Written by: Certified Arborist August Hoppe, WI-0477A
Our weather patterns have been changing. In Wisconsin we have been experiencing wetter springs and falls, often with extended dry periods in summer. These trends influence growth and health of our landscape trees. Trees have evolved over the years to be highly adaptable to extremes in weather. They are remarkably resilient to gradual changes that take place as the season progresses. Trees have developed processes to handle extreme cold, excessively hot, dry and wet weather. Cold hardiness of trees is a perfect example. Layers of bark insulate the cambium from brutal cold temperatures, water is shed out of plant cell tissues to prevent freezing, and leaves are shed off. These aspects help a tree deal with a long, cold winter. Conversely in hot dry periods, trees adapt by closing their gas exchange holes, known as stomata. This prevents water transpiration to the most exposed leaves, but a trade-off is that it slows down growth, which can have an effect on the tree in the long term.
While trees have shown the ability and genetic response necessary to deal with a variety of conditions, these processes are largely undertaken gradually. Trees do not like immediate changes, and extreme fluctuations can cause vulnerabilities for trees, especially when they occur in the spring and fall and interrupt the normal sequences of the tree.
Last fall you may have noticed numerous trees that did not shed their leaves. This was because of a rare cold snap in October that brought extreme cold, before most trees were ready to handle it. This cold weather froze leaves and disrupted the normal process of fall leaf senescence. The leaves were not able to develop abscission layers, a normal process that gradually weakens the leaf attachment to the twig. The leaves essential flash froze right on the trees.
Colder springs have been occurring more frequently as well. This alone is not a problem for most trees, as it generally just means that spring growth is delayed. However, if warm weather occurs in spring and new growth starts to develop, cold snaps can cause frost damage that can kill new plant tissue.
Increased frequency of intensity of storms has effects on trees too. Strong winds can obviously break branches and limbs on trees - a good reason to prune them properly, while saturated soil can reduce the stability of the root ball that anchors trees in the ground. Wet soils have less oxygen, and root systems can not grow and develop properly. Increased precipitation (2019 was the wettest year on record) has led to a rise in fungal diseases on many of our landscape trees. Crabapple and spruce trees immediately come to mind.
You may feel powerless against the whims of Mother Nature, but there are some things you can do to make your trees more resilient against a changing climate. Select new trees and planting locations carefully. Look at hardiness and disease resistance when picking out a new tree or shrub. Avoid areas that are frequently too wet, or plant water-tolerant species in those locations. Prune trees when they are young to develop strong central leaders that hold up best under storm conditions. Strengthen your existing trees with a tree health program. Programs often include applications to prevent disease establishment and below-ground root treatments such as fertilization or growth regulators to improve root system health.