Fire severity as a determinant factor of the decomposition rate of fire-killed black spruce in the northern boreal forest

Several attributes might influence the decomposition process of fire-killed trees. Here, we tested various tree-and plot-level variables on the decomposition rate of fire-killed black spruce (Picea mariana (Mill.) BSP) in the northern boreal forest. Data were collected from 474 individuals burned 17 years prior to sampling. Mean decomposition rate was relatively slow (k = 0.013) and was lowest for severely burned snags (k = 0.001) and highest for lightly burned logs (k = 0.027-0.036). Vertical position and fire severity were the most important variables influencing the decomposition rates, while plot-level variables were marginally significant. Both predictors strongly influenced the moisture content of fire-killed trees. Logs with greater contact with the ground and lightly burned trees had higher moisture content and faster decomposition rates. Very severely burned trees had lower moisture content because of faster bark shedding. This hampered the decomposition process by slowing the snag falling rate. Higher decomposition rates in lightly burned trees may result from greater colonization by early xylophagous species. By having a considerable impact on the decomposition of woody debris, fire severity may strongly influence many post-fire biological processes related to the woody necromass as well as carbon emission from burned stands.