Patterns of Total Ecosystem Carbon Storage with Changes in Soil Temperature in Boreal Black Spruce Forests

To understand how carbon (C) pools in boreal ecosystems may change with warming, we measured above- and belowground C pools and C increment along a soil temperature gradient across 16 mature upland black spruce (Picea mariana Mill. [B·S.P]) forests in interior Alaska. Total spruce C stocks (stand and root C) increased from 1.3 to 8.5 kg C m⁻² with increasing soil summed degree-days (SDD > 0°C at 10 cm) across sites, whereas soil C stocks decreased from 11.9 to 6.3 kg C m⁻² with increasing SDD. Spruce C and organic soil C, which combined represent maximum C accrual since the last fire, increased with soil heat sums until 600 SDD, and then plateaued with increasing SDD across sites (R ² = 0.61, P = 0.002; second-order polynomial regression). The sum of soil and total spruce C (total ecosystem C, TEC) reached its maximum in the middle-range of soil temperatures measured (approximately 600 SDD), and was lower in the coolest (139 SDD) and the warmest (914 SDD) forests. The opposing trends between above- and belowground pools resulted in C shifting from the soil to spruce biomass with warmer soil temperatures. A shift in C distribution from below- to aboveground pools, as temperature increases, has implications for the vulnerability of C lost in boreal forest wildfires. The strongly negative relationship between surface mineral soil C stocks and increasing temperatures warrants further research into the potential loss of deep mineral soil C stocks with continued warming, especially in forests presently underlain with permafrost.