Fire regimes describe information about fire occurrence patterns and their variation, enabling us to describe pattern in fire and its effects on ecosystems. This framework, and the included tables in this section, are adapted from Sugihara et al. (2006) and describe alliance fire characteristics using seven attributes of fire pattern grouped into temporal, spatial, and magnitude variables. Temporal attributes include seasonality and fire return interval. Spatial attributes include fire size and spatial complexity of the fires. Magnitude attributes include fireline intensity, fire severity, and fire type. For a more detailed discussion, see Sugihara et al. (2006).
The fire characteristics information included in the text and the tables was obtained through a combination of literature search, expert knowledge, and opinion obtained from 2000 to 2002 at a series of regional workshops supported by the Joint Fire Science Program. The tables summarize data sources including tree-rings with fire-scars, charcoal deposits in sediment cores, fire records and stand age distributions. This information was considered with the physical environment for each alliance and knowledge of the life history and fire relationships of the characteristic plant species to develop an alliance’s specific fire regime. The resulting fire regimes describe the fire conditions which are necessary to support and maintain each alliance over long time periods.
The following is a key to the information listed in each full alliance description. As this is a complete summary of California fire attributes, not restricted to the known alliances discussed in this manual, there are some attributes that are not currently known for any of the described alliances. The number in brackets following each heading in the key indicates the number of full alliances known with this attribute.
Temporal Fire Regime Attributes
Fire Return Interval
Fire return interval describes how often fires occur over several fire cycle
Truncated Short Fire Return Interval.  All of the area that burns does so with short fire return intervals. Long intervals will allow the establishment and growth of species that will convert the area to another vegetation type. Many oak woodlands, montane meadows, grasslands, and other Native American-maintained types are typical of this fire return interval pattern.
Short Fire Return Interval.  Most of the area burns at short fire return intervals, but there is a wide range including a small proportion of longer intervals.
Figure A1. Schematic representation of major patterns of wildfire return intervals in California
Truncated Medium Fire Return Interval.  The area that burns does so within a range of fire return intervals that has both upper and lower limits that are defined by the life histories of characteristic species. Intervals outside of that range result in conversion to another vegetation type. This is a variation of the previous pattern with upper and lower boundaries on the length of fire return intervals. Many of the closed-cone pine and cypresses are examples of plants where fires must occur within a specific range of intervals for them to regenerate. If fires are too close or too far apart in time, the plants cannot persist.
Medium Fire Return Interval.  Most of the area burns at medium return intervals but occasional strong deviation will not usually facilitate conversion to another vegetation type. This set of fire return interval distributions includes a variety of means, ranges and shapes. The presence of a relatively wide range of intervals within the regime is characteristic.
Truncated Long Fire Return Interval.  In all of the burned area, intervals are long (typically greater than 70 years) and fires burning over the same area within a few years or even decades do not occur without conversion to another vegetation type. This return interval pattern is characteristic of vegetation with discontinuous fuels or very short burning seasons such as most very arid deserts, sand dunes, alpine, and subalpine areas. Plant species are generally not adapted to fire.
Long Fire Return Interval.  In most of the burned area, fire return intervals are long. Fires burning over the same area at shorter intervals can occur, but account for only a small proportion of the overall burned area. This pattern is characteristic of areas that are geographically isolated, do not normally have a fuel layer which will carry a fire, have discontinuous fuels or very short burning seasons, or lack ignition sources.
Seasonality is a description of when fires occur during the year. California in general has warm dry summers and cool moist winters, but season alone does not determine when and how the vegetation is likely to burn. Other factors including elevation, coastal influences, topography, characteristics of the plants, ignition sources, and seasonal weather patterns also influence the fire season. Season of burning is especially important biologically because many California ecosystems include species that are only adapted to burning during a limited part of the year.
