- Metric Name:  Density – Trees Per Acre

- Data Vintage:  2022

- Unit Of Measure: Live trees/acre

Trees per acre (TPA) is a common forest structure measurement that provides a useful index of forest and habitat condition. Many other metrics can be derived from having accurate estimates of trees per acre.- Creation Method:  The F3 model generated several raster surfaces of trees per acre as estimates of tree density on the landscape. This raster surface represents all live trees greater than 1” dbh (TPA). Reference conditions can be generated from contemporary reference sites for mature forest conditions outside of the WUI.

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. 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).

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