Drought summary for Minnesota's forests
By Brian Schwingle, Central Region forest health specialist
The entire state of Minnesota, minus Wabasha, Winona, and Houston counties in the southeastern toe of the state, experienced a serious drought this year from May through July. It was hard on trees, and the effects will last for several years.
The six-month growing season from April through September was the ninth driest on record. May through July in the central third of Minnesota was the third driest on record. The 2021 drought hit the northern third of Minnesota the hardest—such dry conditions up north have not occurred since 1976. Compounding the 2021 impact on trees, northeastern Minnesota has experienced several consecutive years of dry growing seasons, not reaching the long-term average growing season precipitation since 2017.
Different tree species can tolerate different levels of drought, but three aspects of drought are true for all trees: (1) hotter temperatures make drought impacts worse; (2) if trees were stressed before 2021, the drought's impact on them was greater than if they were otherwise unstressed; and (3) severe drought stress lasts several years.
Since the impact of drought is worse on previously stressed trees, this is bad news for some trees in urban areas, campgrounds, and pastures, already stressed from compacted soils. One example is stressed urban maples in central Minnesota that developed dieback in prior years and developed even worse dieback in June and July of 2021. Another set of stressed trees that suffered disproportionately because of the drought were those growing near wetlands flooded in recent years. Many of those previously flooded trees died quickly this summer.
Look for more information on the impacts of drought in this issue.
Drought impacts on red pine
By Megan O'Neil, Northwest Region forest health specialist
A pocket of mature red pine attacked by bark beetles.
Almost the entire state of Minnesota was impacted by drought in the spring and summer of 2021. In the parts of the state most affected we can already see the result—stressed and dying trees. This article focuses on how drought affects red pine (also known as Norway pine) and what you can do and avoid doing to help these drought-stressed trees.
Red pine is the state tree. It can be found growing throughout the northern part of Minnesota, and is an important species, both economically and ecologically. It grows fast and straight in plantations and grows well in sandy soil, making it a desirable ornamental tree in some locations. In addition, red pine has few serious insect and disease issues.
Drought-stressed trees are not able to fend off attacks from insects and diseases that would normally not cause serious problems. During and after a drought, red pine is more susceptible to invasion by native bark beetles. These little beetles live under the bark and leave tiny, round exit holes anywhere on the tree. Bark beetles are found throughout our forests, and in normal conditions, they infest damaged trees and live in logging slash; healthy trees are not affected. During a drought, however, bark beetles are able to overcome the trees' defenses and eventually cause mortality by creating galleries under the bark that cut off the flow of water and nutrients throughout the tree. The beetles also carry blue stain fungi that accelerate the death of the tree and turn the wood blue-green, which can lower its value.
Red pine log covered in exit holes.
Pine bark beetle galleries.
Red pine logs on a landing showing blue stain caused by bark beetle infestation.
Another issue we see in drought-stressed red pine is Armillaria root rot. This is a native fungus that lives in the soil and decays the roots and lower trunk of the tree, eventually causing the trees to die. A healthy tree is able to resist colonization by Armillaria, but natural resistance is easy to overcome in a drought-stressed tree, which can be quickly killed.
For the most part, doing nothing is the best thing to do for your red pine plantation. Active management, such as thinning during a drought and in the year following, can stress the trees even more and make them more susceptible to insect and disease problems. Keeping trees heathy by making sure they are thinned properly before a drought ensures the best chance for survival. It's important to follow a management plan when working in red pine plantations. The Minnesota DNR has further guidelines on how to reduce potential damage of bark beetles.
Red pines in yards or other urban settings are susceptible to the same problems listed above and are usually even more stressed by being planted in an unnatural environment such as a lawn or near a house or other building. It is important to maintain tree health by eliminating the use of lawn chemicals and to mulch properly around yard trees. During a drought, consider watering the red pine around your house with a drip hose. Visit the UMN Extension's watering established trees and shrubs webpage to learn more about how and when to water your trees.
Drought impact on oaks
By Brian Schwingle, Central Region forest health specialist
From July through August this year, tops of scattered red oaks in previously thinned forests turned orange and died. Oaks that were already stressed and dying from flooding or construction damage completely died in July, and isolated oaks in forests with shaded, weakened crowns perished. We expect even more stressed and unstressed oaks to die in 2022 and 2023 because of the 2021 drought.