Spring-Summer-Fall Fire Season (SSF).  The longest fire season type that occurs in California has fires burning well distributed from May to November. It occurs in vegetation with early spring warming and drying and in which fire is primarily carried in rapidly curing herbaceous plants. The spring-summer-fall fire season type occurs in low elevations and deserts that cure early in the spring and persists until wetting rains occur in the late fall.
Summer–Fall Fire Season (SEF).  This is the characteristic fire season type for many of the lower and middle elevation, montane conifer forests of the state. Fires are primarily carried in herbaceous, duff and needle layers. Most of the area burns from July to October.
Figure A2. Schematic of major patterns of wildfire seasonality in California
Late Summer Short Fire Season (LSS).  This is the shortest fire season type that occurs in California. It is characteristic of alpine and subalpine vegetation where there is a very short period late in the summer where the vegetation is dry enough to burn. The climate excludes fire for the remainder of the year. Although lightning is abundant, fuels are mostly sparse and discontinuous, resulting in few fires.
Late Summer-Fall Fire Season (LSF).  This is the characteristic fire season type for central and southern coastal California chaparral. Fire occurrence and size, greatly influenced by Santa Ana and north winds, most commonly occur in the late summer and early fall. A time when live fuel moisture levels are at their lowest. Most of the area that burns does so from September to early November.
Other Seasonal Distributions. The seasonal distributions just described account for most alliances, but a few other distributions are found such as early fall (EF) , spring-summer (SS) , or summer-fall (SF)  without specification of SEF or LSF.
Spatial Fire Regime Attributes
Fire size is the characteristic distribution of area within the fire perimeter. This is not always the same as the total amount of area burned by the fire because it also includes unburned islands and the entire mosaic of burned and unburned areas. The size a fire attains is determined by fuel continuity, site productivity, topography, weather, and fuel conditions at the time of the fire.
Small Fire Size.  Most of the area that burns does so in fires smaller than 10 ha, with larger fires accounting for much less of the total area burned.
Truncated Small Fire Size.  All of the burned area is in small fires, usually less than 1 ha. This is characteristic of areas with very discontinuous fuels.
Medium Fire Size.  Most of the area that burns does so in medium-sized fires that ranges from 10 to 1,000 ha. Smaller and larger fires do occur, but account for a small proportion of the total area burned. This fire size pattern is characteristic of vegetation that has patchy fuel conditions, is limited stand size, has limited burning periods, or fuel continuity.
Figure A3. Schematic representation of wildfire sizes in California
Large Fire Size.  Most of the area that burns is in large fires that are greater than 1,000 ha. This pattern is characteristic of vegetation occurring over extensive areas, with fires typically spreading in continuous fuel layers.
Spatial complexity, or patchiness, is the spatial variability of different fire severity levels within the fire perimeter.
Low Spatial Complexity (L).  Most of the area within the perimeter of the fire is homogeneous with few unburned islands and a relatively narrow range of severity producing a course-grained vegetation pattern.
Moderate Spatial Complexity (M).  Most of the area within burn perimeter has an intermediate level of complexity. Burned and unburned areas and severity levels produce a mosaic of fine and course-grained vegetation patterns.
High Spatial Complexity (H).  Most of the area burns in a highly complex pattern of burned and unburned areas and severity levels producing a fine-grained vegetation pattern.
Figure A4. Schematic representation of types of spatial complexity in California wildfires
Multiple Spatial Complexity.  Most of the area burns in fires that are of two distinct types, one has a complex burn pattern of burned and unburned areas and severity levels producing a fine-grained vegetation pattern, and the other has a mostly uniform pattern of burned area and severity levels and produces a course-grained vegetation pattern. This type is characteristic of vegetation where two distinct fire types occur with flaming fronts in two different fuel layers.
Magnitude Fire Regime Attributes
Fireline intensity (or simply “intensity’) is a measure of energy release per unit length of fire line.
