Changes in the parkland-boreal forest boundary in northwestern Ontario over the Holocene

Changes in vegetation were tracked from a well-dated sediment core from a boreal lake, Lake 239, at ∼200-year resolution over the Holocene. This presently oligotrophic lake is located ∼100-km east from the present-day parkland-forest ecotone in northwestern Ontario. Near-shore sediment core transects from Lake 239 have previously shown this lake was at least 8-m lower than present in the mid-Holocene, or ∼58% less lake volume in comparison to today. Large shifts were expected in the terrestrial vegetation if the low lake levels were related to climate. The core from Lake 239 shows increases in the relative abundance and concentration of pollen such as Cupressaceae and Ambrosia, indicating a more open boreal forest between ∼4500–8000 cal yr BP. Pollen-based inferences of average, summer and winter temperatures suggest that temperatures were on average up to 1–2 °C warmer than today, with winter temperatures up to 4 °C warmer. The pollen inferences also suggest enhanced precipitation, likely in the summer, but with an overall increase in evaporation and evapotranspiration resulting in reduced effective moisture. To assess regional climate changes, pollen-based reconstructions of temperature and precipitation were developed and synthesized from sediment cores from eight previously published lakes, from which pollen sites were available to both the west and east of Lake 239, spanning present-day prairie lakes to forested lakes up to 300 km east of the prairie-boreal ecotone. All sites show shifts in pollen assemblages that indicate a warm mid-Holocene period; prairie sites west of the Experimental Lakes Area (ELA) show mid-Holocene decreases in precipitation relative to today, whereas sites near or east of ELA show consistent increases in precipitation, but with increased temperatures and enhanced evaporation during the mid-Holocene.

► Increases in Cupressaceae and Ambrosia indicate an open boreal forest between ∼4500–8000 cal yr BP.
► Pollen inferences indicate temperatures up to 1–2 °C warmer, with winter temperatures up to 4 °C warmer.
► Inferences also suggest enhanced precipitation, but with increased evapotranspiration resulting in reduced moisture.
► Pollen sites to the west/east all show shifts in pollen assemblages that indicate a warm mid-Holocene period.