Study Finds No Evidence for Widespread Southern Pine Decline

Planted loblolly pine stand, about 35 years old, has been thinned and prescribed burned multiple times. Photo by David Stephens, courtesy of Bugwood.org.
Planted loblolly pine stand, about 35 years old, has been thinned and prescribed burned multiple times. Photo by David Stephens, courtesy of Bugwood.org.

A study by University of Georgia (UGA) and U.S. Forest Service scientists finds that there is no evidence for the widespread occurrence of southern pine decline recently reported as impacting the southern pine region. The results are published online in the journal Forest Ecology and Management.

Four southern pine species – loblolly, longleaf, shortleaf, and slash – form the foundation of forest industry in the South, the region often referred to as the “woodbasket of the world” because of its prominence in global forest products markets. Stands of these same species provide numerous ecological values including water quality, plant and animal habitat, and recreation. The presence of widespread southern pine decline would have important and costly implications for the management of millions of acres of southern pine forests.

For the study led by UGA researchers Drs. David Coyle and Kamal Gandhi, scientists reviewed the history and symptoms ascribed to southern pine decline and used data collected by Forest Service Forest Inventory and Analysis (FIA) to determine the current extent and threat posed by the syndrome in the southern pine landscape. Forest Service collaborators on the study included Kier Klepzig, Frank Koch, William Otrosina, and William Smith from the Southern Research Station, and John Nowak and Steve Oak from Forest Health Protection.

Declines are marked by widespread occurrences of tree weakness and death, and involve multiple factors – site characteristics, land use history, insect and disease outbreaks – that together can affect tree health at the stand level. The addition of stressors such as soil type, genetics, drought, and fire can lead to a progressive deterioration that in turn results in tree mortality at the landscape level, pushing species into a decline and death spiral. In the case of southern pine decline, root-feeding weevils and their associated fungi have been suggested as key stressors.

“Over the last century, we’ve seen reports of unexpected levels of tree mortality at the landscape level in yellow-cedar, aspen, oak, and more recently in southern pines,” says Klepzig, SRS assistant director and researcher. “Given the importance of forestry in the southern pine-dominated region, we decided a closer examination of the described decline syndrome was warranted.”

Results of the study showed that, if southern pine decline is occurring, it is not apparent at the landscape level (i.e., the level detected by the FIA plot sampling process.) Only a small percentage of non-harvested pine plots across the Southeast showed abnormally high mortality and these plots were identified by FIA field crews as having been disturbed by common agents such as insects, fire, weather, or invasive plants. The study also found no evidence for root-feeding insects as causative agents for widespread pine mortality.

The study has important implications for forest management. The practice of thinning forest stands has been shown to actually increase populations of root-feeding weevils by providing more of the stumps that weevils prefer for laying their eggs. Focusing on the weevils as a component of southern pine decline might lead to less forest thinning, which has long been established worldwide as a beneficial practice that increases the resistance of pine stands to outbreaks of southern pine beetle and other insects.

Managing  forests in the Southeast at the landscape level is inherently challenging, since any given landscape is itself is a patchwork of forests with different management histories and current objectives where many other factors – weather, soils, land use history – can affect the health of forest stands. The good news from this study is that local managers can continue to use the standard practices honed and tested over time by forest research and others – site selection, tree species selection, vegetation control, prescribed burning, and thinning – to improve forest health and sustainability.

Access the full text of the journal article.

For more information, email Kier Klepzig at kklepzig@gmail.com.

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