Low Intensity (L).  Most of the area burned has fires that are low intensity with flame lengths less than 1.2 m. A smaller proportion of the area burns at moderate to high intensity levels. Fire remains on the surface and occasionally consumes understory plants.
Moderate Intensity (M).  Most of the area burned has fires with moderate intensities with flame lengths from1.2 to 2.4 m. Fire usually remains on the surface, although there could be complete consumption of understory plants.
High Intensity (H).  Most of the area burned has fires that are of high (to very high (VH) intensities) with flame lengths over 2.4 m. A smaller proportion of the area burnsat low to moderate intensity levels. Some crowning, spotting, and major runs are probable. These intensities usually result in complete consumption and mortality of entire individual plants.
Figure A5. Schematic representation of wildfire intensity patterns in California.
Multiple Intensity.  Most of the area burned has fires that are mostly of two types, low intensity surface fires and high intensity crown fires. A smaller proportion of the area burns at moderate or very high intensity levels.
Fire severity is a description of fire effects applied to a variety of landscape components, including vegetation, soil, geomorphology, watersheds, wildlife habitat, and human life and property. Fire severity is not always directly a result of fire line intensity, but results from a combination of fire intensity, residence time, and moisture conditions at the time of burning. This treatment of severity emphasizes the effect that fire has on the vegetation, especially the species that characterize the vegetation type.Low Fire Severity (L).  Most of the area burns in low severity fires that produce only slight or no modification to vegetation structure and most of the mature individual plants survive. A small proportion of the area burns at higher severity levels.
Moderate Fire Severity (M).  Most of the area burns in fires that are moderately stand modifying, with most individual mature plants surviving. A small proportion of the area burns at lower and higher severity levels.
High Fire Severity (H).  Fires kills the above ground parts of most plants over most of the burned area. Most mature individual plants survive below ground and sprout. A small proportion of the area burns at lower and higher severity levels.
Figure A6. Schematic representation of wildfire severity in California.
Very High Fire Severity (VH).  Fires are mostly stand replacing over much of the burned area. All or nearly all of the individual mature plants are killed. A smaller proportion of the area burns at lower severity levels.
Multiple Fire Severity.  The area burned is mostly divided between two distinct fire types, low severity and high to very high severity. A smaller proportion of the area burns at moderate severity levels.
Fire type describes the combination of flaming fronts that are characteristic of a vegetation type. The term “crown’ in this context refers to individual plant crowns in the case of a passive crown fire and to highest canopy layer in the other cases.
Surface Fire (S).  Fire spreads along the soil surface.
Passive Crown Fire (PC).  Fire spreads along the surface with occasional patches of crown fire (torching).
Active Crown Fire (AC).  The fire spreads through crown fuel with supporting surface fire.
Independent Crown Fire (IC).  The fire spreads through crown fuel ahead of the surface fire.
Ground Fire (GF).  Burning within the ground, ground fire is a significant contributor to fire effects, but are not usually part of the flaming front.
Figure A7. Schematic representation of types of wildfires in California.
Surface-Passive Crown Fire (S-PC). Most of the area that is burned does so with a surface fire.Although as much as 30% of the area may experience torching of individual trees or shrubs or groups of trees or shrubs, the flaming front is primarily a surface fire. Organic layers are burned by ground fires, and small amounts of active crowning can burn stands of trees.
Passive-Active Crown Fire (P-AC).  Most of the burned area has fire that is a combination of surface fire-supported passive and active crown fire. Active crown fire is dependent on and synchronous with a surface fire and is the most common type of sustained crown fire.
Active-Independent Crown Fire (A-IC).  In California forests, independent crown fires are very rare but do occur occasionally in combination with active crown fire. When this type of fire does occur, the crown fire burns independently of the surface fire and advances over a given area prior to the occurrence of the surface fire.
Multiple Fire Type.  Both surface fire and crown fire are characteristic of these ecosystems. Each fire type occurs in a complex spatial mosaic within the same fire under different fuel, topographic, and weather conditions.