Drought, Insects, and Oak Decline
New patterns emerge in forest declines
Recent research by university and U.S. Forest Service scientists suggests that the traditional sequence of events and factors involved in forest decline may be changing in relation to climate conditions. To look more closely at patterns of decline linked to drought and insect attacks, the researchers analyzed the unprecedented oak death event that took place in the Ozarks and Ouachita Mountains of Arkansas, Missouri, and Oklahoma (Ozark region) from 1999 to 2003.
Laurel Haavik from Ohio State University led the study recently published in the journal Forest Ecology and Management, with Sharon Billings from the University of Kansas, Jim Guldin from the Forest Service Southern Research Station (SRS), and Fred Stephen from the University of Arkansas as co-authors.
Forest declines result from several interacting factors such as drought, damaging frost, or insect defoliation, and manifest as regional diebacks of one or a few closely related tree species. Oak decline, which involves a forest’s dominant oak species, is fairly common and usually the result of some combination of a poor history of management practices, climatic or site conditions, and insects (usually defoliating) and pathogens.
Considered a “novel” decline, the event in the Ozarks involved the interaction of drought with a native insect pest, the red oak borer, and aging, over-crowded forests that resulted from a century of poor management practices and fire suppression. In the past, the red oak borer, which does not defoliate trees, was not implicated as an agent of Ozark region oak declines, but rather “emerged” in this role over the past few decades.
“Most studies of forest decline have focused on nonnative insect species invading and destroying conifer forests,” says Guldin, project leader of the SRS southern pines ecology units. “There is a lot less known about native bark beetles and wood-borers in hardwood systems and how changing climate conditions – such as more frequent and severe drought – will affect their interactions with host trees and contribute to forest decline.”
The researchers chose the oak decline event in the Ozark region for several reasons. “It was novel, the single largest-scale oak mortality event reported in the area since European colonization in early 1800s,” says Guldin. “There was almost no defoliation, which was unusual, and the insect involved, the red oak borer, had never been known to reach epidemic levels or to play a part in oak decline. It was also well documented and researched, giving us the opportunity to explore how the roles of forest history, drought, and insect outbreaks in forest decline may be changing.”
The researchers used spatial and temporal studies to piece together the relationships among the different factors at different scales and times, from the level of beetle populations in trees to regional climate patterns, and to generate a point-by-point summary of the sequence of events in the oak decline event. Based on the case study, the researchers reached conclusions about forest decline in general, including:
- Changing climatic conditions and human influences have created hardwood forests currently vulnerable to disturbance.
- Novel decline situations often lack the agents traditionally associated with decline. Many emergent insects and pathogens in oak and hardwood declines are native species that have not caused noticeable levels of tree mortality in the past.
- Defoliation is sometimes absent, which implies that drought alone – and high temperature drought in particular – may debilitate trees to a point where secondary agents like red oak borer become more aggressive.
- Carbon balance dysfunction — a long-term disorder in oaks tied to drought episodes and affecting the root system — may be the cause of oak death during decline.
“In this case study on oak decline, we developed a complete step-by-step understanding of what happened and how different factors interacted with one another,” says Guldin. “Drawing in this way on what is known can help predict future scenarios and guide new areas of study as what have been considered ‘novel’ declines become more common.”
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For more information, email Jim Guldin at firstname.lastname@example.org