Stable Isotopes Reveal the Drivers of Post-Wildfire Natural Regeneration of Interior Douglas-Fir Seedlings in British Columbia

Wildfires are increasing in frequency and severity due to climate change, posing challenges to forest ecosystems, including the southern interior of British Columbia, Canada. Interior Douglas-fir (Pseudotsuga menziesii var. glauca) is a species of great cultural, ecological, and economic importance, necessitating the investigation of post-wildfire regeneration amidst this changing wildfire regime. This study examines interior Douglas-fir seedling regeneration across three burn severity levels (low, moderate, high) 5 years post-wildfire at a site in interior British Columbia. Natural regeneration and seedling traits were measured in 2022 and paired with stable isotope analyses (delta 13C, delta 15N, delta 18O) and foliar nutrient assessments. We employed linear mixed-effects models to assess the impact of burn severity and light, water, and nutrient factors on seedling biomass. Results indicate higher seedling density in low severity sites but larger individual biomass in moderate and high severity sites. Light availability was the primary factor limiting individual seedling biomass, with greater delta 13C and biomass in high severity sites, suggesting that reduced canopy cover enhances photosynthesis and water use efficiency. Despite higher solar exposure, seedlings in high severity sites did not show increased drought stress according to leaf delta 18O and stem water contents, likely due to reduced interception and competition for soil water by overstory trees. Biomass growth was not linked to leaf nutrient status, indicating nutrient availability, particularly N, did not limit seedling biomass. While light availability is the current primary growth-limiting factor for regenerating interior Douglas-fir seedlings in this study, increased frequency and intensity of heat waves and droughts associated with climate change may increase water stress, emphasizing the need for long-term monitoring and adaptive management to support the regeneration of interior Douglas-fir.