Trees provide clean air and water, wildlife habitat, and beauty. Trees are also vital to local economies.
“In 2015, Kentucky had 193 hardwood sawmills,” says USDA Forest Service research forester Tom Brandeis. “That same year, Tennessee had 226 hardwood sawmills.”
Each state produced more than 700 million board feet of hardwood lumber in 2015.
Higher quality hardwood lumber is used to make furniture, cabinets, molding, hardwood floors, and many other products. Lower-quality hardwood lumber is used to make things like railroad ties, crane decks and pallets. For trees in the forest, a complex grading system sums up a stem’s potential, with grade 1 being the best quality.
“Field crews have to see the tree the way a sawyer would see it at a sawmill,” says Brandeis. “They assess the size, the branching, the shape, the straightness, and the defects – which can be very difficult to see.”
Is the supply of higher graded timber decreasing? A variety of evidence suggests it may be.
“We kept hearing it anecdotally,” says Brandeis. “Loggers were having trouble finding grade 1 timber. Our recent FIA publications for these States have suggested that the percentage of grade 1, relative to lower grades, has declined.”
Forest Inventory and Analysis, or FIA, is a forest inventory program. USDA Forest Service and state agencies cooperate to conduct annual forest inventories in every state of the U.S.
“There’s a wealth of FIA data,” says Brandeis. “It can answer all sorts of questions about our forest resources.”
FIA field crews measure plots every 5 to 10 years, depending on the state. The repeated measurements give valuable insight into trends, but also mean that the remeasurement data are correlated – factors that influence the first measurements also affect subsequent measurements.
“We have the statistical tools to account for this,” says Brandeis. “We can make statistically valid comparisons and really follow the trends.”
Brandeis and colleagues used a repeated measures analysis to test for differences in the proportion of each tree grade from 2001 to 2013. The study was published in the journal Forest Ecology and Management.
The analysis showed that the percentage of total volume in higher-graded, commercially valuable hardwood timber has declined. In Kentucky, the volume of grade 1 saw-logs fell, and in Tennessee, grade 2 saw-log volumes fell. The scientists identified a potential increase in lower quality (tree grade 4) saw-log volume in both states.
However, there were sudden increases and decreases in the percentage of saw-log volumes. “This doesn’t make biological sense,” says Brandeis.
Unfortunately, the quality assessment and quality control data could not explain the fluctuations.
“We need strong quality assessment and quality control to back up what our data appear to be saying,” says Brandeis. “These types of questions have important implications.”
Indeed, a declining supply of high-graded timber would have serious economic effects. In Kentucky and Tennessee, trees are typically cut in partial harvests where about half the stand volume is removed.
Partial harvests often mean the best trees are cut and the lower quality trees are left. Brandeis used logistical regression to confirm this trend in a General Technical Report.
The GTR shows that grade 1 trees were most likely to be harvested, while lower-graded trees were much more likely to be left.
“I don’t think these findings will surprise many people,” says Brandeis. “A lot of loggers, landowners, and state foresters have suspected some degree of hardwood sawtimber resource degradation. All I did was document it and show statistically significant trends.”
Brandeis’ statistical explorations also show how FIA data can be used, and how the statistical significance of long-term trends can be properly tested.
For now, overall sawtimber volumes are increasing. However, there are few or no markets for lower quality trees in many areas.
“If there were markets, the lower quality trees could pay their way out of the woods,” says Brandeis. “That would help support silvicultural efforts to manage these hardwood stands.”
Ecologically, the forests are not showing signs of decline due to logging. “We’re not seeing the loss of species due to harvesting, and we’re not cutting down all the old trees,” says Brandeis. “Those big old trees are still out there, they’re just a lower grade for lumber.”
For more information, email Tom Brandeis at firstname.lastname@example.org.