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Fire regimes of montane riparian communities in California and southwestern Oregon

Table of Contents:

• provide up-to-date information to the management community on historical fire regimes and contemporary changes in fuels and fire regimes,
• supplement information on individual species' adaptations and responses to fire provided by FEIS Species Reviews, and
• enable LANDFIRE to incorporate the latest science on historical fire regimes into data revisions and identify regions and plant community types lacking fire history data.

This review covers fire regimes of woodland and forested riparian communities in California and adjacent southwestern Oregon. Due to lack of documentation in the literature, little information is presented on fire regimes of willow scrub, coastal sage scrub, and chaparral riparian communities. Fire regimes of riparian communities in California's Central Valley and desert regions will be covered in other FEIS Fire Regime Syntheses.

Reviews cited in this synthesis are as follows: [14,23,41,62,65,69,85,86,95,97,100].

Terminology: For defining the limits of riparian areas, this review follows usage of Gregory and others [32] and Dwire and others [23]. In a widely cited passage (for example, [23,25,33,83]), Gregory and others [32] define riparian areas or ecosystems as "three dimensional zones of direct physical and biotic interactions between terrestrial and aquatic ecosystems, with boundaries extending outward to the limits of flooding and upward into the canopy of streamside vegetation" [32]. Dwire and others [25] define the stream-riparian corridor as the stream channel, adjacent floodplains, and the transitional upland fringe. Other terms used in this review link to the FEIS glossary.

SUMMARY
This section summarizes fire regime information available in the scientific literature as of 2015. Details and documentation of source materials follow this summary.

Appendix A summarizes data generated by LANDFIRE succession modeling for the Biophysical Settings (BpSs) covered in this review. The range of values generated for fire regime characteristics in California montane riparian communities is:

DISTRIBUTION AND PLANT COMMUNITY COMPOSITION

Figure 1. Distribution of California montane riparian communities based on the LANDFIRE Biophysical Settings (BpS) data layer [51]. Numbers indicate LANDFIRE map zones. LANDFIRE did not map every BpS in this group. Click on the map for a larger image and zoom in to see details.

Riparian ecosystems of California have highly dynamic fluvial, fire, landslide, and other disturbances [70]. Bisson and others [14] concluded that large-scale flooding and wildfire disturbances are inevitable in riparian and aquatic systems, and that these disturbances are "often beneficial over long periods".

Riparian ecosystems are disproportionally important ecologically, considering the relatively small area they occupy on landscapes [85]. Riparian vegetation provides important habitat and food for a variety of wildlife and helps filter runoff before it reaches watercourses [23,31]. Functional floodplains store water—increasing potential for late-season flows—recharge aquifers, and reduce velocity of spring flows [85].

California's montane riparian plant communities are often highly diverse (e.g., [1,4,70]). They are usually more species rich—and have more complex and denser vertical and horizontal structure—than uplands [1,4,34]. High site quality can lead to higher plant productivity and/or faster rates of succession in riparian than in upland areas [3]. For example, a white fir-sugar pine-incense-cedar forest along Suwanee Creek, Sequoia National Park, had a "lush herbaceous" understory, but there was little understory vegetation 70 to 100 feet (20-30 m) from the stream [68]. In Pinnacles National Park, riparian communities contained 14% of all plant taxa known to the Park, including "crowded willow thickets" and the liana Pacific poison-oak [34], even though riparian communities comprise < 8% of Park vegetation [90]. In the Santa Monica Mountains they comprise <1% of the landscape but contain nearly 20% of the vascular plant flora of the region [70]. Stream channel and other habitat modification, recreational use, and nonnative invasive species lower diversity of California's riparian communities [70].

Foothill riparian woodlands are dominated by hardwoods, conifers, or mixes including white alder, bigleaf maple, Pacific dogwood, interior live oak [100], gray pine, Pacific ponderosa pine, and/or sugar pine [37,89]. Higher-elevation riparian communities in the Sierra Nevada are dominated by water birch, quaking aspen, willows [19,28,39], coast Douglas-fir [55], giant sequoia, incense-cedar [19], white fir [19,28,39], red fir [19], and/or Sierra lodgepole pine [19,28,39]. In the North Coast Ranges, riparian mixed-evergreen forests are dominated by white or red alder, tanoak, Pacific madrone, bigleaf maple, California bay [89], coast live oak [37], coast Douglas-fir, grand fir, western hemlock, Sitka spruce, Port-Orford cedar, and/or redwood [37,59]. Bigcone Douglas-fir and incense-cedar are dominant riparian conifers in the Central and South Coast ranges [35], and oaks, especially coast live oak and canyon live oak, become more important than in the North Coast Ranges [89]. Coulter pine/chaparral communities occur in riparian corridors on the Los Padres National Forest [19].

