Impacts of an extreme early-season freeze event in the Interior Pacific Northwest (30 October-3 November 2002) on Western juniper woodlands

Abstract

In mid-autumn 2002, an exceptional 5-day cold spell affected much of the interior Pacific Northwest, with minimum temperatures averaging 13[degree]C below long-term means (1953-21)112). On 31 October, minimum temperature records occurred at 98 of the 106 recording stations, with records lowered in some locations by 9[degree]C. Calculation of recurrence intervals of minimum temperatures shows that 50[percent][percent] of the stations experienced a >500-yr event. The synoptic conditions responsible were the development of a pronounced high pressure ridge over western Canada and an intense low pressure area centered in the Intermountain West that promoted strong northeasterly winds. The cold spell occurred near the end of the growing season for an ecologically critical and dominant tree species of the interior Pacific Northwest--western juniper--and followed an extended period of severe drought. In spring 2003, it became apparent that the cold had caused high rates of tree mortality and canopy dieback in a species that is remarkable for its longevity and resistance to climatic stress. The cold event altered western juniper dominance in some areas, and this alteration may have long-tern impacts on water budgets, fire intensities and frequencies, animal species interrelationships, and interspecific competition among plant species.