- Metric Name:  Dead Carbon

- Data Vintage:  2021

- Unit Of Measure: Mg C/ha

Dead carbon includes dead and down (litter, duff, fine, coarse, and heavy fuels, including 1000+ hour logs) which are inherently unstable due to prevailing fire and decay processes, and a destabilizing factor in the fire-adapted forests of the Sierra to the extent that they contribute to uncharacteristic fire behavior. In addition to that dead carbon, this metric includes the carbon from the canopies of small trees, which is readily released during fire (specifically, trees less than 10 inches in diameter). Standing dead carbon is also included, representing the slower leak from the landscape carbon stock. As a result, this metric is a proxy for unstable carbon: fire liable carbon on the landscape which is more vulnerable to combustion.- Creation Method:  The F3 model generated several different raster surfaces in non-overlapping predefined size classes to estimate the small size live tree (those less than 10” DBH) branchwood and foliage plus unmerchantable portions of stemwood above 4-inch diameter (BMCWN_x), plus the standing dead estimates for all size classes (including stems, branches, and foliage still present) from the FVS Fire and Fuels extension carbon report (Standing_D). The model also generated several raster surfaces of fuel loading estimates of the coarse woody debris by non-overlapping predefined size classes: including 1, 10, 100, and 1000-hour fuels (FLOAD_1-5); and estimates for coarse woody debris of heavy fuels by non-overlapping predefined size classes greater than the 1000-hour fuel sizes (greater than or equal to 6” and less than 8”; FLOAD_6-9) and for litter and duff.

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:  The 2021 values described below for Total Dead/Down Fuels and for Standing Dead and Ladder Fuels, were summed and converted to Mg C/ha to derive this metric.

No adjustments were made for 2021 to the Total Dead/Down Fuels (FLOAD_x, LITTER, DUFF), due to uncertainties in conversions based on the limits with which change detection information can quantify the individual components of this metric. For areas with disturbance 2019-2021 (defined as eDaRT MMI greater than or equal to  10% canopy cover loss), total dead/down fuel values are not represented for 2021 (i.e., NULL). For areas undisturbed 2019-2021, it is a reasonable assumption that total dead/down fuels did not change significantly over the course of two years.

Values for 2021 Standing Dead and Ladder Fuels (Standing_D, BMCWN_x) 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 tree biomass, depending on location.

Adjustments for the standing dead trees raster (Standing_D) took the difference between 2019 and 2021 live volume (as estimated using MMI) converted to short tons/acre using a conversion factor of 32.1 cubic feet/ton and the result was summed with 2019 standing dead. 

Values of undisturbed areas of Total Dead/Down Fuels (FLOAD_x, LITTER, DUFF) were added to the non-overlapping predefined size classes for the small size live trees (less than 10” DBH) branchwood and foliage plus unmerchantable portions of stemwood above 4-inch diameter (BMCWN_x), which had been adjusted for 2021 using MMI percent adjustments. This total biomass was halved converting to carbon values and added to the adjusted standing dead and the result converted to Mg C/ha.

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

[(sum(FLOAD_1-9, LITTER, DUFF, BMCWN_0, BMCWN_2, BMCWN_7)/2) + Standing_D] * 2.2417023114334

In cases where any individual input to the formula is NULL, the resulting sum cannot be computed and is therefore also NULL.

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