Forecasting mountain pine beetle overwintering mortality in a variable environment

A landscape-scale ecophysiological model of mountain pine beetle (MPB) overwintering mortality was developed, validated and communicated. The model was field-validated in Alberta during the winters of 2006-07 and 2007-08. Key model assumptions were tested experimentally during the winter of 2007-08. The model was operationalized for forecasting in Alberta in early 2008 and key outputs (maps and time-series) communicated to the public through a new Natural Resources Canada website. The model describes daily overwintering mortality in the above-snow component of the MPB population. Field and lab tests suggest the model should perform reasonably well during winters characterized by at least one severe cold snap. In 2006-07 the overwintering mortality rate across Alberta was predicted to be 79%, in close agreement with the observed survival rate of 81%. The model suggested that most of the mortality occurred in a single pulse late November 2006, when temperatures across the province dropped suddenly to a winter low between -32°C and -38°C. The following year, in 2007-08, a higher level of mortality was predicted and observed, largely a result of the severe cold snap of late January 2008, when temperatures dropped to a winter low between -35°C and -47°C. In both winters, mortality was predicted and observed to be much higher in northern than in southern Alberta. Despite the overall predictive power of the model, there is substantial unexplained variation in observed mortality. Model performance in relatively mild conditions also remains to be tested. These are two issues that require further research. Putting these observations in context, a retrospective analysis of historical beetle winter weather in Alberta (1951-2008) indicates that the last two winters represent a temporary reversion back to "normal" (i.e., 1980s-style) winter climatic conditions. A return in the coming years to a positive warming trend in winter temperatures would pose a risk of increased potential of outbreaks and eastward range expansion.