Accelerated Nutrient Cycling and Increased Light Competition will Lead to 21st Century Shrub Expansion in North American Arctic Tundra

Climate change under RCP8.5 in the 21st century will enhance tundra shrubification.

The Science

Next-Generation Ecosystem Experiments (NGEE)–Arctic scientists from LBNL used an ecosystem model, ecosys, to mechanistically represent the processes controlling recent and 21st century changes in plant functional type (PFTs) across North American Arctic tundra. The productivity of deciduous and evergreen shrubs was modeled to increase across much of the tundra, particularly in Alaska and tundra-boreal ecotones. The increased canopy cover of shrubs reduced incoming shortwave radiation for low-lying plants, causing declines in net ecosystem productivity of graminoids and nonvascular plants.

The Impact

This study mechanistically modeled and explained the driving factors that control shrubification and its future trajectory under Representative Concentration Pathway (RCP) 8.5 scenario. LBNL scientists highlighted the importance of capturing the basic processes of how Arctic PFTs compete for irradiance, water, and nutrients, which are key mechanisms of how plant functional types may change under future climates. Their modeling approach also highlights the need to understand and model differences in functional traits of Arctic PFTs. Short-term experiments may not capture decadal-scale changes in carbon cycling driven by plant compositional changes.

Summary

Many large-scale land surface models do not represent biological and physical processes important to predicting how future changes in climate and environment will drive PFT changes, and thus they cannot mechanistically explain the dynamic factors that control these changes. The modeling approach applied here is driven by PFT-specific functional traits important for predicting high-latitude vegetation competition under a changing climate (e.g., carbon dioxide fixation kinetics, leaf optical properties, phenology, morphology, and root traits). Modeled differences in PFT functional traits determine the strategy of resource acquisition and allocation that drive growth, resource remobilization, and litterfall, and therefore each PFT’s dynamic competitive capacity under changing growing conditions. Deciduous and evergreen shrub productivity (i.e., shrubification) was modeled to increase over the 21st century across much of the tundra, particularly in Alaska and tundra-boreal ecotones.

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 Office of Science, of the under Contract No. DE-AC02-05CH11231 as part of the Next-Generation Ecosystem Experiments (NGEE)–Arctic project.

References

Mekonnen, Z. A., W. J. Riley, and R. F. Grant. "Accelerated nutrient cycling and increased light competition will lead to 21st century shrub expansion in North American Arctic tundra". Journal of Geophysical Research: Biogeosciences 123 ((5)), 1683–1701  ((2018)). http://dx.doi.org/10.1029/2017JG004319.