Divergent responses of shrub cover in upland and lowland tundra ecosystems to climate change and fire disturbance

The expansion of shrubs across the Arctic tundra may fundamentally modify land-atmosphere interactions. However, it remains unclear how shrub expansion is linked with key environmental drivers as climate and fire-regime changes continue to accelerate in the Arctic. Here we harnessed 40+ years of high-resolution (~1.0 m) aerial and satellite imagery to estimate shrub cover change in 96 study sites across burned and unburned upland (ice-poor) and lowland (ice-rich) tundra ecosystems in Alaskan tundra ecosystems. Results reveal that summer precipitation was the most important climatic driver (r = 0.62, P < 0.001), responsible for 29.8% of shrub expansion in upland tundra between 1977 and 2016. Upland tundra shrub expansion was enhanced by wildfire (P < 0.001), exhibiting a positive correlation with fire severity (r = 0.84, P < 0.001). Three decades after fire occurrence, upland tundra shrub-cover increased to 1271.6 ± 107.7 m² ha^-1, 5.2 times that identified in adjacent unburned upland tundra (242.8 ± 71.5 m² ha^-1). In contrast, shrub cover markedly decreased in lowland tundra following fire occurrence, which triggered thermokarst-associated water impounding and resulted in 52.4% loss of shrub cover over three decades. No correlation was found between lowland shrub cover with fire severity (r = 0.01). Mean summer air temperature (MSAT) was the principal factor driving lowland shrub-cover dynamics between 1951 and 2007. Warmer MSAT facilitated rapid shrub expansion in unburned lowland tundra (r = 0.78, P < 0.001), but accelerated shrub-cover losses in burned lowland tundra (r = - 0.82, P < 0.001). These results highlight divergent pathways of shrub-cover response to fire disturbance and climate change, depending on topographic positions and soil moisture regimes. Our study offers new insights into the complexity of tundra shrubification and land-atmosphere interactions as warming and burning intensify in high latitudes.