- Metric Name: Stand Density Index
- Data Vintage: 2021
- Unit Of Measure: Number of trees per acre expressed as an equivalent density in a stand with a quadratic mean diameter of 10 inches
Stand density index (SDI) helps vegetation managers to identify levels of site utilization and competition to determine management scenarios to meet objectives and is often used for forest health-oriented treatments. SDI was also proposed by North et al., (2022) as an operational resilience metric for western fire adapted forests. This metric is a quantitative measure that relates the current stand density to the size class distribution of the stand. Reineke uses quadratic mean diameter, a weighted mean, to estimate the stand size class, whereas the Zeide method (also known as the summation method) uses Dr (Reineke’s diameter). For additional details on both calculations, see the Essential FVS Guide.- Creation Method: FVS generated estimates of the stand density index metric using either the Reineke 1933 or the Zeide 1983 index calculations for all trees greater than or equal to 1.0” dbh based on max SDI derived from FIA plot data. Then the F3 model imputed the SDI calculations to the landscape.
2019 to 2021 Update: SDI values were adjusted for 2021 following the same procedure as outlined for density – trees per acre (described below).
Tree density values for 2021 were adjusted independently for each predefined non-overlapping 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