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 post fire (PF, 90 to 120 years since burn) plots under black spruce (Picea mariana (Mill.) BSP) on fine-textured, poorly drained lacustrine sediments in the Copper River Basin, Alaska. We found that although live stem and seedling density and organic layer thickness (OLT) was not significantly different between PF and LS plots (28 ± 7 cm and 31 ± 10 cm, respectively), soil organic carbon (SOC) stocks (30 ± 10 kg m-2 and 46 ± 12 kg m-2, respectively), and permafrost depth (90 ± 28 cm and 56 ± 12 cm, respectively) remain significantly different. 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. Soil properties on PF plots appear to be on a trajectory of recovery towards LS plots with respect to SOC stocks, permafrost depth, and organic layer composition, but remain different despite nearly 100 years since fire disturbance and therefore potentially sensitive to changes in future fire frequency or climate.