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  1. Abstract

    Increasing area burned across western North America raises questions about the precedence and magnitude of changes in fire activity, relative to the historical range of variability (HRV) that ecosystems experienced over recent centuries and millennia. Paleoecological records of past fire occurrence provide context for contemporary changes in ecosystems characterized by infrequent, high-severity fire regimes. Here we present a network of 12 fire-history records derived from macroscopic charcoal preserved in sediments of small subalpine lakes within a c. 10 000 km2landscape in the U.S. northern Rocky Mountains (Northern Rockies). We used this network to characterize landscape-scale burning over the past 2500 yr, and to evaluate the precedence of widespread regional burning experienced in the early 20th and 21st centuries. We further compare the Northern Rockies fire history to a previously published network of fire-history records in the Southern Rockies. In Northern Rockies subalpine forests, widespread fire activity was strongly linked to seasonal climate conditions, in contemporary, historical, and paleo records. The average estimated fire rotation period (FRP) over the past 2500 yr was 164 yr (HRV: 127–225 yr), while the contemporary FRP from 1900 to 2021 CE was 215 yr. Thus, extensive regional burning in the early 20th century (e.g. 1910 CE) and in recent decades remains within the HRV of recent millennia. Results from the Northern Rockies contrast with the Southern Rockies, which burned with less frequency on average over the past 2500 yr, and where 21st-century burning has exceeded the HRV. Our results support expectations that Northern Rockies fire activity will continue to increase with climatic warming, surpassing historical burning if more than one exceptional fire year akin to 1910 occurs within the next several decades. The ecological consequences of climatic warming in subalpine forests will depend, in large part, on the magnitude of fire-regime changes relative to the past.

     
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  2. Abstract

    Changes in vegetation in North America indicate Holocene shifts in the latitudinal temperature gradient along the western margin of the North Atlantic. Tree taxa such as oak (Quercus) and birch (Betula) experienced opposing directions of change across different latitudes consistent with changes in temperature gradient steepness. Pollen‐inferred temperatures from 34 sites quantify the gradient changes and reconstruct a long‐term northward steepening in summer and southward steepening in winter. From 4.8 to 3.8 ka, an oscillation in tree distributions interrupted the long‐term trends as a steep temperature gradient developed north of 43.5°N. The shift likely limited cold outbreaks to the south, producing anomalously high summer temperatures at 42–43.5°N, and enabling a northward expansion of oak forests. The forest and temperature gradient changes appear consistent with orbital and ice sheet forcing as well as millennial variability in the North Atlantic pressure field analogous to the North Atlantic Oscillation on interannual time scales.

     
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  3. Abstract The wettest portion of the interior of western North America centers on the mountainous region spanning western Montana, Idaho, British Columbia, and Alberta. Inland ranges there capture the remnants of Pacific storms. Steep east–west hydroclimate gradients make the region sensitive to changes in inland-penetrating moisture that may have varied greatly during the Holocene. To investigate potential hydroclimate change, we produced a 7600-yr lake-level reconstruction from Silver Lake, located on the Montana–Idaho border. Ground-penetrating radar profiles and a transect of four shallow-water sediment cores that were dated using radiocarbon dating and tephrachronology revealed substantial changes in moisture through time. An organic-rich mud unit indicating wet and similar to modern conditions prior to 7000 cal yr BP is overlain by an erosional surface signifying drier than modern conditions from 7000–2800 cal yr BP. A subsequent time-transgressive increase in water levels from 2800–2300 cal yr BP is indicated by a layer of late Holocene muds, and is consistent with glacier expansion and increases in the abundance of mesic tree taxa in the region. Millennial-scale trends were likely driven by variations in orbital-scale forcing during the Holocene, but the regional outcomes probably depended upon factors such as the strength of the Aleutian Low, Pacific sea-surface temperature variability, and the frequency of atmospheric rivers over western North America. 
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  4. Abstract

    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‐year 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 palaeohydrology.

    A trend towards 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 to 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.

    Synthesis. Palaeorecords 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 palaeorecord 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 century suggest that northern Rocky Mountain subalpine ecosystems could remain resilient to future increases in fire activity, provided continued ecosystem recovery within decades.

     
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  5. Abstract. The use of the climatic anomaly known as the “4.2 ka event” as thestratigraphic division between the middle and late Holocene has prompteddebate over its impact, geographic pattern, and significance. The anomalyhas primarily been described as abrupt drying in the Northern Hemisphere atca. 4 ka (ka, thousands of years before present), but evidence of thehydroclimate change is inconsistent among sites both globally and withinNorth America. Climate records from the southern Rocky Mountains demonstratethe challenge with diagnosing the extent and severity of the anomaly.Dune-field chronologies and a pollen record in southeastern Wyoming revealseveral centuries of low moisture at around 4.2 ka, and prominent low standsin lakes in Colorado suggest the drought was unique amid Holocenevariability, but detailed carbonate oxygen isotope (δ18Ocarb) records from Colorado do not record drought at the sametime. We find new evidence from δ18Ocarb in a smallmountain lake in southeastern Wyoming of an abrupt reduction in effectivemoisture or snowpack from approximately 4.2–4 ka, which coincides in timewith the other evidence of regional drying from the southern Rocky Mountainsand the western Great Plains. We find that the δ18Ocarb inour record may reflect cool-season inputs into the lake, which do not appearto track the strong enrichment of heavy oxygen by evaporation during summermonths today. The modern relationship differs from some widely appliedconceptual models of lake–isotope systems and may indicate reduced winterprecipitation rather than enhanced evaporation at ca. 4.2 ka.Inconsistencies among the North American records, particularly in δ18Ocarb trends, thus show that site-specific factors can preventidentification of the patterns of multi-century drought. However, theprominence of the drought at ca. 4 ka among a growing number of sites in theNorth American interior suggests it was a regionally substantial climateevent amid other Holocene variability. 
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