Abstract

Vegetation composition shifts, and in particular, shrub expansion across the Arctic tundra are some of the most important and widely observed responses of high-latitude ecosystems to rapid climate warming. These changes in vegetation potentially alter ecosystem carbon balances by affecting a complex set of soil–plant–atmosphere interactions. In this review, we synthesize the literature on (a) observed shrub expansion, (b) key climatic and environmental controls and mechanisms that affect shrub expansion, (c) impacts of shrub expansion on ecosystem carbon balance, and (d) research gaps and future directions to improve process representations in land models. A broad range of evidence, including in-situ observations, warming experiments, and remotely sensed vegetation indices have shown increases in growth and abundance of woody plants, particularly tall deciduous shrubs, and advancing shrublines across the circumpolar Arctic. This recent shrub expansion is affected by several interacting factors including climate warming, accelerated nutrient cycling, changing disturbance regimes, and local variation in topography and hydrology. Under warmer conditions, tall deciduous shrubs can be more competitive than other plant functional types in tundra ecosystems because of their taller maximum canopy heights and often dense canopy structure. Competitive abilities of tall deciduous shrubs vs herbaceous plants are also controlled by variation in traits that affect carbon and nutrient investments and retention strategies in leaves, stems, and roots. Overall, shrub expansion may affect tundra carbon balances by enhancing ecosystem carbon uptake and altering ecosystem respiration, and through complex feedback mechanisms that affect snowpack dynamics, permafrost degradation, surface energy balance, and litter inputs. Observed and projected tall deciduous shrub expansion and the subsequent effects on surface energy and carbon balances may alter feedbacks to the climate system. Land models, including those integrated in Earth System Models, need to account for differences in plant traits that control competitive interactions to accurately predict decadal- to centennial-scale tundra vegetation and carbon dynamics.

Highlights

  • Northern high-latitude regions have experienced rapid warming in recent decades (Berner and Heal2005, IPCC 2013)

  • Several lines of observational evidence indicate that rapid climate warming over the past few decades has resulted in (a) shifts in phenology (Myneni et al 1997, Tucker et al 2001, Verbyla 2008, McManus et al 2012, Prevéy et al 2019); (b) thawing of permafrost (Brown and Romanovsky 2008, Schuur et al 2015, Hugelius et al 2020); (c) thermokarst development (Schuur et al 2007, Jones et al 2015, Turetsky et al 2020); (d) more frequent and intense wildfire events (Flannigan et al 2009, IPCC 2013); and (e) alteration of landscape thermal dynamics, hydrological (Liljedahl et al 2016, Teufel and Sushama 2019), and nutrient cycling (Xue et al 2016, Sarneel et al 2020)

  • Parameters, and benchmarking we describe modeling needs most relevant to simulate changes in vegetation composition that lead to tundra shrub expansion and alter the ecosystem carbon balance

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Summary

23 April 2021

Original content from this work may be used under the terms of the Creative Commons. States of America 7 Department of Renewable Resources, University of Alberta, Edmonton, Canada.

Introduction
Observed shrub expansion across the Arctic tundra
Impacts of shrub expansion on ecosystem carbon balance
Findings
Conclusions
Full Text
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