In April 2017, the U.S. Forest Service Office of Sustainability and Climate hosted a webinar focused on drought and forest ecosystems. Scientists discussed how the ecological effects of drought and a changing climate interact with those of wildfire, insects, disease, and other stressors. They also described some tools and approaches that land managers can use as management response options. Researchers used case studies from a variety of forest types, in both the western and eastern United States.
Excerpts from the summary
Droughts can result in reduced growth rates, defoliation, and increased stress in trees, with accompanying ecological, economic, and social effects. Droughts have been increasing in frequency and severity over the last 50 years in much of the United States (Peters et al. 2014), although droughts were even more prevalent prior to 1900.
Drought and Wildfire
Higher temperatures and increased drought can lead to more frequent wildfires and greater area burned.
Drought and Insects
Some insects and diseases act as defoliators and growth reducers, reducing gross and net primary productivity, as well as net ecosystem productivity (Hicke et al. 2012a). Other insects or diseases lead directly to tree mortality, which produces longer-term effects over years and decades as dead trees decay and successional species take their places.
Historical Forest Management
In the 20th century, large areas of forest were harvested over a short period of time, and wildfire suppression became standard policy in most locations. This produced forests that were denser (sometimes with more than ten times more trees per acre), more homogeneous and evenly aged, with fewer large fire-resistant trees (Agee & Skinner 2005, Nowacki & Abrams 2008). These factors led to more competition for scarce resources, particularly in times of drought.
Combined Effects of Drought
The combination of past management activities, stressors, and disturbances has altered the structure and function of some forests. The replacement of a dominant overstory (e.g., large, fire-resistant trees) with smaller, denser trees that are then stressed or killed by drought, insects, or disease, followed by an increase in wildfire, has led to extensive areas of tree mortality