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Mixed-Severity Fire Definition Inconsistency: Some western vegetation models were developed using inconsistent definitions of mixed-severity fire.
Background & Advice:
Fire severity types should be defined and used consistently by all modelers. Early in the project, it was learned that, after the models for some western mapping zones were delivered and used in the LANDFIRE (LF) production process, some regional modeling leads and modelers had interpreted the definition of "mixed-severity fire" differently, which may have impacted the way the models were constructed and ultimately the model outputs.
For LF, mixed-severity fire is defined as an area of relatively uniform 25-75 percent top-kill. Some regional modeling leads and modelers interpreted mixed-severity fire as an area containing patchy combinations of unburned areas and totally burned areas, in effect averaging to a mixed-severity condition. This latter interpretation is consistent with the definition of mosaic fire defined in version 1.2 of the FRCC Guidebook as "any landscape-scale mixed fire that has scattered patches across the fire perimeter, resulting in a mosaic of burned and unburned patches" (Hann and others 2004). FRCC Guidebook version 1.3 specifically addresses this issue by clarifying that mixed-severity fire refers only to post-fire effects on vegetation, not to the spatial pattern.
As soon as the discrepancy in the mixed-severity fire definitions was identified by the LF team and recognized as inconsistent with the LF production process, every effort was made to ensure that vegetation models were built using the LF definition. However, users should be aware of the potential inconsistency in the application of the mixed-severity fire definition in LF models from mapping zones 8, 9, 10, 12, 13, 14, 15, 16, 17, 18, 19, 23, 24 and 28. Furthermore, if models from these zones were adopted for use in other zones (as noted in the Comments field in the model description), the potential misapplication of mixed-severity fire may still exist.
It appears that the misapplication of mixed-severity fire occurred most often in grass systems and secondarily in shrub systems. In many cases it appears that modelers inappropriately applied mixed-severity in cases where they would have chosen replacement fire under the appropriate LF definitions. The implication of using mixed-fire inappropriately in cases where replacement fire should have been used is that it would tend to push models towards the low- and mixed-severity fire regime groups (I and III) rather than the high-severity fire regime groups (II and IV). These changes in the modeled fire regime group could then be carried over to the fire regime group map, which is created using the probability and severity of disturbances from the VDDT model.
Hann, W.; Shlisky, A.; Havlina, D.; Schon, K.; Barrett, S.; DeMeo, T.; Pohl, K.; Menakis, J.; Hamilton, D.; Jones, J.; Levesque, M.; Frame, C. 2004. Interagency Fire Regime Condition Class Guidebook. Last update October 2007: Version 1.3. [Homepage of the Interagency and The Nature Conservancy fire regime condition class website, USDA Forest Service, U.S. Department of the Interior, The Nature Conservancy, and Systems for Environmental Management]. [Online]. Available: https://www.frames.gov/partner-sites/frcc/frcc-home.