Highlights

• Douglas-fir seedling recovery varies with burn severity and salvage logging.
• Salvage logging lowers biomass in high severity sites with already sufficient light.
• Water stress increases with severity and salvage, yet biomass remains high.
• Higher %N is linked to improved water-use efficiency in seedlings.
• Findings support adaptive, severity-based post-wildfire management.

Abstract

1. Exacerbated by climate change, wildfires in British Columbia, Canada, have increased in extent and severity, impacting forests, including commercially valuable species like interior Douglas-fir (Pseudotsuga menziesii var. glauca). Post-wildfire salvage logging aims to mitigate financial losses and accelerate regeneration, though its ecological impacts remain uncertain.
2. 2. This study was conducted in the Alex Fraser Research Forest, where a 2017 wildfire burned approximately 1000 ha. Combined with 2023 seedling biomass and %N measurements, we used linear mixed-effects models to examine the physiological responses of regenerating interior Douglas-fir seedlings to burn severity and salvage logging, using carbon, nitrogen, and oxygen stable isotope analyses (δ¹³C, δ¹⁵N, and δ¹⁸O) to assess water-use efficiency (WUE_i), photosynthesis, water stress, and nitrogen cycling post-disturbance.
3. 3 Higher seedling biomass was found in high-severity, not-salvaged sites. Moderate-severity, not-salvaged sites had lower δ¹³C and δ¹⁸O values compared to high-severity (salvaged and not-salvaged) and moderates-severity, salvaged sites. Higher leaf %N was positively correlated with δ¹³C values across treatments, indicating enhanced water-use efficiency.
4. 4. The statistically significant interactions between burn severity and salvage logging and their influence on seedling biomass, δ¹³C, and δ¹⁸O emphasize the key role of microclimatic conditions in post-fire recovery. In high-severity sites, salvage logging did not enhance seedling biomass, likely due to already sufficient light availability. In moderate-severity sites, salvage logging had small, positive effects on seedling biomass that were not statistically significant. Higher leaf nitrogen content appeared to boost WUE_i across treatments. These findings support tailoring post-wildfire management to burn severity, with minimal intervention in high-severity areas and selective salvage in moderate-severity sites.