Forest Insect and Disease Newsletter

Severe winter burn of conifers

image: Winter injury on white spruce Cook Co. May 14, 2012

Click to enlarge

image:  Winter injury in white spruce plantation Portage Brook Cook Co.

Click to enlarge

image: Near Thunder Bay Ontario

Click to enlarge

image: Winter injury Cook Co MN

Click to enlarge

Some winter burn or winter injury of conifers occurs every year. The buds are almost always healthy and by July the damage is covered up by the new growth and mostly forgotten. This year, however, some very severe winter injury including mortality has occurred in the northern portions of St Louis, Lake and Cook Counties. Aerial photos from Canada look even worse.

The damage in northern Cook County is most severe on white spruce with less amounts of damage on balsam fir and jack, white and red pines. The buds on most of the pines are healthy and the trees are now growing new shoots and needles. A lot of the white spruce also have live, healthy buds, but the buds on some of the white spruce are all dead. The needles on the white spruce with the most severe winter burn have all turned brown and are now falling off the tree. Since all the buds on these trees are dead, the tree will die. Many of the white spruce with winter burn that have healthy buds will survive as long as rainfall is adequate this summer.

Winter burn in the forest occurs when the needles lose too much of their moisture during the winter. This happens when the needle temperature gets above freezing and needles dry out, while the root system is still frozen in the soil and the moisture can't be replaced in the needles. Sun reflecting off the snow onto the needles above the snow line can also warm up the needles, increasing water loss.

Parts of NE Minnesota such as Cook County went through last summer and into the winter in drought. They were still listed as being in severe drought by the Long-term Palmer Drought Severity Index in early May; the trees couldn't get much water out of the soil to start with and then we had many days of warm temperatures during the winter when needles dried out. On top of that, we had record-setting warm weather starting in March when the ground was still frozen, so we ended up with unusually severe winter burn and tree mortality in some locations.

 

The drought goes on and on and on and...

At least the rains in the central part of the state have alleviated the long-term drought conditions. So much so that in many counties we are expecting the recent precipitation to amplify the leaf disease situation. In the northwest, soil moisture conditions have deteriorated and the drought has deepened.

Weekly Precipitation and Departure from Normal Maps Produced by the MN Climatology Group

Minnesota Drought Maps as of May 21, 2012

US Drought Monitor Map- Minnesota as of May 29, 2012