relict stands of the Mediterranean fir Abies pinsapo. J Ecol 98:592–603. https://doi.org/10.1111/j.1365-2745.2010.01645.x CrossRefGoogle Scholar
- Lutz JA, Larson AJ, Furniss TJ, Donato DC, Freund JA, Swanson ME, Bible KJ, Chen J, Franklin JF (2014) Spatially nonrandom tree mortality and ingrowth maintain equilibrium pattern in an old-growth Pseudotsuga–Tsuga forest. Ecology 95:2047–2054. https://doi.org/10.1890/14-0157.1 CrossRefPubMedGoogle Scholar
- Moeur M (1993) Characterizing spatial patterns of trees using stem-mapped data. For Sci 39:756–775Google Scholar
- R Core Team (2018) R: A language and environment for statistical computing. R Foundation for Statistical Computing, Vienna, Austria. Version 3.4.4. https://www.R-project.org/
- Wickham H (2011) The split-apply-combine strategy for data analysis. J Stat Softw 40:1–29Google Scholar