Holocene Vegetation and Climate Dynamics at Two Treeline Lakes, Northwest Territories

Abstract

Understanding how northern ecosystems responded to past climate change is critical for predicting ecological responses to modern climate change. This thesis reconstructed vegetative responses to Holocene (natural), and modern (anthropogenic) climate change. Sediments were analyzed from Queen’s Lake and McMaster Lake, located ~ 2 kilometers (km) from one another and ~25 km north of the boreal treeline in the shrub tundra, Northwest Territories. Sediments were analyzed for pollen, sedimentary DNA (sedaDNA), organic content, and magnetic susceptibility. Radiometric ages were calculated to infer temporal ecological shifts. In the early Holocene, ~10,500 calibrated years before present (cal yr BP), the climate was cold, organic content in lake sediments was low, and the landscape included herbaceous and shrub tundra. By ~8000 cal yr BP, organic content increased in response to gradual warming. By ~7300 cal yr BP, Picea pollen abundance began to rise at Queen’s Lake, indicating northward advancement of the boreal treeline. Picea pollen continued to rise throughout the mid-Holocene, peaking at ~5600 cal yr BP. This aligns with the DNA record, with Pinaceae DNA being detected at ~5850 cal yr BP, confirming the local presence of trees at the study site. A mix of shrubs, grasses, moss, ferns, and wetland taxa, were also identified in the pollen and DNA records in the mid-Holocene, suggesting that a structurally complex and dynamic vegetative ecosystem had developed. This ecosystem supported large herbivores and predators, with Canis lupus DNA found at ~5850 cal yr BP, and Cervidae and Rangifer tarandus DNA found between ~5380 – 2360 cal yr BP. Following Neoglacial cooling in the late Holocene, the treeline retreated southward to its modern position. However, these results may be limited by poor sedaDNA preservation. Modern warming is occurring at a faster pace than mid-Holocene warming. Despite rapid warming, modern pollen records do not exhibit meaningful or straightforward shifts. Subtle vegetation changes are captured at Queen’s Lake, with an increase in Betula pollen in ~1980. Meanwhile, McMaster Lake shows a decline in shrubs and an increase in Pinus pollen in ~1960. Further monitoring is required to fully understand the fate of boreal ecosystems under a changing climate.