Wildfires strongly influence forest ecosystem processes, including carbon and nutrient cycling and vegetation dynamics. As fire activity increases under changing climate conditions, the ecological and biogeochemical resilience of many forest ecosystems remains unknown. To investigate the resilience of forest ecosystems to changing climate and wildfire activity over decades to millennia, we developed a 4800-yr high-resolution lake-sediment record from Silver Lake, Montana, USA (47.360° N, 115.566° W). Charcoal particles, pollen grains, element concentrations, and stable isotopes of C and N serve as proxies of past changes in fire, vegetation, and ecosystem processes such as nitrogen cycling and soil erosion, within a small subalpine forest watershed. A published lake-level history from Silver Lake provides a local record of paleohydrology. A trend toward increased effective moisture over the late Holocene coincided with a distinct shift in the pollen assemblage c. 1900 yr BP, resulting from increased subalpine conifer abundance. Fire activity, inferred from peaks in macroscopic charcoal, decreased significantly after 1900 yr BP, from one fire event every 126 yr (83–184 yr, 95% CI) from 4800–1900 yr BP, to one event every 223 yr (175–280 yr) from 1900 yr BP to present. Across the record, individual fire events were followed by two distinct decadal-scale biogeochemical responses, reflecting differences in ecosystem impacts of fires on watershed processes. These distinct biogeochemical responses were interpreted as reflecting fire severity, highlighting (i) erosion, likely from large or high-severity fires, and (ii) nutrient transfers and enhanced within-lake productivity, likely from lower-severity or patchier fires. Biogeochemical and vegetation proxies returned to pre-fire values within decades regardless of the nature of fire effects. Paleo records of fire and ecosystem responses provide a novel view revealing past variability in fire effects, analogous to spatial variability in fire severity observed within contemporary wildfires. Overall, the paleo record highlights ecosystem resilience to fire across long-term variability in climate and fire activity. Higher fire frequencies in past millennia relative to the 20th and 21st centuries suggest that northern Rocky Mountain subalpine ecosystems could remain resilient to future increases in fire activity, provided continued ecosystem recovery within decades.

Data are paleoenvironmental proxies from a lake-sediment core collected at Silver Lake, Montana, USA, in 2017 (47.360° N, 115.566° W). Sediments ages were estimated from a bayesian age-depth model developed in rbacon using dates from Pb210 analysis of near-surface sediments and radiocarbon dating of terrestrial macrofossils or concentrated charcoal. Sediment subsamples were taken contiguously at 0.5-cm intervals throughout the core, representing an average of 10 years per sample, for macroscopic charcoal analysis, XRF analysis, and elemental and stable isotope analysis of C and N. Pollen grains were extracted and identified at c. 100 yr intervals to characterize the relative abundances of terrestrial pollen types over time. Additional data from published studies were also used in the analysis: 

All analyses are conducted in R v. 4.2.1.