Data from: Fire-regime complacency and sensitivity to centennial- through millennial-scale climate change in Rocky Mountain subalpine forests, Colorado, U.S.A.

1. Key uncertainties in anticipating future fire regimes are their
sensitivity to climate change, and the degree to which climate will impact
fire regimes directly, through increasing the probability of fire, versus
indirectly, through changes in vegetation and landscape flammability. 2.
We studied the sensitivity of subalpine forest fire regimes (i.e., fire
frequency, fire severity) to previously documented climate variability
over the past 6000 years, utilizing pollen and macroscopic charcoal from
high-resolution lake-sediment records in Rocky Mountain National Park,
Colorado. We combined data from the four lakes to provide composite
records of vegetation and fire history within a 200 km2 study area. 3.
Rates of forest burning were relatively complacent to millennial-scale
summer cooling and decreased effective moisture. Mean return intervals
between fire episodes, defined over 500-year periods, generally varied
between 150-250 years, consistent with tree-ring-based estimates spanning
recent centuries. Variability around these long-term means, however, was
significantly correlated with variability in summer moisture (i.e., more
burning with drier summers), inferred from existing lake-level and
supporting paleoenvironmental records. 4. The most pronounced change in
fire regimes was in response to decreased subalpine forest density ca.
2400 cal. year BP, itself a response to regional cooling. This indirect
impact of climate was followed by a decrease in charcoal production per
fire, a proxy for crown-fire severity, while the long-term rate of burning
remained unchanged. Over the last 1500 years, increased summer evaporation
and drought frequency were associated with increased fire severity,
highlighting a direct link between fire and climate. 5. Synthesis:
Subalpine forest fire history reveals complacency and sensitivity of fire
regimes to changing vegetation and hydroclimate over the past 6000 years.
Complacency is highlighted by non-varying fire frequency over millennia.
Sensitivity is evident through changes in biomass burned per fire (and
inferred fire severity), in response to climate-induced changes in forest
density and, more recently, increased summer drought. Overall, the paleo
record suggests that (i) fire severity may be more responsive to climate
change than fire frequency in Rocky Mountain subalpine forests, and (ii)
the indirect impacts of climate on vegetation and fuels are important
mechanisms determining fire-regime response to climate change.