- Metric Name:  Large Tree Carbon

- Data Vintage:  2021

- Unit Of Measure: Mg C/ha

Large trees in this metric were calculated as the sum of branch and stemwood plus foliage for trees over 20 inches in diameter. This is intended to represent the most stable (possibly other than soil) component of the carbon pool, and can be an indicator of the carbon stock’s resilience/stability. For this metric, higher values generally indicate more stability, and upward trends in this value may be interpreted as generally increasing resilience of the aboveground C pool.- Creation Method:  The F3 model generated several different raster surfaces to estimate the biomass of stemwood in non-overlapping predefined size classes (BMSTM_x) and for the branchwood, foliage, and the unmerchantable portion of stemwood above 4” in the same non-overlapping predefined size classes (BMCWN_x).

A recent paper (Bernal et al., 2022), suggests that due to drought/temps expected beyond 2040, the Sierra Nevada may not be able to support carbon loads of aboveground live trees over 20 Mg C/ha (note that they report biomass values, not carbon values). Carbon values are generally assumed to be half of biomass (See CAL FIRE’s “AB 1504” methodology, Christensen et al., 2019). Conversion from short tons per acre (the default F3 output units) to Mg/ha requires multiplication by 2.2417023114334.

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 biomass loss, following the formula:

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

Although the assumption of direct correlation between canopy cover and biomass 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 large tree biomass, depending on location.

Values for each of the non-overlapping, predefined, large tree size class for stemwood (BMSTM_x) rasters and for branchwood, foliage, and unmerchantable portion of stemwood above 4” (BMCWN_x) rasters were adjusted for 2021 following the same procedure using eDaRT MMI.

This layer for the Large Tree Carbon metric is derived from F3 layers (2021) using the following formula:  

[(sum(BMCWN_25, BMCWN_35, BMCWN_40, BMSTM_25, BMSTM_35, BMSTM_40)/2)* 2.2417023114334]

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