https://doi.org/10.1029/2024JG008359

Arctic shrub productivity trends display variability at multiple spatial scales. Fine‐scale studies
have generally observed the greatest shrub expansion in landscape positions that accumulate water and
nutrients. While considerable work has focused on the mediating effect of these resources on growth responses
to warming, less is known about the mechanisms constraining recruitment‐driven expansion. Given the low seed
viability of many Arctic shrubs, spatial patterns of seed dispersal may play an important role in constraining
fine‐scale variability of shrub recruitment. This variability may also be driven by ground cover suitability,
though these relationships are understudied in undisturbed sites. Here, we developed models representing seed
accumulation mechanisms around Alnus alnobetula (green alder) patches within the taiga‐tundra ecotone of the
Northwest Territories and compared these with observations of seed and seedling density. We also investigated
relationships between seedling abundance, topographic position, and ground cover. Observed patterns of
recruitment were complex, with preferential expansion occurring beneath alder patches only on the steepest
slopes. Seed accumulation models representing overland flow, wind, and source distance were important
predictors of seedling recruitment. This provides indirect evidence of localized seed limitation around patches,
suggesting future recruitment may not respond as expected to changing environmental conditions. Sphagnum
cover also predicted recruitment, indicating the importance of seedbed conditions for establishment. We
propose that developing models of shrub expansion that include both dispersal and environmental constraints
may increase our ability to predict patterns and rates of expansion. Such predictions are necessary to understand
future biosphere‐atmosphere interactions in a rapidly changing Arctic.

https://doi.org/10.1029/2024JG008359