A riparian ponderosa pine/chaparral community along the upper Klamath River. Image by Tupper Ansel Blake, U.S. Fish and Wildlife Service.

Riparian shrub communities include riparian coastal sage scrub [35], chaparral [21], and willow scrub [37]. Riparian coastal sage scrub is a mix of shrubs that prefer mesic sites, such as American black elderberry and lemonade sumac, and drought-tolerant species such as chaparral whitethorn and birchleaf mountain-mahogany [35]. Riparian chaparral is also composed of a mix of mesic species, such as white alder and California sycamore, and drought-tolerant species such as hollyleaf cherry [21] and whitethorn chaparral (Fryer 2015 personal observation). Willow scrub occurs at low to high elevations and is composed of early-seral species that colonize sand- and gravelbars [37]. Dominant species include Brewer's willow [59], Pacific willow, dusky willow, coastal willow, bigleaf maple, and/or white alder [37]. Willows also dominate many montane scrub communities; other dominants include blue elderberry, Pacific dogwood, red-osier dogwood, and/or thinleaf alder [19,37]. In southern California, riparian scrub merging into desert scrub contains mixes of phreatophytes such as California sycamore, Goodding's black willow, red willow, and Fremont cottonwood; and desert shrubs such as desert willow, catclaw acacia, and stretchberry [37,96].

Fire influences plant community composition, stand structure, and environmental conditions of these riparian communities [53]. In some areas, riparian refugia may provide critical seed sources for plant species that recover slowly after fire. In the southern Sierra Nevada, for example, gray pines on an alluvial site survived the 2002 McMally Fire on the Kern River watershed, but the wildfire killed gray pines on upland slopes. Maximum age of gray pines in the area decreased significantly with distance from stream-riparian corridors (P<0.001) [75].

Most riparian vegetation is resilient to fire [62]. Adaptations of most riparian species to flooding (thick bark, sprouting, masting, and/or water- and wind-dispersed seeds) also enable riparian species to recover quickly from fire [23,43,69]. For details on species adaptations, follow the links in Appendix B to FEIS Species Reviews.

HISTORICAL FIRE REGIMES
Flooding, not fire, is the most important disturbance in California's stream-riparian corridors [1,49,53]. Agee [1] wrote that toward its core, a "typical mid-sized forest riparian zone" shows decreasing disturbance by fire and other upland processes but increasing disturbance by fluvial processes. Results from a study on the lower mid-Klamath River indicate that flooding had greater effects on stand structure, plant community composition, and abundance of vegetation and fuels than fall prescribed fire [49]. Across the landscape, fire and wind disturbances are most frequent in montane riparian areas near 1st- and 2nd-order streams [1]. The Pacific Southwest Research Station, Forest Service, U.S. Department of Agriculture, provides a review and synthesis of the historical range of variability of California riparian communities [73].

Flooding, fire, and wind are synergistic disturbances [3]. Riparian and aquatic ecosystems are structured, in part, by fire effects upslope. For example, windthrow into stream channels often follows severe fire [7]. Wildfires affect hillslope erosion, stream sedimentation, and large woody debris recruitment into riparian and aquatic systems. Sediment and debris are delivered in pulses after large wildfires. Over time, sediment and wood decay, break up, and are transported downstream, but these transported resources are replenished with new deposition after the next fire [14].

Fire history of California and other western riparian ecosystems is poorly known [23,91]. Because riparian communities are so variable, generalizations about their fire regimes are difficult to make [30,63]. Limited research on western riparian systems indicates that fires are often less frequent and of lower severity than in uplands [23]. In the Klamath Mountains, for example, median fire-return intervals were twice as long in riparian than in upland zones [80] (see Table 3). However, this general pattern varies by region, forest type, topography, and fire history [23] (see Fire type and severity).

