Topography and functional traits shape the distribution of key shrub plant functional types in low-Arctic tundra

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 <i>in situ</i> trait measurements to elucidate the distribution patterns and primary controls of two representative deciduous-tall-shrub (DTS) genera, <i>Alnus</i> and <i>Salix</i>, in low-Arctic tundra. We show that topographic features were a key control on DTSs, creating heterogeneous, but predictable distributions of <i>Alnus</i> and <i>Salix</i> fractional cover (fCover). <i>Alnus</i> 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, <i>Salix</i> 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 <i>Alnus</i> and <i>Salix</i> changed with patch size, where larger patches were more specialized in resource requirements than individual plants of <i>Alnus</i> and <i>Salix</i>. 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 <i>Alnus</i> and <i>Salix</i> 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.