- Metric Name:  Total Fuel Exposed to Fire

- Data Vintage:  2021

- Unit Of Measure: Short tons/acre

This is the sum of standing dead, ladders, and the dead and down, documented above. This metric quantifies the total amount of biomass available to contribute to the extreme fire intensity and spread rates that lead to high severity fire (Stephens et al., 2022).

This metric is also applicable to the Air Quality pillar, in that total fuel load is a value often required in smoke management plans to get Rx fire projects approved.- Creation Method:  The F3 model generated several raster surfaces; to estimate the small size live trees (those less than 10” dbh) branchwood and foliage plus the unmerchantable portions of stemwood above 4-inch diameter (BMCWN_x), to estimate fuel loading of coarse woody debris in non-overlapping predefined size classes (FLOAD_x), to estimate both litter and duff, and to estimate the standing dead for all size classes (including stems, branches, and foliage still present) from the FVS Fire and Fuels extension carbon report (Standing_D).

2019 to 2021 Update:  The 2021 values (described below) from the Standing Dead and Ladder Fuels and from the Total Dead/Down Fuels, were summed to derive this metric.

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 predefined size classes, may result in over- or under-estimates of actual ladder fuels, depending on location.

Adjustments for the standing dead trees raster (Standing_D) took the difference between 2019 and 2021 live volume (as estimated using eDaRT 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. This adjusted value was then 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.

Values for 2021 Total Dead/Down Fuels (FLOAD_x, LITTER, DUFF) were not adjusted due to uncertainties in conversions based on the limits with which change detection information can quantify the individual components of the metric. 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. 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).

This layer for the Total Fuel Exposed to Fire metric is derived from F3 layers (2021) using the following formula:  

[sum(Standing_D, BMCWN_0, BMCWN_2, BMCWN_7, FLOAD_1-9, LITTER, DUFF)]

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