Adapted from the Climatology Work Group
Significant precipitation shortfalls were reported across Minnesota over the period from late-July through the end of November. See maps below. Large sections of north central and northeast Minnesota are said to be undergoing 'Moderate' to 'Severe Drought' by the U.S. Drought Monitor. Stream flow and lake levels are very low due to the ongoing impact of precipitation deficits accrued during the 2010 growing season and spotty rainfall that fell during this growing season. In southern and western counties, large areas are also depicted in the 'Moderate and Severe Drought' categories. Much of this region reported significant late-summer and autumn precipitation shortfalls (maps below). Precipitation totals for the past eighteen weeks are less than three inches in many areas, a negative departure from the long-term eighteen-week average of five to nine inches. When compared with the same eighteen-week period in the historical database, the 2011 precipitation totals rank among the lowest on record.
In addition to that, the fall months were warmer than usual. October of 2011 is ranked in the top ten warmest months on record and November is ranked the warmest November to date. Temperatures were 4 to 6 degrees above normal across the state. In biological terms, it is likely that these warmer temperatures facilitated root growth in all species. Warmer temperatures increased transpiration in conifers and combining that with soil water deficits, the effect was to create physiological stress.
Going into the winter, trees will be drought-stressed all across Minnesota. That will mean that they are more vulnerable to opportunistic insects such as bark beetles in conifers, bronze birch borers, two-lined chestnut borers in oak and bronze aspen borer in trembling aspen. If the spring weather is warm and dry, the opportunists will flourish, build populations and start to cause mortality by the autumn of 2012. If the spring weather is cool and rainy, the trees will have an opportunity to rebuild their water reserves and repel the opportunists. We'll just have to "wait and see".