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

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.

Microbial contribution to post-fire tundra ecosystem recovery over the 21st century

“…At the northern Anaktuvuk River site, evergreen shrubs were observed and modeled to be significant contributors to ecosystem biomass and NPP at the beginning of the 21st century and after the onset of wildfire 17 . The initial simulated evergreen shrub expansion is consistent with recent studies 65,66 , and could be attributable to relatively low post-fire nutrient availability, which favors the more conservative strategy of evergreen shrubs 36,67 . In ecosys, evergreen shrub traits imply a more conservative PFT that are slower growing and have slower leaf turnover.…”

“…The elevated NPP is predominantly attributable to the growth of graminoids and the expansion of shrubs, which become the dominate PFT by mid-century. Shrub expansion is attributable to elevated air temperature and increased soil nutrient availability 36 . The latter stems from increased organic matter depolymerization and mineralization under warming soils that release both nitrogen and phosphorus 57 .…”

“…However, the leaf nutrient concentrations are dynamic and dependent on nutrient availability, which feeds back onto modeled carboxylation rates and electron transport. Greater investment in nutrient uptake drives higher CO 2 fixation rate in deciduous, relative to evergreen, shrubs 36 .…”

“…However, the sequence of events that facilitate a reversion to ecosystem steady-state post-fire, including the links between microbial and plant communities, remain difficult to demonstrate empirically. In addition to the effects of fire, shrub expansion under a warming climate 34,35 can change the composition of belowground communities 36 . Shrubs tend to produce litter with higher carbon to nitrogen ratios, encouraging the growth of fungi with lower nitrogen requirements relative to bacteria 37 .…”

Impacts of Permafrost Degradation on Carbon Stocks and Emissions under a Warming Climate: A Review

“…Permafrost C stocks, biodegradability, and emission of permafrost carbon are baselines of simulating permafrost C feedback to climate warming. Their studies can help make alerts for the risks of environmental changes and assure the economic and productive safety for policy makers [115][116][117][118][119][120][121][122][123][124][125][126][127][128][129].…”