Long-term interactions among climate, fire, and biogeochemical cycling in a Rocky Mountain subalpine watershed

Background/Question/Methods

Increasing fire activity in western U.S. forests has been tightly linked to warmer and drier growing seasons, raising concerns about long-term feedbacks among climate, fire, vegetation, and biogeochemical cycling. To improve understanding of the ecosystem impacts of fire over long time scales, we examined a 4,800-year-long sediment record from Chickaree Lake, a subalpine lake in Rocky Mountain National Park, Colorado. Here, we present a high-resolution (~ 4 yr / sample) sediment record of fire, inferred from macroscopic charcoal, and carbon (C) and nitrogen (N) dynamics, inferred from elemental and isotopic analyses, in an ecosystem shaped by stand-replacing fire. Nitrogen losses associated with disturbance and high N availability increase the ratio of heavy-to-light N isotopes (δ¹⁵N), while low N availability tends to decrease the δ¹⁵N of N pools. Therefore, we hypothesized that stand-replacing fires would be recorded in sediment δ¹⁵N as short-term increases caused by N losses, followed by longer-term decreases reflecting early successional demand. The magnitude of these shifts, in addition to changes in %C, %N, and magnetic susceptibility, may offer a proxy of fire severity. We further hypothesized that high fire frequency would cause centennial-scale declines in ecosystem N, represented by a downward trend in δ¹⁵N.

Results/Conclusions

Fossil pollen indicates that the watershed was dominated by Pinus contorta throughout the record. Fire return intervals (FRI; yr/fire) inferred from statistically identified peaks in charcoal accumulation rates varied from 20 to 370 years. Millennial scale mean FRIs increased from 77 yr/fire (95% CI 42-118) between 4800 and 4000 yr before present (yrBP), to 156 yr/fire (95% CI 70-273) from 1000 yrBP to present. Fire frequency decline coincided with a regional increase in effective moisture, inferred from independent paleoclimate records, consistent with other records from subalpine forests. Roughly half of the inferred fires were associated with significant declines in %C and %N and sharp increases in sediment magnetic susceptibility, an indicator of mineral soil erosion, suggesting high-severity fires within the watershed. Post-fire reductions in C and N were as high as 88%, and 76% respectively. Sharp increases and decreases in δ¹⁵N during the period of peak fire frequency were consistent with our short-term hypothesis. Moreover, a pronounced downward trend in δ¹⁵N from 4800 to 3800 yrBP suggests an N cycle progressively tightened by repeated N losses to fire. Our results suggest that changes in fire frequency may delay recovery of forest N stocks, potentially affecting subalpine productivity over centuries.