Characterization of spring thaw and its relationship with carbon uptake for different types of southern boreal forest

Abstract

The timing of spring thaw is of great biological and hydrological significance in seasonally frozen boreal forests, and may significantly vary for different forest types because of variability in canopy architecture, forest floor characteristics, and soil types. This study explores inter-site variability of spring thaw and its controlling factors between two boreal coniferous (jack pine and black spruce) and one deciduous (aspen) forest sites located in Western Canada. The linkages between spring thaw and carbon uptake were also investigated. Long-term (1998 to 2015) observations of snowfall, snow water equivalent (SWE), snow temperature, soil temperature, soil water content, and CO2 fluxes were analyzed. Average annual peak SWE was 112±27, 98±22, and 93±23 mm for aspen, jack pine and black spruce sites, respectively. Snow accumulation was 15% to 20% higher for the deciduous forest site than for the coniferous forest sites despite similar seasonal snowfall of 117–122 mm. The onset of snowmelt, soil thaw start date and the time to complete snow ablation were quite similar across sites but the differences in soil thaw end date were large. Completion of soil thaw at the aspen site was about 18 and 30 days earlier than the jack pine and black spruce sites, respectively. An earlier soil thaw completion at the aspen site was likely driven by the higher sub-canopy net radiation because of the leafless canopy in spring. Between the two coniferous forest sites, soil thaw completion was delayed by about 2 weeks at the black spruce site where the thicker forest floor (ground cover i.e. moss/lichen and organic soil layer) caused higher ice content and also provided better insulation to the underlying soils. Onset and net annual carbon uptake were strongly correlated with snowmelt start and end dates as well as with soil thaw start and end dates for the coniferous forest sites but the correlations were not statistically significant for the aspen site. This study advances our understanding about the variability of spring thaw, its dynamics and controls, and its relationship to carbon uptake for different forest types in the southern boreal forest.