- Metric Name:  Quadratic Mean Diameter

- Data Vintage:  2021

- Unit Of Measure: None

Tree diameter (in inches) at breast height (dbh) for determining tree size. Quadratic mean diameter (QMD) is computed by squaring individual tree diameters, computing their average, and then taking the square root. The result is that QMD represents the diameter of the tree of the mean basal area. QMD is generally preferred over the (arithmetic) mean diameter because it is less influenced by very small trees (which can be highly variable in density from one site to the next) and it captures the fact that an inch of diameter growth means more for tree biomass on larger trees than on smaller trees.- Creation Method:  The F3 model generated several quadratic mean diameter (QMD) raster surfaces; for all live trees (QMD_TOT) and by predefined tree size categories (QMD_x).

2019 to 2021 Update:  Tree density values for 2021 were adjusted independently for each diameter size class (10-inch bins) using the Ecosystem Disturbance and Recovery Tracker (eDaRT), described in the Introduction. All eDaRT events beginning August 1, 2019 through November 30, 2021 were identified, and the corresponding Mortality Magnitude Index (MMI) values for these events was summed, giving the estimated fractional canopy cover loss per 30m pixel over that time period. The MMI value for canopy cover loss was used as a direct proxy to estimate TPA loss, using the formula:

2021 TPA = 2019 TPA – (2019 TPA * MMI/100)

Although the assumption of direct correlation between canopy cover and TPA should be viewed with caution, it serves as a reasonable approximation for representative mixed conifer forests in the Sierra Nevada affected by the recent drought (Slaton et al. 2022). The assumption that canopy cover loss, as estimated using eDaRT MMI, was equitably distributed among the predefined size classes may result in over- or under-estimates of actual tree density per individual size class, depending on location.

QMD was then recalculated for 2021 using adjusted tree densities and by assigning trees in each size class to the respective mid-point diameter of that class.

- Credits:  F3 data outputs, Region 5, MARS Team
USDA Forest Service - Region 5 - Pacific Southwest Region