There has been much debate about the success and usefulness of measures of biodiversity health in forest stands. Most of the diversity approaches have focused on taxon-based species evaluations. Because of the limited knowledge and debate concerning taxon-based species indicators it has been suggested that structure-based indicators be tested and developed as another measure of forest biodiversity health. This study uses the quadratic mean diameter (QMD) to define forest stand structure at the sample unit level. The QMD is a measure of the average stand diameter but uses the diameter of the average basal area of the forest stand instead of true arithmetic mean of tree diameters (DBH). I used empirical data from the USDA Forest Service, Forest Inventory and Analysis (FIA) program to monitor the changes in the QMD between two measurement periods. Tennessee was chosen for this case study. The trends were based on a measurement of approximately 3300 sample units in 2005 and a remeasure of these same sample units in 2015. The assessment determined the amount of forest area by QMD classes along with changes in the amount of forest area over the 10-year measurement period in these specific QMD classes.
Results/Conclusions
Across Tennessee there were approximately 5,650,000 (±36,725 C.I.) ha of forest land in both forest inventory measurements (no significant change). Between the 2005 and 2015 measurement periods the QMD of all trees (≥ 2.54 cm DBH) increased from 16.0 cm (SEM 0.1) to 16.6 cm (SEM 0.1). All of the increase came from the overstory trees (≥12.7 cm DBH) with the overall state average increasing from 24.4 cm in 2005 to 25.6 cm in 2015. There was no change in the QMD average in the sapling layer. This shift to larger QMD classes is good news for Tennessee forests, especially since trees are structural species (play a role in providing the physical environment for the ecosystem). Particularly encouraging was an increase in forest land area of 91,000 ha in the 20.0 – 25.0 cm QMD class along with a 39,000 ha increase in the ≥ 30.0 cm QMD class. The QMD is an easy indicator to apply and understand and provides a good entry-level description of the structural status of forests in large scale forest inventories. Describing and documenting baseline states of ecosystem components, such as the QMD, is an important step in the assessment of ecosystem risk assessment.