Mind the gap? Pines love it!


The bigger the gap
the more pines grow

Especially when site treatment
keeps hardwood growth slow

But if a mixedwood stand
Is the forest we desire

Advanced hardwood regen
Should be allowed to grow higher.

USDA Forest Service scientist Don Bragg collaborated on a study recently published in Forest Ecology and Management that considered the effects of gap size and site treatment on the structure and composition of a stand in the Kisatchie National Forest of Louisiana.

Aerial photograph of the study site in the Kisatchie National Forest in 1992 showing the study plots as circular gaps. Lighter colored trees are hardwoods and darker ones are pines. USDA Forest Service photo.

The research took place on a productive site in a mixedwood stand. “Mixedwood” means the forest had both hardwoods and softwoods, where neither comprised more than 70-80 percent of the stand.

Mixedwood stands can be more desirable than single species stands, such as pine plantations. They often support more diverse wildlife populations and offer enhanced resilience to forest pests while still generating valuable timber for multiple markets.

Originally established in the 1990s by now retired USDA Forest Service scientist Mike Shelton, the mixedwood study area was reexamined by Colby Mohler, a graduate student, and Matt Olson, then an assistant professor of silviculture at the University of Arkansas at Monticello.

“When installed, the goal of the study was to use a type of multi-aged management practice using different harvest opening sizes and post-harvest release treatments to get the best results for regenerating a mixed composition stand,” says Bragg.

Three types of harvest openings—gaps—ranging in size from 0.25 to 1 acre were randomly placed across the treated stand.  Following the cutting of the overstory, workers applied three commonly used understory treatments to test their influence on tree regeneration.

“Initially they were interested in getting oak, especially red oak, and sufficient pine in the mix,” says Bragg, “but there was also concern that hardwoods would overtop the pines and convert the stand from mixedwood to hardwood.”

Example of a large gap with high potential to recruit pine. USDA Forest Service photo.

Competition for often scarce resources, such as light, moisture, and nutrients, by the tree seedlings, as well as from other vegetation, shapes the future stand. While logging opens the stand and helps to prepare the soil for new tree seedlings, many other factors also affect the relative success of naturally regenerating mixedwood forests. For instance, seed production and seedbed condition are also important.

After nearly 25 years of growth, the researchers found that light-demanding species, such as loblolly and shortleaf pines and sweetgum were less common in the smaller, more shaded gaps. Even still, under this particular set of treatments for this mixedwood stand, most gaps became pine dominated. Oaks and other desirable hardwoods did somewhat better in smaller gaps with no understory competition treatments, suggesting that mixedwood management is possible if less effort to favor pines is made.

Read the full text of the article.

For more information, email Don Bragg at don.c.bragg@usda.gov.

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