📘 Read the full study in the Journal of Forest Science, Vol. 70 (2024): https://doi.org/10.1093/forsci/fxae020 or contact Dr. Dehai Zhao and Dr. Bronson Bullock for more information.
In the southeastern United States, loblolly and slash pines are the economic backbone of the forest industry. Many mills and landowners use weight-based systems for their loblolly and slash timber transactions. However, converting between weight and volume requires accurate green density values, but how accurately are we estimating their green weight and volume?
In a 2024 study by Dr. Dehai Zhao, Dr. Bronson Bullock, and other researchers at University of Georgia, the authors developed updated equations for whole-tree green density—defined as the ratio of green weight (with bark) to stem volume (inside or outside bark)—for loblolly and slash pine. Their findings showed that older plantations have higher green density than newer, intensively managed ones, which as implications for timber valuation, biomass estimation, and operational planning.
The researchers analyzed data from over 3,400 destructively sampled trees, combining legacy datasets from plantations established in the 1970s–80s with newer data from intensively managed plantations planted in the 1990s and beyond. Trees were sampled across the southeastern U.S., and measurements included diameter at breast height (DBH), total height, green weight, and stem volume (both inside and outside bark).
A weighted nonlinear seemingly unrelated regression (WNSUR) was used by the team to develop predictive equations for outside-bark volume, inside-bark volume, green weight with bark, and green density ratios.
🌲Key Findings
- Green Density Varies with Tree Size: Larger trees are denser when measured over outside bark, but less dense when measured over inside bark.
- Older Plantations Have Higher Green Density: Trees from legacy plantations had significantly higher green weight to volume ratios than those from newer plantations. This trend was consistent across both species and suggests that intensive silviculture (e.g., fertilization, genetic improvement) may reduce wood density.
- Updated Equations Improve Accuracy: The new models outperformed fixed conversion factors commonly used in the industry. While green density is influenced by many factors (e.g., moisture, bark content, site quality), these equations provide a more reliable basis for operational use.
🌲Implications for the Forest Industry
For forest managers, timber buyers, and mill operators, these findings offer practical benefits:
- More accurate volume-to-weight conversions for timber sales and transport.
- Improved biomass estimates for carbon accounting and sustainability reporting.
- Better alignment between field measurements and inventory models.
The study also highlights the importance of updating operational tools to reflect changes in silviculture. As plantations become more intensively managed, relying on outdated conversion factors could lead to systematic errors in valuation and planning.
