- Metric Name:  Density – Snags

- Data Vintage:  2021

- Unit Of Measure: None

The number of standing dead trees (snags) on the landscape is important to forest managers; high densities of standing dead trees are known to contribute to extreme fire events while snags of certain sizes provide critical habitat to wildlife. For this metric, the snag density for all species and all decay classes with diameters of 20” dbh and greater have been estimated.- Creation Method:  The F3 model generated several raster surfaces of snags per acre for all species and all decay classes in non-overlapping, predefined size classes. For this metric, the three largest, predefined non-overlapping size categories have been included: 20-29.9”, 30-39.9”, and greater than or equal to40”.

2019 to 2021 Update:  Values for 2021 were adjusted 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. Each of the predefined non-overlapping size category trees per acre rasters (TPA_x) were adjusted following the same procedure. The MMI value for canopy cover loss was used as a direct proxy to estimate TPA loss, using the formula:

2021 TPA_x = 2019 TPA_x – (2019 TPA_x * 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 loss per individual size class, depending on location.

This loss of live trees per acre (TPA) between 2019 and 2021 was then added to the 2019 estimate for snag density (of the same size category; SNG_x) from F3. The layers for Snag Density were each derived from F3 layers (2021) using the following formula:  

(2019 TPA_x – 2021 TPA_x) + 2019 SNG_x

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