Fuels Development |
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What are the prototypical fuels development sequences in the Santa Monica Mountains?
A static postfire fuels development sequence was compiled from local knowledge of chaparral postfire succession and translated into the context of existing fuel models, including the National Forest Fire Laboratory fuel models (Anderson, 1982), some United States Forest Service custom fuel models ( Weise and Regelbrugge, 1997), and custom fuel models developed at the University of California, Santa Barbara. Although there are many vegetation cover types in the Santa Monica Mountains, the predominant natural vegetation falls into one of four categories: grass; coastal sage scrub; northern mixed chaparral; and chamise chaparral. Each of these classes has a unique sucessional sequence of fuel types which can be roughly identified using the stand age.
In addition to areas of natural vegetation, disturbed areas, such as grazed pastures, firebreaks, and wildland urban interface, were also assigned a fuel type. However, unlike natural vegetation disturbed areas are assumed to remain static and exhibit no successional pattern of fuels development. In cases where the vegetation cover type is a grassland or savanna area, then the ultimate fuel condition of the area is also assumed to remain static, perpetually grassland. The fuel model used to depict pastures, firebreaks, and other grass areas is the standard NFFL short grass model, also known as NFFL 1. The fuel model used to depict wildland urban interface is a custom model, known as UCSB 20.
Northern mixed chaparral, chamise chaparral, and coastal sage scrub are all assumed to begin their successional sequence as grass-dominated fuels. However, the fire behavior of each cover type rapidly diverges as the stand matures. Details on the attributes associated with each of the fuels models in the diagram below can be found in the links off of the Fuels Analysis page. The manner in which each diverges is depicted in the diagrams and text to follow.
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In successional sequence for Northern Mixed Chaparral, two subtypes, mesophytic dominated and sclerophytic dominated, are shown. For the Santa Monica Mountains, Northern Mixed Chaparral consists primarily of some type of Ceanothus species. Although Ceanothus species type is not distinguished in the vegetation cover type used to generate the fuel map, the argument for distinguishing between the two on the basis of differences in fire behavior is strong. The higher leaf area index of stands dominated by shrubs with sclerophytic leaves, such as Ceanothus megacarpus and Ceanothus crassifolius means that they are better absorbers of radiant heat energy during a fire, and consequently can be expected to exhibit more extreme fire behavior than stands dominated by shrubs with mesophytic leaves, such as Ceanothus spinosus and Ceanothus oliganthus. In general, the average total biomass, not just fuel biomass, of Ceanothus-dominated chaparral in the Santa Monica Mountains ranges from 50 to 90 Mg/ha. Future fuel datasets should develop fuel models unique to each type of Ceanothus species. The fuel models used to represent stands of Ceanothus species include USFS 18 and USFS 16.
The limited ground-data that exists appears to reveal two regimes of chamise chaparral as well. One regime is characterized by a maximum stature of less than 1.5 meters and is generally confined to rocky, unproductive slopes where chamise, Adenostoma fasciculatum, is almost the only species present. The second regime is characterized by individual shrubs with a comparatively larger maximum stature, in excess of 2.0 meters, located in more productive and diverse sites. Since the dichotomy in chamise stands was not present in the vegetation cover type used to generate the fuel map, these features could not be incorporated. In general, the average total biomass, not just fuel biomass, of Adenostoma-dominated chaparral in the Santa Monica Mountains ranges from 20 to 50 Mg/ha. As in the case of Northern Mixed Chaparral, future fuel datasets should develop custom fuel models unique to each regime of chamise chaparral observed. The fuel models used to represent stands of Adenostoma species include USFS 17 and USFS 15.
The final natural vegetation cover type left to address is coastal sage scrub. Stands of coastal sage scrub are characterized by a heterogeneous mixture of one of the following three subshrubs: Artemisia californica; Salvia mellifera; and Salvia leucophylla. A relatively thick layer of herbaceous material including various subspecies of Eriogonum as well as native and exotic grasses are also a common component of fuels in these areas. Large shrubs, such as Malosma laurina, Rhus ovata, and Rhus integrifolia, occur in coastal sage scrub stands, but comprise a much smaller fraction of the total cover than shrubs in the hard chaparral stands described above. In general, the average total biomass, not just fuel biomass, of coastal sage scrub stands in the Santa Monica Mountains ranges from 15 to 35 Mg/ha. In comparison to Ceanothus and Adenostoma dominated chaparral, a greater fraction of this biomass is available as fuel, probably in excess of 80%. In contrast to the coastal sage scrub fuel model developed by the United States Forest Service, USFS 18, the coastal sage scrub of the Santa Monica Mountains contains far less fuel biomass. For less mature stands an alternative model, UCSB 21, with reduced fuel loadings is appropriate.
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