Group Selection Harvest for White Oak Regeneration

sky-canopy-opening
This photo shows the sky and surrounding trees of a small opening. In this study, larger canopy openings increased the chances of white oak survival. Photo by USDA Forest Service.

Oaks are keystone species in forests across the eastern U.S. However, oak reproduction has been declining since at least the 1970s — old oaks still dominate the overstory, but a younger generation is not coming in behind.

White oak, in particular, is valuable to wildlife and wood-dependent industries such as barrel, furniture, and cabinet making. Scientists are trying to find out what influences white oak reproduction and survival so managers can sustain it into the future.

USDA Forest Service scientist Marty Spetich examined how site factors and removal of competitors can influence the survival of white oak seedlings over time. The results were published in the journal Forests.

“Because of my experience as a district forester for the Indiana Department of Natural Resources back in the 1980s, I saw a need for management-based research. A goal of this study was to get information to managers in a practical, usable format so they can maximize seedling success of white oak,” says Spetich.

Some species are more tolerant of shade and can keep oaks from growing into older, larger trees. Studies have shown that group selection methods – where trees are removed to create openings that let in more light – can improve regeneration and survival of white oak.

Seedlings were more likely to survive in large openings with chemical control of understory vegetation. Photo by Dave Graney, USDA Forest Service.

This study tested that strategy in the Boston Mountains of the Ozark-St. Francis National Forest in Arkansas. Overstory trees were harvested to create openings of different sizes. Woody understory vegetation known to suppress white oak was removed by either mechanical or chemical means, while control plots received no understory treatment. The study measured other factors that influence survival or growth of oak like the height of border trees on opening edges, slope, and aspect.

Spetich followed the fates of four thousand oak seedlings before, during, and twelve years after the removal of trees and understory vegetation. “Other opening size studies have examined oak seedlings long after harvesting and then measured group openings,” explains Spetich. “In this study, each seedling was tagged and measured before, during, and after the group selection harvest, so we know how the treatments impacted the seedlings over time.”

Spetich applied a logistic regression model to explore how the survival of seedlings over time was related to each of the measured factors: initial seedling diameter, different understory vegetation removal methods, border tree height, canopy opening size, slope, and aspect.

The results point to a list of factors that managers could expect to bolster white oak survival. Initial size of seedlings mattered. Those that were larger in diameter than one centimeter (slightly less than a half inch) were more likely to survive.

White oak seedlings weren’t as likely to survive in smaller openings with no understory vegetation control. Photo by Dave Graney, USDA Forest Service.

Seedlings were better off in larger canopy openings with shorter bordering trees. Chemical understory removal was more likely to garner white oak survival than mechanical or no treatment of the understory. White oak seedlings also survived in openings smaller than those typically required to successfully regenerate red oak. These differences could be helpful in situations where white oak regeneration is preferred over red oak.

Site factors were also important – seedlings growing on flatter ground and facing northwest or southwest had the best chances of surviving.

“The specific conditions that maximize white oak survival in small group openings on these sites really weren’t known before this study,” says Spetich. “Managers now know what conditions and locations maximize white oak survival in small openings.”

Read the full text of the article.

For more information, email Marty Spetich at marty.spetich@usda.gov.

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