Forest wildfire effects on coarse woody detritus in the US Pacific coast states

Background/Question/Methods

A thorough understanding of forest wildfire effects at landscape-level scale is necessary to understand the impact of wildfire on carbon budgets and ecosystem recovery. While there is general agreement that forest wildfires decrease live tree biomass and increase snag biomass, the impact of forest wildfire on coarse woody detritus (CWD) has been controversial since it is often assumed to be negligible. We estimate the impact of forest wildfire on CWD using spatially balanced probability samples of all lands in the US Pacific states and of federal lands only collected by the Forest Inventory and Analysis (FIA) program and the Current Vegetation Survey (CVS), respectively. FIA plots that were burned within five years prior to plot measurement were spatially matched with unburned plots resulting in 441 blocks of burned and unburned plots. Differences in CWD biomass between burned and unburned plots were analyzed with mixed effects models where wildfire disturbance, ownership group, elevation, and climate variables were treated as fixed effects and a block as random effect. The CVS data provided repeated measurements of 8312 plots of which 165 plots had burned between measurements. Models were fit for the change in CWD biomass. Combustion factors were determined as ratios of means.

Results/Conclusions

Based on the FIA data analysis, CWD biomass was significantly lower on burned than unburned plots (p < 0.0001). No significant difference in CWD biomass was detected between burn severity classes (p > 0.80). CWD biomass in decay classes 2 and 3 was significantly lower on burned than unburned plots (p < 0.0001), while there was no significant difference in CWD biomass in decay class 1 (p = 0.33). Based on the CVS data analysis, CWD biomass increased on average by 2.1 Mg/ha (1.5, 2.7) on unburned plots, while it decreased on average by 11.7 Mg/ha (7.2, 16.0). CWD biomass in decay classes 2 and 3 decreased significantly on burned plots, while there was a significant increase in CWD biomass in decay class 1. Biomass combustion factors for low, moderate, and high severity fires were 0.23, 0.28, and 0.31 based on the FIA data and 0.30, 0.32, and 0.47 based on the CVS data analysis, which exceed previously reported values. The results suggest that losses in biomass and carbon due to CWD consumption during wildfire should not be ignored in carbon accounting.