Crowded and Thirsty: Fire exclusion leads to greater drought sensitivity in mixed-conifer forests

Wildfires were a frequent source of disturbance in forests of the Western United States prior to Euro-American settlement. Following a series of catastrophic wildfires in the Northern Rockies in 1910, the U.S. Forest Service adopted a broad wildfire suppression policy that has resulted in forests thick with small trees. These crowded trees compete for nutrients and water and experience increased drought stress in summer.In recent decades, many trees have died following drought, bark beetle outbreaks, and severe wildfire. A link between this mortality and increasing susceptibility to drought was suspected, but a direct connection hadn’t been confirmed. To learn more, researchers with the USDA Forest Service Pacific Northwest Research Station and Oregon State University used a stable carbon isotope approach to test how individual trees in a mixed-conifer forest in the Deschutes National Forest responded to the exclusion of wildfire. Their analyses revealed that trees had become more drought stressed, and stands that had a basal area greater than 100 square feet per acre in 1910 are more drought stressed today. Conversely, stands that were less densely stocked in 1910 still exhibit signs of drought resis-tance and are thus less susceptible to insects and pathogens. These findings are critical for land managers when deciding how and where to allocate resources for forest restoration in dry mixed-conifer forests to mitigate the effects of drought.