California Indians used riparian areas as sources of water, pelts, fish, and plant materials, and for camping [45,49,86]. The extent of their use, and how often they burned riparian areas, is not well known [86] (but see Anderson [5] and Lake [49]). Keeter [45] wrote that within the North Fork of the Eel River Basin, Indian-set fires "would have resulted in a shorter frequency and rotation period" "than if fires had resulted only from natural causes". In northern coastal California, fire-scarred redwoods registering the most frequent fire-return intervals were nearest Indian villages or in plant-material collecting areas near campsites (Norman 2007 cited in [49]). Indian-set fires were likely of low severity and intensity and were set year-round. For example, winter and early spring fires were used to encourage growth of western bracken fern, a food source [49]. Summer and fall fires stimulated growth of shrubs, such as willows, used for basketry [36]. Traditional burning practices were curtailed as Indian populations were decimated due to disease and war and as survivors were relocated to reservations [45,49,84].

Mining activities caused many wildfires from the mid-1850s to 1870s. Indian ignitions were few by that time, although some traditional burning still occurred on tribal lands [49].

The following sections provide details on fire regime patterns in California's montane riparian communities.

Fire ignition
Ignitions in California are from humans and lightning [52]. Humans start >80% of California's fires on contemporary landscapes [95]. Lightning ignitions tend to increase with distance from the coast and elevation [81]. Studies across southern California's National Forests showed ignitions from 1980 to 2009 were positively associated with steepness of slope (P<0.001). They decreased with distance from roads and development (P<0.0001 for both variables), suggesting human influence [27]. In the Santa Monica Mountains, fire records from 1919 to 1980 showed that almost every fire was started by humans. Lightning ignitions are rare in the area because they occur in the wet season and are out of phase with Santa Ana foehn winds [64].

Dwire and others [23] stated that ignitions are less frequent in riparian than upland areas. Agee [1] speculated that this is due to moister, less flammable fuels in riparian zones. Montane riparian areas generally have more deciduous trees than upland areas, where more resinous and flammable conifers usually dominate [1].

Fire season
Fire season in California runs from summer to fall. Historically, fires would burn until extinguished by fall precipitation [2,49]. In riparian corridors, fuel moisture often remains too high for sustained fire spread until late in the fire season. Often, breaks in fuel continuity slow or prevent fire spread in stream-riparian corridors [23]. Most fires occur in late summer and fall in riparian shrublands and woodlands of the South Coast Ranges [43,64]. Historically, wildfires in the northern Sierra Nevada occurred primarily from late summer to early fall in both riparian and upland areas (88% and 79% of fire scars, respectively) [92]. Fire season runs from late August to late December in the Santa Monica Mountains, peaking in mid-September [72]. Late annual floods may delay fire season in riparian zones [62], while a dry season may advance it. The 2007 Angora Creek wildfire started on 24 June, an unusually early start to a dry fire season in Lake Tahoe Basin.

Fire frequency
Limited studies suggest that forested montane riparian areas generally burn less frequently than uplands. Reigel and others [69] found no fire history studies conducted on the Modoc Plateau region as of 2006. They speculated that fire-return intervals were longer in riparian zones of this region than in adjacent uplands, and that when fires did occur, they were likely of mixed, mostly moderate to high severity. Fire-return intervals are "moderate to long" in riparian corridors of the South Coast Ranges bioregion [43]. Skinner [79,80] suggested there is more variability in fire frequency and severity in forested riparian areas than in uplands. Because riparian areas are moister than upland areas, some fires may leave no scars on riparian trees. Consequently, fire histories using fire scars may underestimate fire frequency in riparian zones [79].

Fire history studies of montane riparian shrublands were not available as of 2015. Coastal sage scrub and chaparral riparian communities may burn at similar frequencies as their upland counterparts [54]. Shrub communities that are wet throughout the year, such as willow scrub, may burn less frequently than adjacent upland communities [54].

Four fire history studies provided quantitative data on fire frequency of forest or woodland montane riparian communities in California and southwestern Oregon. They are detailed below and summarized in Table 3.

Sespe Wilderness: In this study, white alder riparian communities on the Sespe Creek Watershed burned relatively frequently and at the same frequency as uplands. The white alder communities burned in large wildfires in 1932 [13] (Matilija Fire, 220,000 acres (89,100 ha)) [44] and 2002 (Wolf Fire, 21,600 acres (8,760 ha)). Two of the 11 sites also burned in a smaller fire in 1975 (on the Potrero John Creek Watershed, ~100 acres (43 ha)). Because this study was conducted by small streams and only 29 white alders were cored to determine stand ages, the authors cautioned that these results cannot be extrapolated across all white alder communities in the area [13].

