A Proxy Record of Drought Severity for the Southwestern Canadian Plains

This paper examines the statistical relationships between Palmer Drought Severity Index (PDSI) data from the southwestern Canadian plains and tree-ring index chronologies from nearby sites. Standardized tree-ring widths from white spruce (Picea glauca) from the Cypress Hills (Alberta and Saskatchewan) and from lodgepole pine (Pinus contorta) from the Bears Paw Mountains (Montana) account for 47% and 39% of the variance in regional July PDSI. The corresponding regressions are the basis for the first reconstructions of PDSI for the Canadian plains. This proxy PDSI record extends from 1597 and demonstrates that, although 1937 was the worst single drought year, the 20th century lacked the prolonged droughts of the 18th and 19th centuries, when decades of July PDSI were consistently below zero. Clusters of drought years in the 1690s, 1720s, 1750s–60s, 1790s–1800s, 1820s, 1850s–60s and 1890s, support the notion of a 20- to 25-year drought cycle for the northern Great Plains. These prolonged droughts lower the resistance of ecosystems and soil landscapes to disturbance from hydroclimatic events, such that thresholds of landscape change are exceeded and the recovery of natural systems can take decades or centuries. These droughts also seriously affect the soil and water resources that support dryland agriculture.

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