Oak Regeneration and Acorn Size

Differences in size and mass affect seedling quality

Oak regeneration is an oft-discussed topic in forestry. The trees aren’t sustainably generating in many upland temperate forests due to a variety of factors — including the fact that they are less competitive than other, more sun-loving species such as tulip poplar.

The northern red oak seed orchard on the Watauga Ranger District of the Cherokee National Forest has provided research and reforestation opportunities for the last 25 years. Image by Stacy L. Clark, USFS.

One way that forest managers are responding to this issue is artificial oak regeneration. This approach consists of sowing acorns in a nursery, growing seedlings for one year, and then planting the best seedlings in areas where the species isn’t regenerating well on its own.

A recent USDA study recognized that this process can be difficult and expensive and looked to reduce the large variation in the quality of oak seedlings in order to reduce costs and improve planting success. Differences in seedling quality are thought to be due to a number of factors like acorn size and genetics, which were examined in the study.

Research forester Stacy L. Clark and University of Tennessee professor Scott E. Schlarbaum investigated how acorn size and mass, specifically, affect northern red oak (Quercus rubra) seedling quality. Their findings were published in New Forests.

A big reason why the scientists chose to study northern red oak was because they had access to a 45-year-old seed orchard on the Cherokee National Forest, which they also helped manage.

“While there aren’t enough resources to study every type of oak, northern red oak is of interest to the timber industry as well as to wildlife managers and conservationists,” explains Clark. “They especially care because it is widespread and can be more difficult to regenerate than other species of oak.”

Clark chose to study acorn size and mass for a reason. “I wanted to really look at what the best minimum acorn size is to yield a quality seedling,” says Clark. “Size class is going to change year to year, region to region, and species to species. Independent of all that, I wanted the absolute best size.”

Doing so could assist in oak regeneration, since scientists could choose the best acorns to plant and hopefully reduce variation in seedling quality in the nursery.

Mass was studied as a way to account for the presence of weevils, an insect that likes to burrow into acorns and eat away at the starches inside. The measurement is more precise than size for quantifying the dimensions of an acorn, since size doesn’t include the possibility of part of the interior being eaten.

Furthermore, there is good reason to think that acorn mass could be connected to seedling size and quality. Acorn mass is associated with cotyledons, a part of the seed embryo that supplies carbohydrates to assist with seedling growth following germination.

The scientists used a float/sink test to increase the overall germination rate of acorns planted in this nursery in Polk County, TN. Healthy acorns tend to sink in water since the acorn flesh is denser than water. Acorns that have been eaten by weevils or diseased float due to partial removal of the inside flesh. Image by Stacy Clark, USFS.

Clark and Schlarbaum first collected acorns from six open-pollinated orchard trees. They then separated the acorns into six size classes based on acorn diameter, which ranged from 1.3 to 2.5 centimeters. They weighed samples from each class to obtain acorn masses.

The scientists then sowed the acorns in a commercial bareroot nursery located in Polk County, Tennessee. The resulting seedlings were grown for one year. To encourage the greatest growth, the seedlings were fertilized continuously throughout the growing season and irrigated as needed according to protocols developed by a retired Forest Service scientist, Paul Kormanik.

The next step was to quantify seedling quality. The scientists measured stem heights, the number of small roots branching from the main root, stem diameter, and diameter of root collars – where the stem of the seedling meets the roots. These data were used to estimate the volume of the stems. It was also noted if a seedling did not survive.

The study found that there was little, if any, connection between size or mass of an acorn and the quality of the seedling it produces. “That was true for almost all six oak families we studied,” notes Clark. “One family showed a somewhat positive correlation between acorn mass, size, and seedling quality. However, it wasn’t strong enough for me to recommend for managers to size their acorns.”

Clark also notes that an external factor could have affected the study. “I think one reason why we didn’t find a connection was because it was a one-time study. Every year is different. It was a long and wet growing season, and I think that it negated the effects of the acorns. The impact of acorn genetics and size would have dissipated as the effect of the environment overwhelmingly influenced seedling quality.”

Read the full text of the article.

For more information, email Stacy L. Clark at stacy.l.clark@usda.gov.

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