Story and photos by Josh Shields, Ph.D.
Forester and Wildlife Biologist, Forestry Assistance Program (FAP) – Manistee and Mason-Lake Conservation Districts
Editor’s note: Earlier this summer, we asked Josh Shields if he could explain to our readers why we were seeing so many brown evergreens in our county and neighboring counties. He was quick to respond and, unfortunately, we weren’t as quick to publish his story, which originally appeared in the Manistee Conservation District newsletter. Reprinted here by permission, with minor edits.
This summer I have been getting many questions about the “browning evergreens.” While it may seem like there is one simple answer, the answer is quite complicated!
In some cases, environmental stressors are the cause. For example, when trees are located close to roads and the brown coloration is mainly found on the portions of the trees facing the road, the likely cause is damage from road salt.
Winter injury is also common, whereby trees typically exhibit browning needles on the portions facing the prevailing winds. This is especially noticeable on trees that grow close to Lake Michigan. With winter injury, there is also sometimes a zone of green needles below the brown needles. This zone is indicative of the snow depth, whereby needles below the snow depth do not turn brown the same way as exposed needles above the snow.
When summer drought is the factor, you will notice a progression. Just a few needles are brown at first, followed by all the needles turning brown, followed by the death of the tree if the tree is not able to access adequate water.
Pests and diseases can also be factors, and environmental factors play a role in this. Non-native invasive species, such as the white pine blister rust fungus and the insect hemlock woolly adelgid, can impact otherwise healthy trees. However, in most cases, pests and diseases are affecting trees that have already been weakened due to environmental stressors.
One well-known example is the statewide decline of Colorado blue spruce. This species was widely planted as a Christmas tree, with tight spacing between tree trunks. However, many of these Christmas tree plantations were abandoned before the trees were harvested, and the trees have continued to grow and compete with one another, causing much stress on each tree. In addition to the stress caused by this competition, there is stress associated with the fact that the climate in Michigan is not the same as the climate in the Rocky Mountains, where Colorado blue spruce originates. (Native evergreen species, as in native to Michigan, are best adapted to our environmental conditions.) In response to this stress, Colorado blue spruce gets attacked by numerous types of needlecast fungi, tip blights, canker diseases, gall adelgids and spider mites.
There are numerous other examples, as well, such as stressed red pine trees in plantations that have not been properly thinned being attacked by native Armillaria spp. root rot fungi and native pine engraver beetles.
Because the browning of evergreens is such a complex issue that depends on numerous factors, it is best to contact a professional and have a discussion, based on the specific case in question, before making any conclusions and proceeding with management.
For more information, contact Josh Shields, forester and wildlife biologist with the Manistee and Mason-Lake Conservation Districts, at 231-889-9666 (Office) 989-220-9236 (Mobile), or via email at joshua.shields@macd.org.
Josh Shields, Ph.D., is also a Certified Forester with Society of American Foresters, Certified Arborist with the International Society of Arboriculture (ISA), Certified Inspector with the American Tree Farm System and Tree Risk Assessment Qualified with ISA. Additionally, he serves as a Certified Commercial Pesticide Applicator in Michigan and a Certified Wildlife Biologist with The Wildlife Society.
