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Virginia Tech Studies the Relationship of Mineral Stain in Red Oak to Growth Site Characteristics

The potential presence of mineral stain can make it difficult to accurately assess the value of any particular stand of red oak trees.

By Peter Hamner, Brian Bond, Lynn Resler
Date Posted: 4/1/2010


            The “Red Oak” classification for lumber and veneer is comprised of seventeen tree species; these species currently account for the majority of timber resources that are available in the Appalachian Mountain region of the United States. Since red oak accounts for such a high percentage of the available forest resource, its cost as a raw material and its value in sawn products greatly impact the profitability of the hardwood manufacturing industry in the Appalachian region.

            For hardwoods, value is typically dependent on appearance, particularly with regard to large clear faces of light colored wood in both lumber and veneer. Therefore, any visual blemishes or discolorations in the wood, such as knots, stains, streaks, spots, and holes, are considered defects that affect value negatively. While some of these defects can be observed and assessed when evaluating a living tree, others such as mineral stain cannot be detected. The potential presence of mineral stain can make it difficult to accurately assess the value of any particular stand of red oak trees.

 

What is Mineral Stain?

            Mineral streak or stain is an area of dark discoloration that occurs within the stem of a living tree. This visual defect commonly occurs in red oaks and other hardwoods. When present, it usually can be seen bordering potions of one or more growth rings, or in small patchy clusters within the sapwood, heartwood, or both (Figure 1). Sawn lumber or slices of veneer containing mineral stain will have dark greenish to black colored streaks appearing on their surfaces (Figure 2). Since higher value lumber and veneer products are directly associated with visually clear faces, the presence of mineral stain can significantly devalue an otherwise potentially high grade, high value product like veneer.

            Mineral stain is not only a visual defect in lumber; it can also lead to processing problems.  Both the primary and secondary wood industries have long recognized that lumber containing mineral streaks is more prone to surface checking and internal honeycomb when it is dried – a fact that has been documented by research.  Mineral streaks are also noted to be harder than normal wood due to the ash content being considerably greater than normal wood. This increased hardness in mineral stained areas can be difficult to cut or shape and can damage the edges of cutting tools.

            The formation of mineral stain in living trees is not well understood, and few explanations exist to account for its development and presence in oaks or other hardwoods. Though some researchers have attributed its onset as a result of abnormal cell physiology occurring from wounds or infections caused by insects, bacteria, fungus, animals, weather, or human activity (logging), there is more evidence to suggest that the occurrence of mineral stain is related to site-specific variables. For example, it has been reported that mineral-free, light colored red oak is more commonly found in certain areas of the country including lower New York, Pennsylvania, southern Indiana, and southern Michigan. These reports suggest that geographic variation in site characteristics may influence mineral staining.

            Poor site quality and/or soil properties are also thought to be probable causes of mineral staining in red oak. Wet, poorly drained soils are often associated with the formation of mineral stains in hardwoods. For example, it has been reported that oaks, sweet gum, ash, and cottonwood contain “much more” mineral stain than similar species growing on better drained sites. It has also been noted that increased incidences of mineral staining in red oak may be related to trees growing on soils with a high content of coal, or trees growing on slopes. Yet, perhaps the most common agreement among foresters and log buyers in the Appalachian region is that red oak trees harvested from areas that are somehow associated with previous or current cow pasture use are the most likely to contain mineral stain. 

            Despite these general observations, it is clear that the causes of mineral stain associated with site quality remain vague. Based on the results of previous research, no definite conclusions have been made to explain the presence of mineral stain in oak or other hardwoods. Mineral stain has been found in young trees and in old trees, in trees on poor sites and in trees on good sites. It has been found in trees with apparent injuries that occurred during their development, and in trees with no apparent injuries. As a result, it is generally agreed upon by both the academic community, and by those with years of experience in the field, that understanding how and where mineral stain occurs in oak and other hardwoods is a complex problem.

            Because the presence of mineral stain is currently impossible to predict in standing trees, a method to determine the likelihood of its presence in red oak stands would greatly improve the ability of landowners, timber managers, foresters, and the primary and secondary hardwood industry to improve quality and value assessments of the standing timber.

 

Study to Determine Geographic Factors

            With the support of the hardwood industry in Virginia and West Virginia, and funding provided by the USDA Forest Service – Wood Education & Resource Center (WERC), a research team at Virginia Tech is carrying out a project aimed at fostering a better understanding of the relationships between red oak growth site characteristics and the presence and severity of mineral stain in red oak trees. To minimize variation, the focus of the study will be limited to Virginia and West Virginia only, although results will likely apply to other geographic regions.

            Research methods that will be used to assess the frequency and severity of mineral stain in this study will involve field data collected from forest harvesting sites that have been targeted to study specific red oak growing conditions. At these sites, data will be collected based on the presence and severity of mineral stain as it can be observed in the end of a cut log (cross-sectional face), and correlated with relevant growth site characteristics. Geospatial information will also be collected and analyzed.

            The goal of this research is to develop a model that will provide landowners, foresters, and the wood industry with the ability to predict the presence and severity of mineral stain in stands of red oak trees. This will improve harvesting decisions and value assessment of red oak trees for land owners, foresters, and industry personnel by filling in an important gap in the knowledge of the relationships between site variables and mineral staining across the Appalachian mountains of Virginia and West Virginia.  Models based on geographic location and selected growth site variables can be used by stakeholders to predict the presence/severity of mineral stain, and thus assign the appropriate value to the raw material and make better harvesting decisions. Part of the goal is to disseminate the results of this work to all stakeholder groups. Being able to predict the presence and severity of mineral stain based on geographic region and growth site variables would allow all interested parties to make better price and harvesting decisions, and ultimately improve the profitability and competitiveness of the hardwood industry. 

 

Does the Log End Indicate the Amount of Mineral Stain in a Log?

            The first portion of this project included a pilot study to validate whether the amount of mineral stain observed in the end of a log is representative of mineral staining throughout the entire log. The study utilized imaging analysis techniques to measure percentages of mineral stain observed of the surfaces of log ends as well as the lumber that was cut from the logs. The amount of mineral stain occurring in the cross-sectional face of the study logs ranged from about 0.5% to close to 8%. The results of the pilot study show that a linear relationship exists between the amounts of mineral stain observed in the end of a particular log and the surfaces of the lumber cut from it. Furthermore, a high degree of correlation was shown to exist (correlation factor =0.96). In other words, the amount of mineral stain present on the cross-sectional face of a log is a good indication of how much mineral stain will be present in the surfaces of lumber or veneer cut from that log.

            The project is continuing through the spring of 2010 and we hope to share the final results this summer. If you have any insights about mineral stain in relation to growth site, please don’t hesitate to contact us.

            For more information please contact Peter Hamner at 540/231-3043 or email: phamner@vt.edu.




 






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