Long-term recovery of soil carbon stocks and permafrost depth lags recovery of organic layer thickness following fire in black spruce forests of the Copper River Basin, Alaska

Abstract

This study investigated differences in forest structure, organic layer thickness, soil organic carbon, and permafrost depth between late-successional (LS) and postfire (PF; 90–120 years since burn) plots under black spruce (Picea mariana (Mill.) B.S.P.) growing on fine-textured, poorly drained lacustrine sediments in the Copper River Basin, Alaska. Although live stem and seedling density and organic layer thickness were not significantly different between PF and LS plots (28 ± 7 cm and 31 ± 10 cm, respectively), we did find a significant difference in soil organic carbon (SOC) stocks (30 ± 10 kg·m−2 and 46 ± 12 kg·m−2, respectively) permafrost depth (90 ± 28 cm and 56 ± 12 cm, respectively). OLT was linearly related to 1 m SOC stocks for LS plots but not for PF plots, and LS plots had a greater proportion of highly decomposed (humic) material in the organic layer. The soil properties of PF plots appear to be on a trajectory of recovery toward those of LS plots with respect to SOC stocks, permafrost depth, and organic layer composition. However, PF plots remain different despite nearly 100 years since fire disturbance and thus they are potentially more sensitive to changes in future fire frequency or climate.