How Do Weather, Insects, and Diseases Impact Forests? It Depends

The report summarizes weather disturbances, including wind, and the range of impacts they can cause. Photo by Joe O’Brien, USFS, courtesy of Bugwood.org.

Forests of the southern U.S. are among the most productive and intensively managed in the world. Disturbances naturally alter forest stands, sometimes creating conditions that benefit plant or animal communities, but can cause major economic losses to landowners. What’s more, the impacts from one disturbance may invite other disturbances – dead and damaged trees can attract bark beetles, wood-boring beetles, or diseases.

USDA Forest Service scientists recently led a research synthesis focused on catastrophic weather-related disturbances, insects, and disease — events that can dramatically change forest composition and structure. James T. Vogt, Don Bragg, and Rabiu Olatinwo, with partners Kamal Gandhi of the University of Georgia and Kier Klepzig of The Jones Center at Ichauway, authored the General Technical Report.

“The goal was to assess the current state of our knowledge around disturbance and subsequent insect and disease activity,” says Vogt. “Do damage levels from disturbance affect your management decisions after the damage occurs? Are those damage levels related to insect and disease outbreaks?”

Not all weather disturbances are created equal, and neither are their impacts. The Report describes the major weather disturbances in the South including wind, ice and snow, hail, and flooding.

Ips engraver beetles may impact trees damaged from weather disturbance. More research is needed. Photo by Erich G. Vallery, USFS, courtesy of Bugwood.org.

The extent of impacts is as variable as the weather events themselves. And then there are factors that may reduce or exaggerate a weather event’s impact – like the species, size, or age of a tree or stand, recent management activities, and tree spacing – and dramatically influence the outcome. For instance, longleaf pine trees less than five years old were the only trees largely undamaged in areas that experienced the strongest winds of Hurricane Michael.

Knowing how the impacts of disturbance can influence the onset of insects or disease could help landowners know what kind of action to take, and how quickly to take it.

In pursuit of patterns, the researchers combed exhaustively through decades of annual Forest Health Protection reports on “Major Forest Insect and Disease Conditions in the United States” (spanning 1955 to 2019) and noted observations where weather damage matched up with reports of insect or disease.

Their search found frequent reports of insects and diseases on stressed or disturbed trees, but those individual observations didn’t readily translate into predictable patterns. The researchers examined existing research studies to look for further connections.

It is often assumed that weather events incite infestations or outbreaks of bark or wood colonizing insects. Vogt explains, “While bark beetles can increase in dead and dying trees following disturbance, there are no published data to support observations that, for example, southern pine beetle reaches outbreak status as a result of weather disturbances. Outbreaks tend to occur in older stands, with high basal area and closely-spaced trees.”

Diseases, like fusiform rust, can weaken trees, making them vulnerable to wind and other disturbances. Photo by Jaeson Huang, USFS, courtesy of Bugwood.org.

“Ips beetles, often called engraver beetles, are probably more of an issue in the South than we give them credit for. We need to learn more about Ips beetle population dynamics and their capacity to cause damage following disturbance events,” adds Vogt.

The research did not show strong relationships between disturbance type or impacts with incidence of insects and disease.

“Responses of pests and pathogens can be complex and dependent on stand structure and composition, previous stressors such as drought, and the timing and severity of disturbance. The response may be quite complex, but the general advice to timber managers remains the same – salvage sooner rather than later whenever possible,” says Vogt. “In other situations, such as land set aside for recreation or hunting, gaps created by disturbance can be beneficial, increasing abundance and diversity of understory plants, creating mixed-age stands, and increasing habitat for a diversity of animals.”

Indeed, insects and disease are vital parts of forest ecosystems. Their role as consumers is important to forest nutrient cycling, which in turn provide opportunities for microbes, symbiotic fungi, or ambrosia beetles that work to break things down even further.

The Report identified some key considerations for managers and landowners:

  • Identify post-disturbance forest or woodland management goals.
  • Expect damaged forests to be colonized by bark and woodboring insects and monitor to track increases in activity and populations.
  • Salvage damaged forests sooner, rather than later. Salvage logging and other management activities, such as prescribed burning, can have variable impacts on species diversity and vulnerability to disease or insects.
  • Replant forests promptly to jumpstart restoration of economic and ecological values.

The Report also sets up some key paths for future research. One goal will be how to extend the useful lifespan of salvage timber. Learning more about how wood degrades over time, and the relationships between disturbance, insects and disease, and forest conditions in the wake of these events, can inform land managers’ decisions.

Read the full text of the General Technical Report.

For more information, email J.T. Vogt at james.t.vogt@usda.gov.

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