Pfrs 1 Summary

[Sanna Pospicil]

Drawingcomparisons between presettlement and current fire regimes can also assist land managers in prioritizing areas for ecological restoration. Fire return interval departure (FRID) analysis facilitates quantification of the difference between current and presettlement fire return intervals (FRIs), allowing managers to target areas at high risk of type conversion due to altered fire regimes (Caprio et al. 1997, Caprio and Graber 2000, van Wagtendonk et al. 2002). Robust estimates of the variability of presettlement FRIs in different vegetation types are a crucial part of FRID analysis, yet most fire history studies are highly localized, making it difficult to apply the results of individual studies to a regional-scale mapping and assessment effort.

Much work has been accomplished in documenting historical fire regimes in various vegetation types throughout California, and while several manuscripts summarize different subsets of this information, they are often either restricted to data derived from tree-ring studies, limited to forest vegetation types in a particular geographic region, or not intended to be comprehensive (e.g., Heyerdahl et al. 1995, Skinner and Chang 1996, Stephens et al. 2007). Thus, scientists and land managers currently lack a single source that summarizes all of the literature pertaining to presettlement fire regimes. The objectives of this paper are to provide an up-to-date comprehensive summary of presettlement fire frequency estimates for California ecosystems dominated by woody plants, and to provide the quantitative basis for fire return interval departure (FRID) mapping across California.

Although the state of California is home to a high diversity of species, vegetation types, and fire regimes (Barbour et al. 2007), similarities among fire regimes and their effects on vegetation generally allow the organization of ecosystems into broad fire regime groups. Published efforts to categorize relationships between fire and vegetation in California include Agee (1993; northern California), Skinner and Chang (1996; Sierra Nevada), Stephenson and Calcarone (1999; southern California), Arno (2000; western US), Sugihara et al. (2006; statewide), Sawyer et al. (2010; statewide), and the LANDFIRE project (2010; Rollins 2009; entire US). Both the Sugihara et al. (2006) and Sawyer et al. (2010) efforts drew from a series of Joint Fire Science Program supported regional workshops held between 2000 and 2002 that reunited fire and vegetation experts from across the state and developed descriptions of fire regime characteristics for California vegetation communities. All of this information also fed the development of the LANDFIRE Biophysical Settings (BpS), which are potential natural vegetation (PNV) types linked to quantitative models of disturbance and succession (Rollins 2009). The disturbance-succession models for the California BpS types (which apply to the pre-Euro-American settlement period) were developed at a series of regional expert workshops sponsored by The Nature Conservancy in 2004 and 2005. After refinement and peer review, the BpS classification was finalized and mapped as part of the national LANDFIRE project (see Rollins 2009 and www.landfire.gov for details).

As an evolutionary outgrowth of the previous fire regime work cited above, the LAND-FIRE Biophysical Settings represents the current state of the art for linking vegetation and pre-Euro-American settlement fire regimes across California. There are more than eighty individual BpS types mapped in California by LANDFIRE, but some are extremely uncommon, and many share similar fire regimes. Using the fire regime information provided for each BpS in the type description (LANDFIRE 2010), and referring to integrative vegetation and fire resources in the literature (see citations above, plus, e.g., Burns and Honkala 1990, Potter 1998, Barbour and Billings 2000, Barbour et al. 2007) and on the internet (e.g., the Fire Effects Information System [ ]), we reduced the BpS list down to a smaller number of pre-Euro-American settlement fire regime groups (PFRs) that we subjectively considered sufficiently different to warrant retention. From our research, we also identified a number of PFRs that were not represented in the BpS classification. PFRs were designed to balance a reasonably small number of fire regime groups with sufficiently high discrimination in fire regime characteristics. We solicited peer review of the PFR list from 27 California fire and vegetation ecology experts and received responses from eleven. After adjustment, our final list included 28 PFRs.

For each PFR, we conducted an exhaustive review of the published and unpublished literature pertaining to mean, median, minimum, and maximum fire return intervals observed prior to significant Euro-American settlement (i.e., the middle of the nineteenth century). Sources included fire histories derived from dendrochronological and charcoal deposition records, modeling studies, and expert quantitative estimates. Priority was given to studies conducted in California, but sources from other states in western North America were included as appropriate for PFRs for which information was limited. When all sources were compiled, the average was taken of all mean, median, minimum, and maximum FRI values to yield a single mean, median, mean minimum, and mean maximum FRI estimate for each PFR. Thus, the minimum and maximum FRI estimates we provide for each PFR are not absolute minima and maxima, but typical mean values that would be expected across the geographical range of each PFR.