Not all the oaks suffering from drought in 2021 will die. Hundreds of bur oak canopies across central Minnesota rapidly changed to brown and dropped many of their leaves from the end of June to early July. Many bur oaks suffering from a year of drought stress will leaf out the following spring as if nothing happened to them. But many drought-stressed oaks are taken advantage of by two opportunistic native pests: twolined chestnut borer and Armillaria root disease.
The insect and disease often work in tandem. Generally, both Armillaria and twolined chestnut borer take more than one year to kill oaks. In extreme cases, oaks can die in one year, but many of the dead leaves cling in the canopy for months. Dead leaves remaining in the canopy is a characteristic symptom of twolined chestnut borer.
Widespread outbreaks of twolined chestnut borer and abundant mortality from Armillaria root disease cannot be controlled. However, both yard-tree owners and forest owners can do things to lessen the blow of drought.
An overwintering twolined chestnut borer larva. Larval feeding tunnels below the bark can be seen in the background.
Several red oaks dying from past flooding, current-year drought, and twolined chestnut borer infestation at the edge of a wetland in Morrison County in August 2021.
Armillaria root disease killed this red oak in Stearns County in late summer 2021.
Yard tree owners can simply water their oaks during dry conditions. This alleviates stress from drought and the problems that follow. Yard trees benefit most from deep watering once a week. An effective tool for watering trees is a drip-irrigation or soaker hose, placed under the edge of the canopy. If your local weather source indicates you're below the monthly average precipitation by an inch or more, slowly irrigate large oaks once every week for a few hours.
For the next few years, don't stress forest oaks any more than they already are by thinning or by driving heavy equipment in the woods when the soil is not frozen. If you want to salvage timber or firewood from oaks that died this year, cut them down during frozen-ground conditions. An oak that died this year probably has hundreds, if not thousands, of overwintering twolined chestnut borer larvae, so it's best to cut oak logs into lumber, kiln-dry, debark, or cut them up into firewood before next spring. This will help dry out the twolined chestnut borer larvae under the bark and prevent them from developing.
After several years of average precipitation, thinning an overcrowded younger oak forest will prepare it for the next drought. If the oaks are old, consider working with a professional forester to decide how best to regenerate your woods for future generations. Oaks don't grow forever, and at some advanced age, they become even more susceptible to environmental stresses, Armillaria root disease, and twolined chestnut borer.
Spruce budworm impact on northeast Minnesota
By Eric Otto, Northeast Region forest health specialist
Spruce budworm with feeding damage and webbing.
The size of the spruce budworm caterpillar may appear insignificant, but its present strength in numbers is not. Since 2018, the native caterpillar has defoliated or destroyed hundreds of thousands of acres of fir and spruce forests in northeastern Minnesota. The average yearly defoliation and mortality is approximately 282,000 acres since 2018. To put that into context, that's almost the size of Lower and Upper Red Lake combined.
Data on spruce budworm damage is collected each year by aerial surveys that map large-scale disturbances across Minnesota. In 2020, the COVID-19 pandemic restrictions prohibited the forest health team from doing aerial surveys, so we estimated the number of impacted acres mostly with ground surveys and a small number of aerial surveys that could be conducted.
The majority of the present outbreak is in Lake County and the western part of Cook County. In this year alone, spruce budworm defoliated and killed 384,000 acres of fir and spruce forests. Most (roughly 303,000) of those acres were defoliated, and about 81,000 acres had a mix of mortality and defoliation. This is the highest number of acres the caterpillar has impacted since 1995, when it damaged 506,000 acres in one year.
The past few years of dry weather, particularly in the spring and early summer, may have magnified the spruce budworm situation. Scientists hypothesize that warm, dry weather increases the survival of budworm larvae. Insect diseases such as fungal pathogens need cool and moist conditions to thrive, so fewer budworm larvae are attacked when the weather is warm and dry. These natural enemies help keep the budworm population at lower levels.
Not only were large numbers of trees damaged or killed, the drought, coupled with spruce budworm damage, possibly played a role in Minnesota's fire season. The drought of 2021 in northern Minnesota led to an extended fire season, and with the large amount of dying and dead fir and spruce trees in northeast Minnesota, people wonder if this is also enhancing the risk of fire. The largest fire in 2021 was the Greenwood Fire, burning about 27,000 acres. The fire was near Isabella, in the heart of the budworm outbreak, with a large amount of dying and dead fir and spruce trees. Dying trees and standing, recently dead trees with needles are fuel through which fire can more easily burn than if trees are healthy or on the ground and decaying. It seems probable that these conditions caused by spruce budworm altered the behavior of the Greenwood Fire.
