Arctic tundra shrubification: a review of mechanisms and impacts on ecosystem carbon balance

Vegetation composition shifts, and in particular, shrub expansion across the Arctic tundra aresome of the most important and widely observed responses of high-latitude ecosystems to rapidclimate warming. These changes in vegetation potentially alter ecosystem carbon balances byaffecting a complex set of soil–plant–atmosphere interactions. In this review, we synthesize theliterature on (a) observed shrub expansion, (b) key climatic and environmental controls andmechanisms that affect shrub expansion, (c) impacts of shrub expansion on ecosystem carbonbalance, and (d) research gaps and future directions to improve process representations in landmodels. A broad range of evidence, including in-situ observations, warming experiments, andremotely sensed vegetation indices have shown increases in growth and abundance of woodyplants, 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 andhydrology. Under warmer conditions, tall deciduous shrubs can be more competitive than otherplant functional types in tundra ecosystems because of their taller maximum canopy heights andoften dense canopy structure. Competitive abilities of tall deciduous shrubs vs herbaceous plantsare also controlled by variation in traits that affect carbon and nutrient investments and retentionstrategies in leaves, stems, and roots. Overall, shrub expansion may affect tundra carbon balancesby enhancing ecosystem carbon uptake and altering ecosystem respiration, and through complexfeedback mechanisms that affect snowpack dynamics, permafrost degradation, surface energybalance, and litter inputs. Observed and projected tall deciduous shrub expansion and thesubsequent effects on surface energy and carbon balances may alter feedbacks to the climatesystem. Land models, including those integrated in Earth System Models, need to account fordifferences in plant traits that control competitive interactions to accurately predict decadal- tocentennial-scale tundra vegetation and carbon dynamics.