Forest Insect and Disease Newsletter

Uptick in conifer mortality is drought-related

We've all noticed it around Minnesota: a few more mature conifers than usual have died in the last couple of years. Is this related to our recent droughts? Yes, definitely. Eight of the last 11 years have been very dry during the summer and fall, plunging many forested areas of the state into severe drought.

Spruce, fir, and tamarack growing along the edge of lakes, wetlands, and ditches have been victims of fluctuating water tables over the last few years. When water is overly abundant, root systems are flooded for weeks lose their lowermost roots. Then, when water is scarce, these root systems don't extend down far enough to reach the low water table. With too much or too little water, tree health and vigor suffers as photosynthesis shuts down and reserve sugars and starches are used up. In a few droughty growing seasons, these trees, especially balsam fir and white and black spruce, die of starvation.

The situation is similar for red, white and jack pine except that root death due to fluctuating water tables is not involved. Photosynthesis was shut down for days and weeks during droughty growing seasons in the past decade. To make up for this, trees used up their reserve sugars and starches in order to remain alive. They couldn't resupply their reserves in the following years because they were also droughty. Eventually, reserves were drawn down too far and they also died of starvation.

For most of Minnesota, the 2013 spring and early summer were wonderful for tree growth and the restoration of sugar and starch reserves. However, the summer and autumn were hot and dry, leaving a prolonged drought in several forested areas of the state as we head into the winter months.

Click to enlarge

Total precipitation departure from normal map Jun 26, 2013- Nov 5, 2013 Total precipitation departure from normal map Jun 26, 2013- Nov 5, 2013