Northern Sierra Nevada: Fire-return intervals were not significantly different between riparian and upland areas in most (75%) of the sites sampled in this fire history study (see Table 3). Three plant communities were studied on sites in Lake Tahoe Basin and on Lassen National Forest: Jeffrey pine, mixed conifer, and white fir. Jeffrey pine tended to dominate south-facing and other dry slopes, while white fir tended to dominate north-facing and other moist slopes. The oldest fire scar found, on a Jeffrey pine snag, dated to 1526. On both riparian and upland sites, most fires occurred from late summer to early fall. Mean fire-return intervals were calculated in 2 ways: using a "broad filter", which included every fire scar on every tree (after 2 trees were scarred) and thus captured information even from small fires; and using a "narrow filter", which included only fires recorded on 2 or more trees on the site and thus included only larger fires. Using the broad filter, the authors calculated mean fire-return intervals of 16.6 years on riparian sites and 16.9 years on upland sites. Using the narrow filter, they calculated mean fire-return intervals of 30.0 years on riparian sites and 27.8 years on upland sites. [92]. Fire-return intervals were shortest on sites with these conditions [92,93]:

• mixed-conifer stands with a high proportion (22.8%-37.6% basal area) of fire-tolerant Jeffrey pine, Pacific ponderosa pine, and sugar pine
• east of the Sierra crest
• at low elevations (<6,400 feet (2,000 m))
• near narrow, incised streams (width:depth ratio <6.2)

In white fir communities, mean fire-return intervals were shorter on riparian thanupland sites (Table 2), although the difference was significant at only 1 of 3 sites (P<0.10). In Jeffrey pine and mixed conifer plant communities, the majorityof mean fire-return intervals were longer on riparian than upland sites, althoughthe differences were significant on only 3 of 27 sites (P<0.10)[92].

This study found no significant difference in mean fire-return intervals before and after 1850 [92,93]. The authors suggested that the continuance of presettlement fire frequencies into the settlement and fire exclusion periods might be due to extensive logging and slash burning during railroad construction [93]. Increased occurrence of fire in both riparian and upland forests across all sample sites was significantly correlated with drought cycles recorded in the Palmer Drought Severity Index. This correlation was stronger in upland areas than in riparian areas (P≥0.05), indicating that upland fire-return intervals were more highly synchronized with summer drought [92].

Although fire-return intervals were similar between riparian and upland zones in this study, fire behavior and effects may differ between the 2 zones. Riparian sites in this study had significantly higher fuel loads, stand densities, ladder fuels, and predicted probabilities of torching and overstory mortality compared to upland sites (P<0.05 for all variables). Based on those variables, the authors considered sites in the Lassen National Forest most fire-prone and sites on the east side of the Lake Tahoe Basin least fire-prone. They suggested that the variability of fuel properties (loading, stand density, etc.) was greater for riparian than for upland sites [94].

Klamath Mountains: Skinner and others [79,81] reported that in the Klamath Mountains, median fire-return intervals were about twice as long in riparian areas near perennial streams as in nearby uplands, but the ranges were similar for riparian and upland areas. The riparian communities were mixed-conifer forests by 1st- to 3rd-order streams [79]. The authors speculated that in riparian areas near small or ephemeral streams, fire-return intervals are likely similar to those of surrounding areas. Riparian areas near perennial streams were often fuelbreaks [79,80,81]. However, they speculated that upper reaches of intermittent streams may burn more severely and at higher intensity than nearby uplands because channels in upper reaches may become chutes that spread fire and burn more intensely than the overall landscape [79].

Rogue River: A fire history study in the Rogue River watershed, southwestern Oregon, showed a pattern of moderate-interval, mixed-severity fires at midelevations and frequent, mostly low-severity surface fires in lower-elevation interior valleys. Fire scars and tree ages in midelevation mixed-conifer riparian sites showed that mixed surface and crown fires of low to high severity occurred at approximately 50- to 60-year intervals until the latter 19th century. Severe fires created large gaps that favored coast Douglas-fir recruitment. Coast Douglas-fir dominated the overstory (82% of total recruitment) from 1740 to 1850, but white fir dominated the overstory (51% of total recruitment) afterwards. Coast Douglas-fir showed pulses of recruitment and rapid growth after fires before 1900 but little recruitment and slow growth with fire exclusion in the 20th century. Slow growth was attributed to successional replacement by incense-cedar and other shade-tolerant species [56].

