Fire regimes of woody riparian communities of the Central Valley, California


Table of Contents:


Citation for this synthesis:
Fryer, Janet L. 2015. Fire regimes of woody riparian communities of the Central Valley, California. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Missoula Fire Sciences Laboratory (Producer). Available: https://www.fs.fed.us/database/feis/fire_regimes/CA_valley_riparian/all.html [].

INTRODUCTION
This Fire Regime Synthesis brings together information from 2 sources: the scientific literature as of 2015, and the Biophysical Settings (BpS) models and associated Fire Regime Data Products developed by LANDFIRE, which are based on literature, local data, and/or expert estimates. This synthesis is intended to:

This synthesis discusses the plant communities and fire regimes of riparian shrublands, woodlands, and forests of the Central Valley. Fire regimes of herbaceous wetlands of the Central Valley are covered in the Pacific marshland Fire Regime Synthesis. Two reviews are cited frequently in this synthesis: [53,60]. Common names are used throughout. For a complete list of common and scientific names of plant species discussed in this synthesis and links to FEIS Species Reviews, see Appendix B.

In this review "presettlement" refers to times prior to the start of the Gold Rush in 1849. Before the discovery of gold in 1848 there were a few ranches in the Central Valley, but otherwise, it was relatively unsettled. The California Department of Water Resources [14] reports that before the Gold Rush and subsequent development in the Central Valley, the "streams ran uncontrolled, and during the wet seasons, large areas became wetlands filled with thousands of waterfowl and other wildlife" [14].
SUMMARY
This section summarizes fire regime information available in the scientific literature as of 2015. Details and documentation of source materials follow this summary.

The Central Valley's riparian woody plant communities are comprised of shrublands and hardwood woodlands and forests. Willows dominate most shrublands; other shrub types are rare. The woodlands and forests are hardwood mix, usually with Fremont cottonwood dominating low terraces, and valley oak dominating upper terraces.

Presettlement fire regimes of these woody riparian systems are unknown. Historically, the Central Valley was a primary population center for California Indians, and tribal peoples used fire in the riparian areas for a variety of purposes. Lightning is uncommon in the region, so humans were likely the primary ignition source for presettlement fires. For this reason, fire season could extend from spring to fall, or even year-round in some years, but most fires were probably set during summer and fall.

Extent of riparian plant communities is greatly reduced from presettlement times due to agricultural development and urbanization. Hydrological function of these systems is greatly compromised, and restoration of historical flooding and anthropogenic fire regimes is only possible on preserves and at small scales. Nonnative invasive plants have altered historical plant community composition and stand structure. Flammable nonnatives including saltcedar and giant reed may increase fire spread rate and intensity in some riparian communities of the Central valley. However, fragmentation of riparian systems from agriculture and urbanization disrupt fuel continuity and fire spread on most sites.

Appendix A lists the Biophysical Settings (BpSs) covered in this review, with links to LANDFIRE documents with more information. LANDFIRE successional modeling did not include fire in simulations because they did not consider fire an important disturbance in these systems. Appendix B provides links to relevant FEIS Species Reviews.

Table 1. Fire intervals and severities in woody riparian communities of the Central Valley, California [36]
Fire interval¹
(years)
Fire severity² (% of fires)
Number of Biophysical Settings (BpSs) in each fire regime group
  Replacement Mixed Low I II III IV V NA³
NA       0 0 0 0 0 2
¹Average historical fire-return interval derived from LANDFIRE succession modeling (labeled "MFRI" in LANDFIRE).
²Percentage of fires in 3 fire severity classes, derived from LANDFIRE succession modeling. Replacement-severity fires cause >75% kill or top-kill of the upper canopy layer; mixed-severity fires cause 26%-75%; low-severity fires cause <26% [7,35].
³NA (not applicable) refers to BpS models that did not include fire in simulations.

DISTRIBUTION AND PLANT COMMUNITY COMPOSITION
The riparian shrublands, woodlands, and forests covered in this synthesis occur on the floodplains and terraces of major streams and rivers of California's Central Valley. The Central Valley is bound by the Klamath Mountains and Cascade Range to the north; the Southern Cascade Range and Sierra Nevada to the east; the Tehachapi Mountains to the south; and the Coast Ranges to the west [53]. It is comprised of the Sacramento Valley in the north and the San Joaquin Valley in the south. Climate of the Central Valley is mediterranean, with mild, wet winters and hot, dry summers. Nearly every year, snowmelt from the Southern Cascade Range and the Sierra Nevada provides moisture during the dry seasons. Most plant communities in this riparian system are floodplain woodlands and forests; the woodlands and forests are interspersed with riparian shrublands, bulrush marshes, and sloughs of freshwater emergent vegetation [38,53]. In places, the California prairie historically extended down to the riverbanks; the prairie has been displaced by nonnative annual grasses [51]. These riparian communities lie at low elevations, from sea level to no more than 400 feet (0-120 m) [60]. Prior to development, an estimated 2 to 3 million acres (0.8-1.2 million ha) of the valley—approximately the area of Connecticut—were seasonally flooded [53]. An estimated 1 million acres (0.4 million ha) of riparian forest occupied the Central Valley in the mid-19th Century. Less than 4% of California's riparian forests remain [27]. Of what is left, few retain their presettlement structure (see Contemporary Changes in Hydrology, Fuels, and Fire Regimes) [60].

