Biogeochemical impacts of wildfires over four millennia in a R ocky M ountain subalpine watershed

Abstract

Summary: Wildfires can significantly alter forest carbon ( C ) storage and nitrogen ( N ) availability, but the long‐term biogeochemical legacy of wildfires is poorly understood. We obtained a lake‐sediment record of fire and biogeochemistry from a subalpine forest in C olorado, USA , to examine the nature, magnitude, and duration of decadal‐scale, fire‐induced ecosystem change over the past c . 4250 yr. The high‐resolution record contained 34 fires, including 13 high‐severity events within the watershed. High‐severity fires were followed by increased sedimentary N stable isotope ratios (δ 15 N ) and bulk density, and decreased C and N concentrations – reflecting forest floor destruction, terrestrial C and N losses, and erosion. Sustained low sediment C : N c . 20–50 yr post‐fire indicates reduced terrestrial organic matter subsidies to the lake. Low sedimentary δ 15 N c . 50–70 yr post‐fire, coincident with C and N recovery, suggests diminishing terrestrial N availability during stand development. The magnitude of post‐fire changes generally scaled directly with inferred fire severity. Our results support modern studies of forest successional C and N accumulation and indicate pronounced, long‐lasting biogeochemical impacts of wildfires in subalpine forests. However, even repeated high‐severity fires over millennia probably did not deplete C or N stocks, because centuries between high‐severity fires allowed for sufficient biomass recovery.