- Metric Name:  Proportion of Maximum SDI

- Data Vintage:  2021

- Unit Of Measure: Proportion, 0 to 1

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. The maximum forest stand density represents an approximate upper limit to the SDI of a site, and tree growth may be limited by competition as SDI approaches maximum SDI. This approximate upper limit on potential site SDI has been considered to be species- and site-specific by several authors using different variables to characterize the stand.- Creation Method:  These raster data present the SDI proportion of the estimated max Stand Density Index (SDI) for both the Reineke (1933) and Zeide (1983) calculations.

2019 to 2021 Update:  SDI values were adjusted for 2021 following the same procedure as outlined for density – trees per acre. 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. These adjusted values for actual SDI were used to calculate percentages in combination with the max SDI values from 2019.

The maximum SDI was calculated as the 99th percentile of observed values for each of five broad climate classes. The classes were derived from the Basin Characterization Model (BCM; Flint and Flint) developed at a 270m spatial resolution. The variables (1981-2010) AET, climatic water deficit, Tmin, and Tmax were rescaled using a linear transformation to a range of 0-100 and clustered into five classes using a k-means algorithm.  

Finally for each pixel, the proportion of maximum SDI is simply calculated as SDI divided by maximum SDI: 

Proportion_MaxSDI  =  SDI/MaxSDI

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