The spruce budworm outbreak will probably continue to have a large impact next year as it moves farther into Cook County toward Grand Marais. Spruce budworm could wreak more havoc if another warm, dry spring occurs in 2022.
Managing spruce budworm can be tricky once defoliation has started. Landowners can pre-salvage vulnerable fir and spruce to capture their economic value if budworm has just started feeding on trees. Additionally, landowners can work with a DNR forester on a stewardship plan or work with their local Soil and Water Conservation District if their forests have a significant number of trees killed by spruce budworm.
To promote healthier and more rigorous forests less susceptible to budworm damage, research has shown that thinning white spruce plantations before outbreaks and reducing the quantity of older balsam fir in natural stands can be beneficial.
Finally, homeowners have a few additional options to protect trees and increase resiliency to budworm feeding. This includes promoting tree vigor with watering and mulching or using an insecticide such as Btk, if practical.
Good news: forests and spruce budworm have coexisted for thousands of years. Our forests have bounced back before, and they will again.
Invasive, deadly oak wilt confirmed for the first time in Cass County
By Rachael Dube, Northwest Region forest health specialist
Oak wilt, an invasive fungal disease that kills all species of oak in Minnesota, has been confirmed for the first time in Cass County on private lands. DNR Division of Forestry staff detected and University of Minnesota Plant Disease Clinic laboratory staff confirmed two infected oak trees, and DNR staff identified three additional pockets using aerial and ground surveys in a southeast township of Cass County. Most of the landowners have decided to control the disease with help from a Morrison County Soil and Water Conservation District grant. Earlier this summer, oak wilt was confirmed for the first time in Crow Wing County north of Brainerd, about 7 miles northeast of the pockets in Cass County.
Oak wilt has slowly been spreading north in Minnesota since the 1940s, and is now located in 41 counties. To prevent further spread, the forest health team reminds residents and visitors statewide to avoid pruning oak trees from April through July and to not move firewood.
Click to enlarge
The shaded area is the oak wilt high-risk zone and shows the known range of oak wilt in Minnesota as of October 2021. List of affected counties.
Forest health staff have observed that oak trees within a 20-mile radius of any known oak wilt location are at high risk of developing the disease, so with this new detection, additional oaks in much of Crow Wing and southern Cass counties are threatened. Moving oak firewood can spread oak wilt over long distances, so use only firewood with the Minnesota Department of Agriculture certified seal to prevent moving oak wilt. Oak wood seasoned for two years is no longer a threat for spreading oak wilt.
A good indicator of oak wilt is a carpet of fallen leaves under an oak in mid-summer, when leaves should still be on trees. Residents in Crow Wing and Cass counties should be on the lookout for this symptom next year and report possible oak wilt to the EDDMapS website or by contacting their local DNR Forestry office. Because oak wilt and other oak health issues have similar symptoms, sharing pictures of the tree is very helpful. We expect to see an increase in oak mortality from twolined chestnut borer attack and Armillaria root disease in the next few years in areas affected by drought.
For more details on oak wilt prevention and what to do with infected trees and wood, see the DNR's oak wilt management webpage. The Morrison County Soil and Water Conservation District, in cooperation with the DNR, has grant funding available from the Environment and Natural Resources Trust Fund to assist property owners on the northern edge of the oak wilt invasion, including Crow Wing and Cass counties.
Attack of the squirrels
By Brian Schwingle, Central Region forest health specialist
Sugar maples in Pine County flagging yellow and orange in August 2021 from squirrel feeding the previous winter.
In early March, we described how the forest health team and Extension Forester John Ball at South Dakota State University were receiving earlier-than-usual reports of squirrel feeding. It turns out that squirrel damage on sugar maples was the most unusual thing I saw in aerial surveys of central Minnesota forests this year. It was hard for me to believe that all the scattered, orange sugar maple tops I saw from the survey airplane were from squirrels, but indeed, several ground checks and reports from foresters confirmed squirrels really went nuts on the sugar maples this past winter.
Squirrels feeding on sugar maple bark in late winter is nothing new. The damage frequently kills branches. Squirrel feeding on the trunk and branches reveals the bright, almost-white sapwood. Over the spring and summer though, the exposed sapwood discolors to the point where it matches the surrounding gray bark and makes it harder to see the initial feeding patch.
I consulted with a couple of wildlife biologists about why this may have happened over the 2020–2021 winter, but no one knew for sure. Perhaps there were more squirrels than normal that had fewer acorns to eat, or maybe sugar maples were a more attractive food source. One thing to know is that sugar maples are hardy, and most of them will survive this aggressive mammalian assault.