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°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°C. Calculation of recurrence intervals of minimum temperatures shows that 50%% 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.

Learning from single extreme events

“…Opportunistic single-event studies sometimes use a hybrid definition (area I in figure 2a, [5]). For example, Knapp & Soule [13] examined the spatial extent of an extreme frost event after it had led to widespread tree mortality in the Pacific Northwest of the United States. Opportunistic single-event studies show what can happen to a system under certain climatic conditions.…”

Topoedaphic and morphological complexity of foliar damage and mortality within western juniper (Juniperus occidentalis var. occidentalis) woodlands following an extreme meteorological event

“…, 2004), and western juniper trees growing in areas where the freeze was not as extreme were not affected (see Fig. 3 in Knapp & Soulé, 2005) despite similar drought conditions. Further, the visible damage occurred after the freeze, and damaged trees were still growing (producing annual growth rings) in 2003 and 2004.…”

“…occidentalis ). Temperatures below −15°C were recorded at numerous climate stations east of the Cascade Mountains from 40 to 45° N, with some stations recording temperatures below −20°C (Knapp & Soulé, 2005). While these low temperatures are common within the range of western juniper during winter, they are extremely rare during autumn.…”

“…In a prior study, we documented the meteorological parameters of the extreme freeze, showing that it was an unusual event with a statistical recurrence interval, calculated for daily low temperature on 31 October 2002, often in excess of 500 years (Knapp & Soulé, 2005). Disturbances of this magnitude are notable from an ecological perspective because they may help explain both species range distributions and complex landscape patterns not easily attributed to smaller and more frequent disturbances.…”

Climate change effects on vegetation in the Pacific Northwest: a review and synthesis of the scientific literature and simulation model projections

“…Runyon et al (1994) reported cold winter temperatures to be almost as important as drought for limiting photosynthesis in juniper woodlands. Extreme cold temperatures can also function as a disturbance agent (Knapp and Soulé 2005).…”