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Continent-wide synthesis of the long-term population dynamics of quaking aspen in the face of accelerating human impacts

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Abstract

In recent decades, climate change has disrupted forest functioning by promoting large-scale mortality events, declines in productivity and reduced regeneration. Understanding the temporal dynamics and spatial extent of these changes is critical given the essential ecosystem services provided by forests. As the most widespread tree species in North America, quaking aspen (Populus tremuloides) is well suited for studying the dynamics of tree populations during a period of unprecedented climate change. Synthesizing continent-wide data, we show that mortality rates of mature aspen stems have increased over the past two-to-three decades, while relative gains in aspen basal area have decreased during the same period. Patterns were pervasive across multiple stand size classes and composition types in western North America biomes, suggesting that trends in demographic rates were not simply a reflection of stand development and succession. Our review of the literature revealed that increased aspen mortality and reduced growth rates were most often associated with hotter, drier conditions, whereas reduced recruitment was most often associated with herbivory. Furthermore, interactions between climate and competition, as well as climate and insect herbivory, had important, context-dependent effects on mortality and growth, respectively. Our analyses of aspen across its entire geographic range indicate that this important tree species is experiencing substantial increases in mortality and decreases in population growth rates across multiple biomes. If such trends are not accompanied by increased recruitment, we expect that the reduced dominance of aspen in forests will lead to major declines in the many essential ecosystem services it provides.

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adapted from Wiken et al. 2011) on the drivers of quaking aspen (P. tremuloides) mortality, growth and recruitment. b Total number of studies published per year by biome(s) over the past thirty years (1989–2019) on the drivers of quaking aspen mortality, growth and recruitment. All studies of deciduous forests also included other biomes, so these articles are instead included in the ‘multiple biomes’ study count. Ecoregions are labeled as follows: (b1) Alaska interior, (b2) boreal cordillera, (b3) boreal plains, (b4) boreal shield, (t1) taiga plains, (t2) western taiga shield, (t3) Hudson Plain, (t4) eastern taiga shield, (p1) prairie parkland, (c1) Canadian Rockies, (c2) Cascades, (c3) Idaho Batholith, (c4) Snake River Plain, (c5) Sierra Nevada, (c6) Great Basin, (c7) Arizona/New Mexico Mountains, (c8) Middle Rockies, (c9) Wasatch and Uinta Mountains, (c10) Colorado Plateaus, (c11) Southern Rockies, (m1) Northern Great Lakes and Southern Laurentians, (m2) Atlantic Highlands, (d1) interior and coastal lowlands, and (d2) Appalachian Plateau

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Acknowledgements

We are grateful to Elizabeth Reikowski for assistance with the literature review and Robert Shriver for valuable discussions about demographic modeling. We thank the many people involved in the planning, collection and management of forest inventories, including the British Columbia Ministry of Forests; Lands and Natural Resource Operations, Alberta Sustainable Resource Development; Saskatchewan Ministry of Environment Forest Service; Manitoba Conservation Forestry Branch; Ontario Ministry of Natural Resources and Forestry; Ministère des Ressources naturelles du Québec; Nova Scotia Department of Natural Resources; New Brunswick Department of Natural Resources and Energy; Newfoundland and Labrador Department of Fisheries and Land Resources; Yukon Energy, Mines and Resources; the Canadian Forest Service; and the USDA Forest Service. We also thank four anonymous reviewers for thoughtful comments.

Funding

This work was supported by funds from the United States Department of Agriculture, The Nevada Department of Wildlife, and the University of Nevada, Reno.

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JHC conceived of and funded the study, TKR developed the methodology and performed the analysis, and TKR and JHC wrote the manuscript.

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Correspondence to Tyler K. Refsland.

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The authors declare that they have no conflicts of interest.

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Data sources are summarized in Table S1 in Online Resource 1.

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Communicated by Forest Isbell.

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Refsland, T.K., Cushman, J.H. Continent-wide synthesis of the long-term population dynamics of quaking aspen in the face of accelerating human impacts. Oecologia 197, 25–42 (2021). https://doi.org/10.1007/s00442-021-05013-7

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  • DOI: https://doi.org/10.1007/s00442-021-05013-7

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