Lower-elevation, interior valley sites were also composed of mixed conifer and dominated by coast Douglas-fir. Coast Douglas-fir recruitment occurred in 2 broad peaks, around 1870 and 1950. These recruitment periods were attributed to 2 stand-replacement fires in an area that, prior to the 19th century, typically experienced frequent, low-severity surface fires. Few fire scars were found on these sites; the authors suggested that this was because fires were so frequent that they caused little scarring to overstory trees. Presettlement structure of this community was likely open coast Douglas-fir-Oregon white oak woodland or savanna, with few shade-tolerant conifers. The interior valley zone is presently a mosaic of mixed-conifer forest dominated by white fir, Oregon white oak woodlands and savannas, and chaparral [56].

Fire type and severity
Montane riparian zones of California and southwestern Oregon experience surface fires, mixed fires, and crown fires [12,48,58]. Crown fires are typical in coastal sage scrub and chaparral [43]. Forests may have surface, mixed, or crown fires [12,48].

Fire severity is often less in riparian than in upland communities due to lower air temperatures from shade, higher relative humidity, and moister fuels [23,62]. However, fuel loads are often higher in riparian than upland communities due to the availability of water. During drought, fire severity and intensity in riparian areas may be equal to or exceed fire severity and intensity in upland areas [23,62].

Topography can greatly affect fire severity. Taylor and Skinner [87] suggested that headwater reaches may act as chutes where fires spread rapidly and burn severely. Steep riparian canyons are more shaded than basins, but they can become tunnels though which windspeed, fire intensity, and fire severity increase. Basins and gently sloped montane riparian areas may have lower windspeeds than valley areas or surrounding uplands and ridgetops. Consequently, downed woody fuels may be less in basins and on gentle slopes, and fires may have decreased rates of spread, severity, flame length, and fireline intensity [23].

Mediterranean riparian communities generally recover from fire more quickly than adjacent upland communities. A review suggested that recovery time in riparian communities is about half that of upland plant communities [95].

There are 4 general patterns of fire severity in western riparian systems [54]:

1. Riparian areas serve as fuelbreaks [54,62]: This is most common where large perennial streams and river valleys create large breaks in fuel composition and continuity [54]. Large riparian areas with flat to moderate slopes, stream-riparian corridors with wide stream channels, gravel bars, and sparsely vegetated areas with wet soils are generally effective fuelbreaks [54]. The effectiveness of a riparian zone as a fuelbreak is proportional to its width, foliar moisture, and humidity [95]. In a mixed-conifer forest on the Plumas National Forest, the 1999 Lookout Fire burned around riparian zones of Third Water Creek but burned into Fourth Water Creek's riparian zones at high severity. First terraces of Third Water Creek were twice as wide (20.7 feet (6.3 m) as those of Fourth Water Creek (10.8 feet (3.3 m)), and terraces of Fourth Water Creek were significantly steeper (P<0.05 for all variables) [48]. Moist fuels and saturated soils may cause a fire to stop at or "jump" the riparian area in most years [54]. Fire severity, soil disturbance, and consumption of understory fuels increases with distance from the water's edge [95]. On the Rusch and Judd Creek watershed in the Klamath Mountains, many fires that burned from 1650 to 1930 were bounded by riparian areas with permanent water [78]. Ability of riparian areas to act as fuelbreaks decreases late in the fire season and during drought years [54,93].

2. Riparian areas burn less severely and/or less frequently than adjacent uplands [54]: This is most likely when the riparian area is wetter than adjacent vegetation [43,54]. This pattern is frequently asserted and well documented by experts (for example, [1,23,61,62]). It is most common on northerly slopes with cold-air drainage [54].

3. Riparian areas burn like adjacent uplands [54]: This occurs in areas where riparian vegetation and terrain are similar to those of uplands; for example, in coastal sage scrub and chaparral riparian communities and low-lying riparian areas with low relief. This pattern may also occur in riparian woodlands and forests during severe fire weather, when a large fire burns across an entire landscape [54], and it is particularly likely in narrow riparian areas with 1st- or 2nd-order streams [1,62]. Despite distinct differences in vegetation composition and topography between riparian and upland areas, fire severities were somewhat similar in riparian and upland areas during the large 2002 Biscuit wildfire in southwestern Oregon; riparian areas had similar levels of overstory fire severity (crown scorch and basal area mortality) compared to uplands but lower understory fire severity (exposed mineral soil and scorch height). Upland areas were mixed coast Douglas-fir-Pacific ponderosa pine and grand fir-white fir; overstories of riparian areas also had red alder, western hemlock, western redcedar, and Port-Orford cedar. First- and 2nd-order streams with steep banks were most common in the area [33].

