Diagnosing Spring Onset Across the North American Arctic‐Boreal Region Using Complementary Satellite Environmental Data Records.

The timing and progression of the spring thaw transition in high northern latitudes (HNL) coincides with warmer temperatures and landscape thawing, promoting increased soil moisture and growing season onset of gross primary productivity (GPP), heterotrophic respiration (HR), and evapotranspiration (ET). However, the relative order and spatial pattern of these events is uncertain due to vast size and remoteness of the HNL. We utilized satellite environmental data records (EDRs) derived from complementary passive microwave and optical sensors to assess the progression of spring transition events across Alaska and Northern Canada from 2016 to 2020. Selected EDRs included land surface and soil freeze‐thaw status, solar‐induced chlorophyll fluorescence (SIF) signifying canopy photosynthesis, root zone soil moisture (RZSM), and GPP, HR, and ET as indicators of ecosystem carbon and water‐energy fluxes. The EDR spring transition maps showed thawing as a precursor to rising RZSM and growing season onset. Thaw timing was closely associated with ecosystem activation from winter dormancy, including seasonal increases in SIF, GPP, and ET. The HR onset occurred closer to soil thawing and prior to GPP activation, reducing spring carbon (CO2) sink potential. The mean duration of the spring transition spanned ∼6 ± 1.5 weeks between initial and final onset events. Spring thaw timing and maximum RZSM were closely related to active layer thickness in HNL permafrost zones, with deeper active layers showing generally earlier thawing and greater RZSM. Our results confirm the utility of combined satellite EDRs for regional monitoring and better understanding of the complexity of the spring transition. Plain Language Summary: The transition from winter to spring in high northern latitudes coincides with warming and thawing temperatures, the release of soil moisture, and the start of the growing season. The pattern of spring transition events is hard to monitor due to vast size and remoteness of Arctic tundra and boreal forest. Here, we used environmental data records (EDRs) from recent satellite observations (2016–2020) to map spring transition events across Alaska and Northern Canada. The EDRs included surface and soil freeze‐thaw status, soil moisture, vegetation productivity and soil respiration, and evapotranspiration. The EDRs were used to classify the timing and order of spring activation in these processes. Spring thawing was closely linked with increased soil moisture, which coincided with the start of growing season. Rising carbon dioxide (CO2) emissions from thawing soils generally preceded ecosystem uptake of CO2 from photosynthesis, reducing the spring carbon sink for atmosphere greenhouse gas emissions. On average, the spring transition spanned ∼6 weeks between initial and final onset events, with earlier timing in boreal forests than tundra. Earlier spring thawing and greater soil moisture levels also coincided with more extensive permafrost thawing. The satellite EDRs were effective in capturing multiple spring transition events and their climate sensitivity. Key Points: Satellite monitoring of complex of spring transition events in high northern latitudesSpring metrics linked with thaw‐related shifts in soil moisture and respiration, productivity, and water and energy fluxesSpring transition spans almost 6 weeks between initial landscape thaw and final growing season onset [ABSTRACT FROM AUTHOR]