Nitrogen-Fixing Shrubs Advance the Pace of Tall-Shrub Expansion in Low-Arctic Tundra

The Science

Researchers used fine-scale remote sensing to model tall-shrub expansion on Alaska’s Seward Peninsula over the last 68 years. The model predicted past expansion well and demonstrated suitable tall-shrub habitat is currently only one-third occupied and well-constrained by permafrost, climate, and edaphic gradients. The model also predicted increases in tall-shrub habitat driven by permafrost degradation and increased wildfire frequency. Analysis of historic imagery also revealed a positive relationship between willow-birch expansion and alder expansion, suggesting that increased nutrient availability from nitrogen-fixing alders can accelerate the rate at which tall shrubs expand into suitable habitats.

The Impact

Graminoids and short-stature shrubs have historically dominated tundra plant communities, but recent warming has caused tall shrubs to become more prevalent. A team of researchers investigated tall-shrub expansion in low-Arctic tundra by modeling past expansion of tall shrubs and predicting how and where future warming will open suitable habitats for tall shrubs. Analysis suggests that nitrogen-fixing alder will accelerate tall-shrub expansion into newly available habitat areas. Species-specific nutrient interactions are therefore important for predicting vegetation dynamics in warming, low-tundra ecosystems.

Summary

Tall deciduous shrubs are critically important to carbon and nutrient cycling in high-latitude ecosystems. As Arctic regions warm, shrubs expand heterogeneously across their ranges, including within unburned terrain experiencing isometric warming gradients. Improved knowledge of local-to-regional scale patterns, rates, and controls on decadal shrub expansion is required to constrain the effects of widespread shrub expansion in terrestrial and Earth system models.

Using fine-scale remote sensing, researchers modeled the drivers of patch-scale tall-shrub expansion over 68 years across the central Seward Peninsula of Alaska. Models show the heterogeneous patterns of tall-shrub expansion are not only predictable but have an upper limit defined by permafrost, climate, and edaphic gradients, two-thirds of which have yet to be colonized. These observations suggest that increased nitrogen inputs from nitrogen-fixing alders contributed to a positive feedback that advanced overall tall-shrub expansion. These findings will be useful for constraining and projecting vegetation-climate feedbacks in the Arctic.

Principal Investigator

Aiden Schore
University of Illinois
[email protected]

Co-Principal Investigator

Mark Lara
University of Illinois
[email protected]

Program Manager

Daniel Stover
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
[email protected]

Funding

This research was supported by the National Science Foundation’s Environmental Engineering Program (EnvE-1928048) and the U.S. Department of Energy’s (DOE) Biological and Environmental Research (BER) program (DE-SC0021094). Support was also received from the Next-Generation Ecosystem Experiments Arctic project, which is funded through BER. Geospatial support for this work was provided by the Polar Geospatial Center under NSF-OPP awards 1043681 and 1559691.

References