4. Riparian areas burn more severely and/or more frequently than adjacent uplands ([54], Agee personal observation cited in [3]): Fire managers have observed this in steep terrain and narrow stream valleys. Such topography often promotes updrafts and convective heat transfer, so the fire spreads rapidly and at high severity upland and up the stream-riparian corridor. This pattern is most likely in middle to upper portions of a drainage—especially on southerly slopes—along small perennial or intermittent channels, and where fuel loads are higher or denser than in adjacent uplands [54].

High fuels loads in riparian areas can increase fire spread and severity by acting as "wicks". Fuel loads higher than those of surrounding vegetation may be due to natural succession, fire exclusion, tree harvesting, or fuel treatments in uplands [54]. During the 2007 Angora Fire on the Tahoe National Forest, heavy dead woody debris in the Angora Creek stream-riparian corridor helped fuel a mixed surface and active crown fire that raced down the corridor and up Angora Ridge [58]. The community type was Jeffrey pine/manzanita-antelope bitterbrush [58,72].

Figure 2. The Angora Creek corridor shortly after the 2007 Angora Fire. Surface and crown fuels were heavy, and most were consumed by this severe wildfire [58]. Forest Service photo.

Fire intensity
Reports of fire intensity in riparian zones are rare. In a mixed-conifer riparian forest on the Dark Canyon Creek Watershed, Blodgett Experimental Forest, a prescribed fire from 21 to 23 October 2002 burned at variable intensity (138-1,165 kW/m, x̄ = 625 kW/m), with flame lengths from 2.5 to 6.6 feet (0.75-2.0 m). It was most intense where there were large accumulations of conifer litter, duff, and other fine fuels. The last fire on the watershed was in 1905 [9], although the mean fire-return interval was 6.8 years from 1649 to 1921 (Stephens and Collins 2004 cited in [9]). Historic fire-return intervals were not available for the riparian forest.

Fire pattern
Patterns of fire spread and consumption in riparian areas are not well described in the literature [23]. Skinner [79] suggested that fires in California's montane riparian zones are likely patchier than in uplands, although these patterns are seldom documented [95]. Riparian areas within conifer ecosystems may be dominated by woody deciduous species including alders, willows, cottonwoods, and/or quaking aspen. Dwire and Kauffman [23] speculated that this change in species composition often results in patchy fires, but they cautioned that as of 2003, few data were available on fuel loads, chemistry, or moisture for even the most common riparian communities. The Lookout Fire on the Plumas National Forest burned in patches of low, moderate, and high severity in riparian zones [48]. On the Seiad Late Successional Reserve in the Klamath Mountains, mean fire size from 1627 to 1987 was around 900 acres (400 ha), with 16 fires burning more than one-third of the approximately 4,000-acre (2,000 ha) study area. Upper reaches of streams, especially seasonal streams, generally burned more severely than lower reaches [78].

CONTEMPORARY CHANGES IN HYDROLOGY, FUELS, AND FIRE REGIMES
Extent of California's riparian systems is greatly reduced from historical ranges [74,100]. Riparian landscapes are estimated at 2% of their extent 300 years ago [38,74].

Changes in hydrology and fuels: Land use and management have altered physical and biological characteristics of many riparian areas in California. Alterations include lowering of surface water, groundwater, and biotic diversity; and changes in floodplain topography, stand structure, and species composition. These human disturbances can profoundly affect fire regimes of riparian areas [23]. Shaffer and others [77] suggested that although the fire-return interval may not have changed due to altered stream flows, fires probably move through valley bottoms and low-gradient riparian zones differently than they did historically. Human alterations in hydrology and to the terrain vegetation have likely increased the probability of severe fires and reduced the capacity of riparian areas to serve as fuelbreaks [23]. Some speculate that lack of flooding on regulated watercourses has contributed to higher fuel accumulations and more severe fires than occurred prior to regulation [26,46]. Legacies of past management within watersheds can affect fuel characteristics and possibly confound the effectiveness of fuel treatments [25,53].

