Postwildfire soil trajectory linked to prefire ecosystem structure in Douglas-fir forest

Changes in soil C and N pools following wildfire are quite varied, but there is little understanding of the causes of the variation. We examined how the legacies of prefire ecosystem structure may explain the variation in soil trajectories during the first decade following wildfire. Five years prior to wildfire in a southwestern Oregon forest dominated by mature Douglas-fir [Pseudotsuga menziesii var. menziesii (Mirb.) Franco], ecosystem structure was experimentally manipulated by thinning or clearcutting for comparison with unthinned forest. Repeated measurements of replicated experimental units were made before wildfire and during the first decade following wildfire. In the unthinned forest, the O-horizon soil C and N pools were decreased to 24-39% of prefire levels by wildfire, then increased to 53-70% during the first year postwildfire by deposition of fire-killed needles from overstory trees. The mineral soil (0-6 cm depth) C pool was decreased by wildfire, then increased in the following decade, while no change in the N pool was detected. In contrast, in the clearcut treatment, the O-horizon soil C and N pools were nearly totally consumed during the wildfire, lacked fire-killed overstory as a source of needle and fine and coarse wood inputs, but regained 20% of prefire masses in the following decade via foliar and root inputs from regenerated shrubs and herbaceous vegetation. Surface mineral soil C and N pools were decreased 35-50% by wildfire and showed no sign of recovery during the following decade. In contrast to wildfire, unburned ecosystem structures showed no changes in O horizon and increased mineral-soil N pool in the clearcut. We propose a conceptual model of soil C and N response following wildfire that includes legacy influences resulting from prefire ecosystem structures: residual live trees that generate continual litterfall and root turnover; fire-killed trees that produce needle-fall, dead roots, and fine- and coarse-wood detritus; and surviving roots and burls that contribute to postwildfire shrub regeneration. Consideration of prefire ecosystem structure and legacies in quantitative models may improve forecasts of postwildfire C budgets at stand to regional scales.