Abstract:
The timing and progression of spring thawing in high northern latitude (HNL) ecosystems coincide with a persistent temperature increase above freezing, snowmelt, thawing, and increased soil moisture, which facilitate the start of the growing season and associated increases in gross primary productivity (GPP), soil heterotrophic respiration (HR), and evapotranspiration (ET). These events help define eco-hydrological spring transition metrics, but the timing, order, pattern, and progression of these key events are uncertain due to the vast size and remoteness of HNL. In this study, we utilized 5-year (2016-2020) satellite-derived environmental data records (EDRs) to assess the pattern and order of spring transition events across Alaska and Northern Canada. Satellite EDRs used for analyzing transition events included two landscape freeze-thaw products derived from Soil Moisture Active Passive (SMAP; L-band) and Advanced Microwave Scanning Radiometer2 (AMSR2; Ka-band) microwave brightness temperature measurements, soil freeze-thaw data derived from machine learning, active layer thickness (ALT) derived from Moderate Resolution Imaging Spectroradiometer (MODIS) land surface temperature, solar-induced chlorophyll fluorescence (SIF) from the Orbiting Carbon Observatory 2 (OCO-2). Other geospatial data used in this analysis included snow depth, root zone soil moisture (RZSM), GPP, HR, and ET. The EDR spring transition maps showed thawing and snowmelt as the precursor to rising RZSM and growing season onset; whereby, RZSM onset coincided with the seasonal increase in HR, GPP, and ET. The spring transition generally occurred earlier in the boreal forest (DOY 85-132) than tundra (DOY 99-157). SMAP observed thawing was closely linked with snowmelt and RZSM onset. In contrast, AMSR2 and soil thawing were more closely associated with the onset of SIF, GPP, and ET. The timing of peak RZSM was significantly correlated with spring thaw timing. The average spring transition duration was 8±1 weeks between initial thawing and the last spring onset event. Spring thaw timing and maximum RZSM were closely related to ALT across the study domain. Our results confirm the utility of the combined satellite EDRs for regional monitoring and a better understanding of the complexity of the spring transition in the HNL. The spring onset metrics also provide effective indicators of changing soil moisture conditions affecting carbon sink activity in a warming climate.