Impacts of Climatic Variation on the Growth of Black Spruce Across the Forest-Tundra Ecotone: Positive Effects of Warm Growing Seasons and Heat Waves Are Offset by Late Spring Frosts

Guillaume Moreau,John Caspersen,Catherine Chagnon,Alexis Achim,David Auty

doi:10.3389/ffgc.2020.613523

Guillaume Moreau, John Caspersen + Show 3 more

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https://doi.org/10.3389/ffgc.2020.613523

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Abstract

Climate strongly limits the physiological processes of trees near their range limits, leading to increased growth sensitivity. Northeastern North America is experiencing considerable warming, so the growth of trees near the northern treeline represents a key indicator of forest responses to climate change. However, tree-ring series and corresponding climatic data are scarce across the forest-tundra ecotone when compared to southern boreal regions, resulting in fewer studies on growth-climate relationships focused on this ecotone. Using daily climatic data, we identified trends in growing season heat accumulation and the intensity of acute climatic events over the last several decades in the southern and the northern parts of the forest-tundra ecotone in northeastern North America, and investigated their influence on black spruce radial growth. We found that black spruce trees responded positively to the increase in growing season temperatures and heat wave intensity, suggesting that growth is currently limited by suboptimal temperatures. While tree growth in the southern region generally benefited from warm spring temperatures, vulnerability to late spring frosts reduced tree growth in the northern region and increased probability of abrupt growth decline. In this region, late spring frosts offset approximately half of the additional growth that would otherwise occur over the course of a warm growing season. This vulnerability of northern trees may result from local adaptations to short growing seasons, which initiate biological activities at colder temperatures in the spring. Overall, our results highlight the need to explicitly incorporate acute climatic events into modeling efforts in order to refine our understanding of the impact of climate change on forest dynamics.

Highlights

As the rate of atmospheric warming increases at high latitudes of the northern hemisphere (ACIA, 2004; IPCC, 2014), arctic and subarctic ecosystems are undergoing major changes (ACIA, 2004)
While late spring frosts have previously been related to severe episodes of forest dieback and growth reductions in temperate forests of North America (Auclair et al, 1996; Bourque et al, 2005; Moreau et al, 2020) and Europe (Menzel et al, 2015; Vanoni et al, 2016; Príncipe et al, 2017; Gazol et al, 2019; Vitasse et al, 2019), this study provides, to the best of our knowledge, the first report of their impact on tree growth dynamics in subarctic environments
By using daily-level climate data, we identified relevant climatic variables influencing black spruce growth and demonstrated the influence of acute climatic events on trees growing near the northern treeline in Nunavik, Canada

Summary

Introduction

As the rate of atmospheric warming increases at high latitudes of the northern hemisphere (ACIA, 2004; IPCC, 2014), arctic and subarctic ecosystems are undergoing major changes (ACIA, 2004). The growth response of trees to a changing climate is considered a key indicator for anticipating how forest structure and function may be altered by climate change (Payette et al, 1989; Callaghan et al, 2002) Because of their remote location, tree-ring series and corresponding climatic data are scarce in northern regions, so there are fewer studies on growthclimate relationships in the forest-tundra ecotone compared with the southern boreal forest (i.e., Goldblum and Rigg, 2005; Huang et al, 2010; Girard et al, 2011; D’Orangeville et al, 2016, 2018; Ols et al, 2018; Marchand et al, 2019; Marquis et al, 2020). This apparent lack of climate-growth relationships is surprising considering the strong warming trends observed over the last decades in northeastern North America (Tremblay et al, 2012), and could be due to the low temporal resolution (i.e., monthly) of the climate data used in the analyses, which may obscure the effects of more discrete climatic events (Graumlich, 1993; Ols et al, 2018; Moreau et al, 2020)

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