Derived mean, median, mean minimum, and mean maximum fire frequencies for each PFR are given in Table 2. Information on median FRIs was lacking for some PFRs, so median values were either taken from expert quantitative estimates of mean FRI (desert mixed shrub, semi-desert chaparral) or were not estimated (coastal fir, shore pine, spruce-hemlock). Because FRI distributions are often skewed (with more short or long intervals, depending on the PFR), median FRI values may be a better approximation of how often a given PFR burned than mean FRIs (Falk 2004).

Much variability is evident among PFRs, with mean FRIs ranging from 11 yr (dry mixed conifer and yellow pine) to 610 yr (desert mixed shrub), median FRIs ranging from 7 yr (yellow pine) to 610 yr (desert mixed shrub), minimum FRIs ranging from 5 yr (bigcone Douglas-fir, California juniper, dry mixed conifer, moist mixed conifer, oak woodland, and yellow pine) to 190 yr (shore pine), and maximum FRIs ranging from 40 yr (yellow pine) to 1440 yr (desert mixed shrub) (Table 2). There was also a great deal of variability within PFRs, as evidenced by differences between minimum and maximum FRIs ranging from 32 yr and 34 yr (montane chaparral and yellow pine, respectively) to 1324 yr (desert mixed shrub). FRI distributions ranged from unskewed distributions with little difference between mean and median FRIs (aspen, bigcone Douglas-fir, dry mixed conifer, lodgepole pine, montane chaparral, oak woodland, subalpine forest), to highly skewed distributions dominated by relatively short FRIs (coastal sage scrub), to highly skewed distributions dominated by relatively long FRIs (pinyon-juniper). Figure 1 graphically depicts the mean, median, mean minimum, and mean maximum FRIs for the 11 most widely distributed PFRs on Forest Service lands in California.

Fire return intervals (FRIs) for the 11 most widely distributed presettlement fire regime groups in California. Solid line is mean FRI, dotted line is mean median FRI, bottom of each bar is mean minimum FRI, top of each bar is mean maximum FRI.

More research is needed in PFRs that currently have little quantitative fire history data available for California (see Table 2), or have high geographic variability in FRIs. The difficulties associated with obtaining high-resolution presettlement FRI data in shrub-dominated vegetation types categorically necessitates further study in most of these PFRs (e.g., big sagebrush, black and low sagebrush, chaparral, coastal sage scrub, curl-leaf mountain mahogany, desert mixed shrub, montane chaparral, semi-desert chaparral, silver sagebrush), and perhaps innovation of new or adaptation of existing fire history techniques. Similarly, PFRs dominated by tree species that are easily killed by fire (California juniper, coastal fir, fire sensitive spruce or fir, pinyon-juniper, shore pine, spruce-hemlock) require further study and application of techniques other than fire scar studies. The PFRs of limited geographical distribution in California (bigcone Douglas-fir, coastal fir, fire sensitive spruce or fir, Port Orford cedar, shore pine, spruce-hemlock, western white pine) are chronically understudied. Other PFRs (shore pine, desert mixed shrub, spruce-hemlock, California juniper, coastal fir, pinyon-juniper, western white pine, subalpine forest) are characterized by high geographic variability in fire frequency (high standard error of FRI statistics), requiring scientists and managers to carefully search for literature from local or similar areas.

Ignitions by indigenous peoples were likely a large component of the presettlement fire record in some PFRs, such as redwood (Greenlee and Langenheim 1990) and oak woodland, and are difficult or impossible to definitively differentiate from lightning ignitions, although fire cause may be inferred from seasonality in some cases (Anderson and Moratto 1996). Some vegetation types in certain areas were probably maintained mostly by presettlement anthropogenic fire regimes, which may have resulted in vegetation type conversions in some parts of the landscape prior to Euro-American arrival. Widespread indigenous ignitions were probably uncommon in other PFRs, however, such as subalpine forest and desert mixed shrub (Anderson 2005). Regardless, no attempt is made in this assessment to differentiate between lightning and indigenous ignitions.

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