U.S. Forest Service and university researchers are working together to understand the escalating decline and death of oaks—especially red oaks—in the Ozark Highlands of Arkansas and Missouri.
Research forest ecologist Marty Spetich from the Forest Service Southern Research Station and scientists from the Forest Service Northern Research Station (NRS), Mississippi State University, and the University of Missouri team up to look at risk factors and possible effects drought plays on oak decline and death.
Under normal conditions, oaks can live a century or more. The early death of oaks affects forest biodiversity, timber values, acorn production, oak regeneration, and wildlife food. In the condition called oak decline, a negative mix of internal and external factors hampers the tree’s ability to thrive, leading to weakness and early death. Prolonged exposure to stresses eventually kills the trees. For example, repeated drought, tree age, location and pest attacks can combine to cause trees’ roots and crowns to slowly die back.
Scientists study these interactions to help managers understand and manage their stands to reduce tree stress. In the first phase of an ongoing study, Spetich and scientists analyzed information collected by Forest Service Forest Inventory and Analysis (FIA) unit, using data collected from 3,945 plots between 1999 and 2006 to map locations and show changes in oak decline and mortality.
The researchers shared findings from the first phase in an article published in 2011. Using statistical models they calculated average death rates for 3 groups of trees: red oak, white oak, and non-oak. They then compared mortality rates in and among the groups. They found that the white oak group and non-oak group mortality rates were relatively steady at around 4 to 5 percent over time, but the mortality rate in the red oak group increased from 8 percent in 1999 to 16 to18 percent by 2006. The researchers found the red oak group mortality was 3 to 4 times higher than the white oak group in this geographic region.
The study group then correlated the map they generated with drought severity data to show the cumulative effects of drought as one cause of oak mortality and decline. They found that mortality usually lagged 2 to 3 years behind a single drought event. Their data also showed that the cumulative impact of droughts on red oak decline and mortality might last up to 10 years, and can play a major part in the large-scale decline and death of red oaks.
The research is rapidly evolving. In late 2012, the scientists published findings from the second phase of the project, where they used data from twice the number of FIA plots as in the first phase. This paper provides even more detail than the first about the effects of oak decline on different oak species in the Ozark Highlands.
The maps and other tools the scientists develop, such as the “Oak Decline and Mortality Hazard Index” will help managers design specific silvicultural treatments to lessen the severity of oak decline and mortality. Knowing regional events such as drought and the potential risk factors associated with oak decline can help predict the likelihood that declining trees will live or die.—Julia Kirschman
For more information, email Marty Spetich at firstname.lastname@example.org