Topography and Functional Traits Shape the Distribution of Key Shrub Plant Functional Types in Low-Arctic Tundra

Daryl Yang^1*Wouter Hantson²Kenneth Davidson³Julien Lamour⁴Bailey Morrison⁵Verity Gale Salmon¹Tianqi Zhang¹Kim Ely⁶Charles Miller⁷Daniel Hayes⁸Stephen B Baines⁹Alistair Rogers⁶Shawn Serbin¹⁰

• ¹Oak Ridge National Laboratory (DOE), Oak Ridge, United States
• ²WSL Institute for Snow and Avalanche Research SLF, Davos Dorf, Switzerland
• ³American Forests Inc, Washington, United States
• ⁴Universite de Toulouse, Toulouse, France
• ⁵University of California Merced, Merced, United States
• ⁶E O Lawrence Berkeley National Laboratory, Berkeley, United States
• ⁷Jet Propulsion Laboratory, Pasadena, United States
• ⁸The University of Maine, Orono, United States
• ⁹Stony Brook University, Stony Brook, United States
• ¹⁰NASA Goddard Space Flight Center, Greenbelt, United States

The expansion of shrubs in the Arctic tundra fundamentally modifies land-atmosphere interactions. However, it remains unclear how shrub distribution and expansion differ across key species due to challenges with discriminating tundra plant species at regional scales. Here, we combined multi-scale, multi-platform remote sensing and in situ trait measurements to elucidate the distribution patterns and primary controls of two representative deciduous-tall-shrub (DTS) genera, Alnus and Salix, in low-Arctic tundra. We show that topographic features were a key control on DTSs, creating heterogeneous, but predictable distributions of Alnus and Salix fractional cover (fCover). Alnus was more tolerant of elevation and slope and was found on hilly uplands (slope >10°) within a specific elevational band (200 - 400 m above sea level [MSL]). In contrast, Salix occurred at lower elevations (50 - 300 m MSL) on gentler slopes (3-10°) and required adequate soil moisture associated with its profligate water use. We also show that niche differentiation between Alnus and Salix changed with patch size, where larger patches were more specialized in resource requirements than individual plants of Alnus and Salix. To understand what constrains the growth of DTSs at locations with low fCover, we developed environmental limiting factor models, which showed that topography limits the upper bound of Alnus and Salix fCover in 69.2% and 48.7% of the landscape, respectively. These findings highlight a critical need to better understand and represent topography-controlled processes and functional traits in regulating shrub distribution, as well as a need for more detailed species classification to predict shrubification in the Arctic.