Biogeochemical response to fire over six millennia in a Rocky Mountain subalpine forest

Background/Question/Methods

Increasing fire activity linked to warm, dry conditions in western U.S. forests has raised concerns about feedbacks among climate, fire, and biogeochemical cycles. Fires disrupt C and N cycling, yet little is known about the ecosystem impacts of fire over multi-decadal to millennial time scales. We used high-resolution lake-sediment records (~5 yr/sample) of macroscopic charcoal, elemental and isotopic composition, and magnetic susceptibility (MS) from subalpine Chickaree Lake, Colorado, to help elucidate the ecosystem effects of fire over decades to centuries. Because the N stable isotope ratio (δ¹⁵N) of bulk organic matter tends to rise with N loss and availability, we predicted that high-severity fires would result in isotopic enrichment followed by decline during forest succession. We hypothesized that post-fire changes in sediment C, N, C:N ratio, and MS would provide indicators of organic matter loss and post-fire erosion. Fire occurrence and return intervals (FRI; yrs/fire) were determined using statistically identified peaks in charcoal accumulation (CHAR). The biogeochemical response to fires was evaluated via Superposed Epoch Analysis of high-severity events within the catchment, inferred from coincident peaks in CHAR and MS, an indicator of soil erosion. We tested the significance of the average responses using Monte Carlo-derived 95% confidence intervals.

Results/Conclusions

The 6,200-year record revealed 1000-year mean FRIs ranging from 81 yr/fire (0.95 CI 38-134) to 138 yr/fire (CI 60-246). Sediment biogeochemistry is consistent with regulation by inputs from the lodgepole pine-dominated forests around the lake, suggesting that fires within the watershed would leave sedimentary signatures. Significant post-fire declines in %C, %N, and C:N reflect consumption of forest organic matter (OM), dilution of sediment OM by erosion of mineral soils, and a dramatic reduction in the flux of high-C:N terrestrial OM to the lake. Multi-decadal recovery of %C, %N, and C:N following fire suggests a record of successional C and N accumulation. High-severity fires (n=11) over the last 4200 years were followed by significant increases in δ¹⁵N, suggesting preferential loss of ¹⁴N to volatilization and post-fire processes and/or an influx of ¹⁵N-enriched soil and charred material to the lake. Post-fire δ¹⁵N was positively correlated with CHAR, indicating that higher-severity events resulted in greater isotopic enrichment. Significant δ¹⁵N decline ~ 40-70 years after fires is consistent with an N cycle tightened by forest aggradation. Our results indicate that high-resolution analysis of lake-sediment biogeochemical proxies offers a promising avenue for investigating the impacts of fire on forest and lake ecosystems.