The last few weeks in January are always some of the coldest in Minnesota. While we are hunkered inside staying warm, trees do not have the benefit of escaping the cold.
Deciduous trees in Minnesota, such as maples and oaks, adapt to the winter by dropping their leaves in the fall. Conifers like spruces and firs retain their needles and can continue to photosynthesize over the winter to help them survive.
Minnesota's climate shapes the kinds of trees that grow in our forests. Minnesota’s growing season simply is not long enough for trees like bald cypress and live oaks to thrive. Because of this, Minnesota has fewer tree species than states to our south. Although the cold limits the range and growth ability for trees, there are several benefits of cold weather for Minnesota trees that other states don’t have.
One of the greatest benefits of cold weather is the impact it has on many insect populations. As an example, the emerald ash borer is currently found in 30 Minnesota counties but has yet to reach the far northern counties in the state. It is of great concern if the insect reaches northern Minnesota because of the expansive ash forests found in the region, representing one billion ash trees in the state.
Emerald ash borer larvae overwinter beneath the bark of ash trees. Research has shown that if the temperature reaches -20 degrees Fahrenheit, 50% of the EAB larvae will die. If the temperature reaches -30 degrees Fahrenheit, 90% of the EAB larvae will die. Compared to the low temperatures that much of northern Minnesota is seeing this week, these temperatures can help to set back the spread of EAB in northern latitudes.
Inside the forest, many different factors influence whether or not EAB larvae can survive. The location of the tree (for example, whether it's found on a north- or south-facing slope), the thickness of the tree’s bark, and the snow depth also influence whether or not EAB will survive at cold temperatures.
The other benefit is that EAB is less vigorous in northern climates compared to more southerly ones. In northern latitudes, EAB larvae are smaller, take longer to develop into insects, and produce fewer eggs.
While cold weather can slow the spread of EAB, there is consensus that the climate alone may not eliminate EAB from Minnesota’s landscape. However, cold temperatures in northern parts of the state have helped to limit the spread of EAB into its expansive ash forests.
For the science behind how trees adapt to winter, Minute Earth has put together an excellent video. To learn more about keeping ash trees healthy for future generations, read Managing Ash Woodlands: Recommendations for Minnesota Woodland Owners, Extension’s resource for woodland owners with ash trees on their land.