The lack of adequate quality assurance/quality control data hinders the assessment of potential forest degradation in a national forest inventory
Abstract
Hardwood lumber harvested from the temperate broadleaf and mixed broadleaf/conifer forests of the east-central United States is an important economic resource. Forest industry stakeholders in this region have a growing need for accurate, reliable estimates of high-quality wood volume. While lower-graded timber has an increasingly wide array of uses, the forest products sectors in those states would be negatively affected if there is ongoing forest degradation due to the relative loss of higher graded timber. The United States national forest inventory provides data that could answer whether the supply of higher graded timber is decreasing despite an overall increase in merchantable wood volume. To study trends over time, however, one must take into account the partial lack of independence within forest inventory and monitoring datasets with repeated measurements on the same permanent plots and the trees within them. By doing this, we demonstrate that the data show significant decreases in the relative saw-log volume found in higher-graded, commercially valuable hardwood trees in the states of Kentucky and Tennessee from 2001 to 2013, most notably a decrease in the percentage of tree grade 1 saw-log volumes in Kentucky and a decrease in tree grade 2 saw-log volumes in Tennessee. We also identified a potential increase in lower quality (tree grade 4) saw-log volume in both states. These findings would be consistent with indirect and anecdotal evidence of degradation in hardwood resource in portions of the region. However, substantial annual fluctuations in the volume percentages by grade led us to question the validity of those observed trends. Quality assurance and quality control (QA/QC) data collected in conjunction with those tree grade data were not sufficient or consistent enough to allow us to verify whether we are observing real trends or data collection anomalies, thereby compromising our ability to provide important information to land managers and decision makers. The occurrence of hardwood tree grade fluctuations over time illustrates the need for robust QA/QC procedures in national forest inventories. More frequent QA/QC data collection and analysis, field data collection training consistency across regions, and potentially simplifying field measures of tree stem quality could provide more clarity and confidence when assessing the condition of forest resources.