As of mid-July 2023, just about all of forested Minnesota is facing abnormally dry or drought conditions. While recent rains have reduced those concerns in some areas, drought conditions could return. This is the third consecutive year of drought across much of the state, and unfortunately, successive years of drought can compound stress on trees. This post includes basic information for woodland stewards concerned about the health of their trees.
How does drought affect trees?
Trees depend on a steady supply of water in the soil to enable photosynthesis. Although mature trees tend to have extensive root systems, prolonged and severe drought can limit water availability even to large, mature trees. Of course, smaller trees with more limited root systems are also at risk. Trees located on sandy soils are at a higher risk of drought impacts compared to finer-textured soils like loams and clays.
When water is limited, trees are forced to close their stomata (the small pores on the leaves) to retain water and avoid drying out. Unfortunately, keeping stomata closed interferes with the tree’s ability to photosynthesize, producing energy needed for growth, defense against insects and disease, and other essential functions.
This, in turn, can make trees more vulnerable to insect pests and diseases that are normally innocuous. For example, native insects like bronze birch borer and two-lined chestnut borer (TLCB) rarely kill trees, but when birches and oaks are stressed, TLCB can push them over the edge. The same is true of bark beetles on red pine: native Ips beetles generally don’t kill red pine when conditions are favorable to tree growth, but when the trees are stressed — say by an extended drought — bark beetles can overwhelm tree defenses and cause extensive mortality.
Diseases like cytospora and rhizosphaera needle cast on spruce can also become more serious during a drought.
What factors other than drought can produce similar symptoms?
Not all tree decline during a drought is drought-related. For instance, if our current drought continues it may coincide with the annual loss of some northern red oak trees to oak wilt, which is typically most visible in midsummer. Oak wilt is a fungal disease spread by sap-feeding beetles. Sadly, of course many ash trees across southern Minnesota are dying from emerald ash borer, a process that will continue with or without continued drought.
What can be done to keep trees and woodlands healthy?
If possible, avoid adding further stress by not pruning, thinning, or wounding trees during a severe drought. (The Minnesota Department of Natural Resources recommends avoiding thinning woodlands for a year after a drought.)
Water yard trees and seedlings with limited root systems if you can. You can also manage woodlands to increase adaptive capacity and ecological resilience: in at least some forest ecosystems, lower stocking (lower vegetation density) has been shown to reduce drought risk and improve forest resilience.
If you have trees killed by bark beetles or oak wilt, remove the affected wood as soon as possible and before the next growing season to avoid further spread.
Managing invasive plants can reduce competition within a forest. In addition, there’s some research that allelopathic properties of some invasive plants can increase because of drought so removing or treating those plants may be particularly beneficial.
Don't forget to keep yourself healthy! Make sure you know the signs of heat-related illness if working in the woods during hot weather, and take precautions to stay healthy and hydrated. And if the drought is getting you down, check out this article: Drought, smoke and climate change getting you down? You’re not alone.
A good source of drought status information
The U.S. Drought Monitor provides weekly updates on drought maps across the U.S., and here in Minnesota.