Integrating Field Observations and Multi-scale Remote Sensing to Understand Tall Shrub Distribution and Environmental Limits in Arctic Tundra

The Arctic region has experienced some of the most rapid climate warming on Earth. In response to this warming and associated permafrost thaw, the abundance of tall shrubs (those that can potentially grow >2 m) has been observed increasing across the Arctic, causing substantial and complex impacts on biodiversity, energy balance, and the biogeochemical cycling of carbon, water, and nutrients. However, large uncertainties persist in addressing the critical question: what drives the patterns and rate of shrub expansion in Arctic tundra? Warming temperatures and permafrost thaw have been linked to the increased shrub cover in the Arctic. However, shrub distribution is also likely limited by other biotic and abiotic factors that do not favor their growth, and changes in these limiting factors could have larger or more direct impacts on shrub expansion. In this study, we combined ground observations and multi-scale remote sensing data (UAS, NASA ABoVE AVIRIS-NG, ArcticDEM, and high-resolution climate data) to investigate the environmental limiting factors (ELFs) of two tall shrub genera (alder and willow) in the Seward Peninsula, Western Alaska. We first developed a partial least squares regression (PLSR) model to predict the fractional cover (FCover) of alder and willow from AVIRIS-NG imagery by integrating UAS and AVIRIS-NG data. Using the developed PLSR model, we mapped the FCover of alder and willow across multiple AVIRIS-NG flights collected across the Seward Peninsula. We then combined these FCover maps with topographic, climate, and soil moisture data to construct ELF models for alder and willow, respectively, and mapped the EFL of alder and willow across the Seward Peninsula. Our results show that the distribution of alder and willow are driven by different environmental variables, where the distribution of alder is strongly affected by topography and energy related factors (e.g., radiation and temperature), while the distribution of willow is strongly tied to water related factors (e.g., topographic wetness and soil moisture). In addition, willow species can extend to more extreme environments than alder. These results suggests that continued climate warming in the Arctic may affect the expansion of alder and willow differently, which lead to distinct distribution patterns of the two genera.