Arctic Tundra Shrubification: A Review of Mechanisms and Impacts on Ecosystem Carbon Balance

A woodier Arctic will alter carbon balances by affecting a complex set of soil-plant-atmosphere interactions.

A review of key climatic and environmental controls on tundra shrub expansion. The figure shows a complex set of soil-plant-atmosphere interactions driven by (1) climate change [increases in surface air temperature, precipitation, and atmospheric carbon dioxide (CO2)]; (2) soil moisture and snow dynamics; (3) topography; (4) permafrost thaw; (5) nutrient dynamics; (6) disturbance (e.g., wildfire); and (7) herbivory, as well as interactions among these factors.
[Image Credit: Reprinted under a Creative Commons Attribution 4.0 International License (CC BY 4.0) from Mekonnen, Z. A., W. J. Riley, L. T. Berner, et al. “Arctic tundra shrubification: A review of mechanisms and impacts on ecosystem carbon balance.” Environmental Research Letters 16(5), 053001(2021). DOI:10.1088/1748-9326/abf28b]

The Science

In this invited review, researchers explored and synthesized information from the literature to facilitate improved representations of tundra shrub processes in models used to assess carbon-climate feedbacks.

The Impact

Uncertainty in land model representations of the processes associated with tundra shrub expansion are uncertain and result in large uncertainties in the magnitude and direction of carbon-climate feedbacks. Prediction of tundra carbon dynamics requires land models that consider the wide array of relevant ecological processes and their interactions. This study explored and synthesized the literature to explain the key climatic and environmental drivers and controlling mechanisms for shrub expansion across the Arctic.

Summary

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. This review synthesizes the literature on (1) observed shrub expansion, (2) key climatic and environmental controls and mechanisms that affect shrub expansion, (3) impacts of shrub expansion on ecosystem carbon balance, and (4) 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 versus 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.

Principal Investigator

William Riley
Lawrence Berkeley National Laboratory
wjriley@lbl.gov

Program Manager

Daniel Stover
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
daniel.stover@science.doe.gov

Funding

This research was supported by the Office of Biological and Environmental Research, within the  U.S. Department of Energy (DOE) Office of Science, under contract no. DE-AC02- 05CH11231 to Lawrence Berkeley National Laboratory, as part of the Next-Generation Ecosystem Experiments (NGEE)–Arctic project.

References

Mekonnen, Z. A., W. J. Riley, L. T. Berner, et al. "Arctic tundra shrubification: A review of mechanisms and impacts on ecosystem carbon balance". Environmental Research Letters 16 (5), 053001  (2021). https://doi.org/10.1088/1748-9326/abf28b.