Greatest alterations in hydrology are due to dams, decline of American beaver populations, mining, and unregulated logging. Dams reduce the magnitude and frequency of downstream floods [11,60,98], which may result in more homogeneous vegetation below dams [11]. Dams have greatly reduced the extent and changed the structure and composition of riparian communities [23]. Decline of American beaver populations has also altered stream flow rates [23,25,30,60]. Historically, beaver dams were far more numerous and collectively, they greatly slowed water velocity. Sediment and debris carried by streams lodged behind the dams, trapping sediments and nutrients along the stream length. Backed-up water raised the water table and increased wetland areas. With removal of American beavers by trapping, many beaver dams eventually failed, and stream energy became confined to narrower channels (review [86]). Narrow channels increased water velocity and tended to separate channels from floodplains [86]. The lateral extent of riparian vegetation likely decreased as the zone of saturation decreased with loss of beaver dams [23]. Placer mining and dredging were also destructive to stream channel function [25,60,86]. Removal of large trees was encouraged by the federal [86] and state [15] governments from the 1940s to the 1970s in order to increase stream water yields. Unselective tree cutting in riparian zones destabilized banks. Many mature and old-growth trees were harvested for timber [76], with little thought given to the importance of trees in stabilizing banks [86].

Fire exclusion has altered stand structure and plant species composition and increased fuel loads in many montane riparian corridors [54,72,88]. Along the Rogue River in southwestern Oregon, fire exclusion resulted in conversion of a coast Douglas-fir forest to white fir. The few coast Douglas-firs recruited in the 20th century had not gained canopy dominance as of 2011 [56] (see Fire frequency for more information on this study). Heavy fuel accumulation is particularly common in early postfire and late-seral riparian forests. Studies in coast Douglas-fir riparian forests of coastal Oregon found more snags and coarse woody debris in young (30-80 years) than mature stands, but fuel accumulation was greatest in old growth. Fuel distribution was patchier in young than in old-growth stands due to more variable fire severity in young stands [82]. Parenti [61] suggested that with fire exclusion in the Sierra Nevada, the frequency of large woody debris deposition into streams, pool frequency, and pool volume are all reduced compared to historical levels. When fire does occur in suppression areas, postfire deposition of large woody debris and sediment is often greater than historical levels. Parenti hypothesized that 1st-order streams have sufficient stream power to transport increased fuel and sediment loads; however, midelevation (around 5,000 feet (1,500 m)) alluvial stream channels cannot transport these loads efficiently because their stream power is too low. As a result, pool volumes are decreased from historical levels as sediment from 1st-order streams is transported and deposited into alluvial streams [61].

Livestock can greatly alter riparian zones. They may reduce fine fuels, alter stand structure and plant species composition [41,47] and increase bank erosion and degradation, sediment loads, concentrations of fecal bacteria, and water temperatures. Nonnative herbs such as Kentucky bluegrass and timothy may increase with heavy grazing [41]. Few studies have explored interactions of grazing and fire in riparian zones [20]. In riparian areas of the Wallowa Mountains, Oregon, Kauffman and others [42] found that the proportion of native sedges and forbs increased and that of nonnative grasses decreased on sites where cattle were excluded for 3 grazing seasons (P≤0.05) [42].

Dwire and others [24] provide a review of fuels management projects in riparian zones of the Interior West, and some of their information is likely applicable to California and southwestern Oregon. The review includes case studies on fuel-reduction treatments in riparian ponderosa pine, willow, and mountain meadow communities and provides guidelines for monitoring fuel-reduction projects in riparian zones [24].

Changes in fire regimes: A trend toward more severe fires in the Sierra Nevada was apparent by the late 20th century. A landscape-level analysis of fires that occurred from 1908 to 2006 on Forest Service lands in the Sierra Nevada showed fire severity, mean and maximum fire size, and area burned annually were "substantially" greater from 1984 to 2006 than from 1908 to 1956. Area burned decreased from the 1930s through the 1950s (x̄ ~74,000 acres (30,000 ha)/year) but has increased rapidly since the 1970s (x̄ ~140,000 acres (55,000 ha) in 2000). From 1984 to 2006, there was a trend of low- and midelevation mixed-conifer types burning at higher severity (25%-40% stand replacement) than high-elevation conifer types such as red fir (13% stand replacement). From 1984 to 2006, 3,553 acres (1,438 ha) burned in riparian zones; 15% of that was stand replacement. The authors noted that because small fires (<100 acres (40 ha)) were underreported before 1950, they were not included in their analysis [57].

