Forest structure and biomass reflects the variable effects of fire and land use 15 and 29 years following fire in the western Cascades, Oregon

Abstract

The mixed severity fire regime of western Oregon forests creates a complex post-fire landscape mosaic with patches of low, moderate and high overstory tree mortality. Conversion of old-growth forests into plantations and post-fire salvage logging are widespread land uses that dramatically affect structure, biomass and carbon stocks. Few studies, however, have quantified the complex responses to wildfire and land management (i.e. logging and post-fire salvage logging) over long time periods. We quantified total aboveground biomass and composition in forest stands following low, moderate, and high severity fires 15 (2002 Apple Fire) and 29 years (1991 Warner Creek Fire) following fire in low elevation, old-growth forests dominated by Douglas-fir (Pseudotsuga menziesii). We also sampled post-fire responses in forest plantations (harvested prior to fire) and salvage-logged sites (harvested after fire) in the same fires. Fire severity had dramatic effects on the partitioning of total aboveground biomass (TAGB). Most of the TAGB in high severity fires was sequestered in dead trees (>43%) and downed wood (>29%) while live trees comprised the largest component of TAGB (>62%) in low severity fires. In spite of differences in overstory mortality, there was no significant difference in the TAGB between the low, moderate and high severity fires 15 years following fire (Apple Fire). Similarly, there was no significant difference between the low and high severity burns 29 years following fire (Warner Creek Fire). Managed forests (salvage and plantations) had significantly lower post-fire aboveground biomass and carbon storage that the natural forests. The TAGB of salvage logged sites was 49% and 42% that of the high severity sites at the Apple Fire and Warner Creek Fire, respectively. The mean TAGB of plantations was lowest of all fire and land use scenarios. At the Warner Creek Fire, TAGB of the plantations were <30% of that of the high severity fire sites (e.g. 326 and 984 Mg ha−1, respectively). This equates to a difference in aboveground carbon in the managed compared to the natural stands of 553 Mg CO2e ha−1 at the Apple Fire and 781 Mg CO2e ha−1 at the Warner Creek Fire. This research highlights the management tradeoffs involving values relating to carbon storage and wood harvest following fires.