A tough year for trees in the Driftless

By Aaron Trezona, Certified Arborist

This year has been difficult for many trees trying to leaf out, with the cause for the bare branches starting with the cold snap last May. This tree found along Ohio Street in Decorah illustrates the struggle trees have had this year, and Veteran Arborist Aaron Trezona explains why. (Driftless Multimedia photo by Kate Klimesh)

Many Driftless Region residents have noticed the many defoliated and struggling trees this growing season. Although we still have many major tree problems like Emerald Ash Borer and Oak Wilt, much of what has been seen is less severe, and due to weather and cyclical pests.

With the warm and dry weather this area had most of the summer, it may seem like a distant memory, but in the second week of May 2023, there was an extended period of cold and damp weather. Much of the area saw two to three inches of rain with daily temperatures struggling to crest 50 degrees, and sunshine was a rare commodity. 

Sudden weather changes and cold snaps are nothing new to the Driftless, but this one came right as new leaves were just emerging from their buds on most of our trees. As a leaf matures, it develops a waxy cuticle to aid in their defense against the elements, insects and other pathogens.  Trees are very vulnerable when the leaf initially emerges from the buds and before this waxy cuticle develops.  

Cold and damp conditions are ideal for the growth of fungi. Anthracnose, a fungus that attacks leaves that remain damp too long, had ideal conditions to run rampant on the vulnerable new leaves this spring. This resulted in many trees being delayed in the development of their canopies. Some trees leafed-out only to shed the fungus damaged leaves in late May. Most of these trees were able to send forth another flush, but some struggled and still have parts of their canopies barren of foliage.  

This year was several times worse for anthracnose than any other I have seen in my 23 years as an arborist. Anthracnose is not typically serious because it only effects the leaves and not the remainder of the tree. Since trees shed their leaves every year, replacing diseased or damaged leaves is not anything particularly stressful to the tree. However, prior to this year, I had never seen a tree completely defoliated by it either. The fungus typically afflicts the interior leaves of thick canopied trees, like Sugar Maple, or trees that have limited air flow through them like trees in the alcoves or courtyards of buildings.  

The vast majority of these trees should make a full recovery. Anthracnose only causes tree mortality when it occurs frequently enough to exhaust the tree of reserved starches, so it no longer has the resources to put leaves out again. The subsequent weather since May’s cold and wet period has hindered this recovery though with drought conditions.  Our native trees have tools to deal with drought that limit the damage the drought could do. They, in many ways, simply go dormant in the period where there is insufficient moisture for basic metabolic processes of photosynthesis. This, of course, delays any restorative processes like replacing shed or damaged leaves.  

These weather-stressed trees become more vulnerable to native pests their normal, natural defenses usually keep at bay. Consequently, many different insect infestations like Bronze Birch Borer (Birches), Two-lined Chestnut Borer (Oaks), and Cottony Maple Scale (Maples) are much more common this year. Finally, with this past winter being mild, two of the most damaging invasive insects lost very few of their larva to the cold. That is why pressure from Japanese Beetles and Emerald Ash Borer is also particularly intense this year. 

The mixture of climate, untimely weather patterns, invasive pests and native pests has made for a very tough year for the trees in the Driftless.  

Not to dismay though, there is a reason why some of the growth rings on a stump are large and others are small. As Aldo Leopold, vaunted naturalist and author from Sauk County, Wisconsin, once wrote: “Halt, cried the sawyer, and we pause for breath.”

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