Climate change: Verkaik and others [95] suggested that increased temperatures and changes in precipitation will likely increase fire frequency and intensity in riparian zones [95]. Climate change may reduce precipitation amounts and change its seasonality, alter flow regimes of watercourses, and reduce moisture of riparian soils [18].

Models predict that fuel loads in riparian corridors will change with climate change. These changes may include [53]:

• changes in large woody debris recruitment, including larger inputs in the short term following fire or blowdowns and possible decreased inputs in the long term
• changes in plant community structure and composition, including less old growth
• changes in transport rate and seasonal redistribution of instream large woody debris in response to shifting flood and drought cycles, and
• local changes in frequency of debris flow, depending on lithology and precipitation patterns

Nonnative invasive plant species: Riparian ecosystems are among the most invaded ecosystems globally [18,67], and this is likely to continue [18]. Many of California's riparian plant communities have undergone dramatic changes in species composition due to nonnative plant invasions [16,22], and highly flammable nonnative species may increase fuel biomass, continuity, and fire severity [10,46,95]. In Yosemite National Park, invasives were significantly more abundant in burned riparian areas compared to burned upland areas (P=0.002). However, within burned areas, abundance of invasive plant species did not significantly increase with fire severity [40].

LIMITATIONS OF INFORMATION
Few studies to date (2015) focused on fire regimes within riparian zones of California and southwestern Oregon. Most studies of fire effects and fire regimes do not differentiate between riparian and upland ecosystems [62,95]. More research is needed on fire regimes of riparian areas and relationships between riparian and upland fire regimes [23].

Since the riparian zone serves as a buffer and filter for the aquatic zone, understanding fire effects in riparian ecosystems is critical in determining fire effects on stream ecosystems. Further studies are needed on how riparian vegetation recovers after fire [95]. There is little research on how management activities designed to prevent, suppress, or recover from fire affect riparian vegetation. Studies are needed to guide such management actions [95].

Little information was available on the effects of prescribed fire in California's and southwestern Oregon's montane riparian communities. A fall prescribed fire on the Blodgett Experimental Forest had little to no short-term effects on riparian vegetation. In postfire year 1, the riparian corridor of a mixed-conifer forest showed no significant differences in volume of large woody debris or recruitment of fine sediment in pools. There were changes in macroinvertebrate communities, but these changes were insignificant by postfire year 1. The streams were 1st and 2nd order; postfire precipitation was below normal [9]. While outside the region covered in this synthesis, a study conducted on the Payette National Forest, Idaho, points out differences that might occur between prescribed fire and wildfire. For upland Pacific ponderosa pine forests and similar forests in the stream-riparian corridor of the Salmon River, there were significant differences in the effects of a prescribed fire conducted on 8 May 2004 compared to effects of the August 2000 Diamond Peak wildfire. In upland areas, changes in fuels, plant and litter cover, and tree scorching were similar in wildfire plots and those burned under prescription. However, the prescribed fire burned significantly less riparian area, and at lower overall severity, than the wildfire. None of the prescribed-fire plots burned at high severity (P=0.05 for all variables) [6].

Considerations for LANDFIRE
LANDFIRE's modeled fire-return intervals for California's and southwestern Oregon's montane riparian communities are mostly within range of those found in this literature review. The range modeled for montane riparian systems in southwestern Oregon (x̄=37 years for BpS 0211520 of the "California montane riparian systems" series) is shorter than the 50- to 60-year range found by Messier and others [56] on their midelevation sites. However, they suggested that interior valley sites had surface fires so frequently that fire-return intervals were not recorded with fire scars [56] (see the Rogue River study). If so, this would probably bring LANDFIRE's estimate within the range inferred for the Rogue River area.

Models for the "California montane riparian systems" (BpS series 11520) could be improved by including surface fires in the percent of fire calculations (see Appendix A and its links to BpS descriptions). Surface fires are known occur in montane riparian zones across California [12,48,56,58]. They are accounted for—and comprise the majority of—fires in LANDFIRE's model for the Sierra Nevada (BpS 0611600). It is important to include surface fires as a component of fire regimes in other montane riparian communities of California and southwestern Oregon as well.

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