Willows dominate most riparian shrublands of the Central Valley [53]. Willow scrub is usually <33 feet (10 m) tall; it occurs on frequently flooded sites and tolerates sustained inundation. These communities often form narrow strips on floodplains. Narrowleaf willow typically dominates, although dusky willow dominates some sites along the Sacramento River. Willow communities often occur on highly scoured sites that lack ground cover, but nonnative grasses and forbs such as ripgut brome, Italian ryegrass, yellow starthistle, and black mustard usually dominate the ground layer of willow communities in quiet backwaters. Giant reed has become highly invasive in some willow scrub communities in the Central Valley; it has excluded willows on some sites (see Nonnative invasive plants) [53].

Other riparian shrub communities are rare in the Central Valley. Blue elderberry savannas sometimes occur on high floodplains, terraces, and in openings within riparian woodlands and forests. Grasses, including beardless wildrye, blue wildrye, and ripgut brome, typically dominate the ground layer of these communities. Trees rapidly invade blue elderberry savannas without disturbance. The historical role of fire in maintaining these savannas, if any, is unknown [53]. However, blue elderberry was historically an important resource for California Indians, and they managed it with fire [32]. As of 2015, these savannas were infrequent along the Sacramento and San Joaquin rivers, but recent restoration activities to promote the federally Threatened valley elderberry long-horned beetle (an obligate elderberry feeder) have increased frequency of blue elderberry along the Sacramento River. Restoration of presettlement stand structure of these savannas remains challenging. Buttonbush scrub communities occur along backwater sloughs and oxbow lakes. These are typically thickets ranging from 20 to 30 feet (5-10 m) tall, but buttonbush sometimes forms small woodlands. The ground layer is usually sparse. Buttonbush communities are rare, having been largely replaced by agricultural lands [53]. Coyotebrush occasionally establishes in patches along floodplains, while California wildrose occasionally establishes in patches along sloughs [53].

Hardwoods dominate the Central Valley's riparian woodlands and forests. Dominant hardwoods include Fremont cottonwood, California sycamore, boxelder, white alder, Oregon ash, and/or valley oak [21,22,24,52,53,57]. An account from the 1800s described Fremont cottonwood, valley oak, and boxelder as dominant along all rivers and major streams explored in the Sacramento Valley (Thompson 1961 in [53]). Typically, Fremont cottonwood dominates the lowest terraces above the scour zone, intermediate terraces are mixed hardwood, and valley oak dominates upper terraces. Blue oak, interior live oak, and/or coast live oak woodlands surround and finger into the valley; these oak woodlands also have riparian components [2]. Willows, blackberries, and/or red-osier dogwood typically dominate the understories of these riparian hardwood systems, while the lianas Pacific poison-oak and California wild grape may extend into the canopies [2,38,53]. The groundlayer vegetation—now mostly nonnative annuals—was thought to be composed of herbaceous perennials [2].

Historically, the Central Valley had the most developed [1,60] and structurally complex riparian forests in the state [6,53]. Elevated water tables, highly fertile soils, and favorable climate produced extraordinarily productive communities [53]. Tree density varied from widely spaced to closed [60], with stand width varying from narrow bands to several kilometers across [53]. A presettlement explorer described a valley oak-California sycamore forest along the Feather River as "thickly wooded, for some two miles in depth, throughout its entire extent". "Its banks are heavily timbered, and some fifty feet in height, coming down abruptly to the water" (Farquar 1932 in [51]). Historically, many riparian trees in the Central Valley were larger than all but the most ancient trees now living. Explorers reported oaks of 6 to 8 feet (1.8-2.4 m) in diameter, 74 feet (23 m) tall, and 125 feet (38 m) in crown width along the Sacramento and Feather rivers, with branches layered continuously from trunk base to crown [51]. Jepson (1893 in [53]) reported the understory of these riparian forests was a "tangle" of California wildrose and California blackberry. California wild grape and Pacific poison-oak often draped up branches of the hardwoods [51]. Surveyors in the 1850s reported that "grape vines form a screen, by which the view of the (Sacramento) river is frequently shut out" (Botanical report to U.S. Senate 1857 in [51]).

Figure 1. Distribution of Californian Central Valley riparian communities based on the LANDFIRE Biophysical Settings (BpS) data layer [36]. Numbers indicate LANDFIRE map zones. Click on the map for a larger image and zoom in to see details.

HISTORICAL FIRE REGIMES
There are no fire studies on presettlement fire regimes of the Central Valley's riparian systems [60]. Tree ring data are not reliable for riparian hardwoods. Mature Fremont cottonwoods and other riparian hardwoods usually have such extensive heart rot that even when fire scars are present, the fire histories of the hardwoods cannot be reconstructed [49,50]. Sediment cores cannot be used for fire histories on floodplains because floodplain sediment is not stable. This is especially true in the Central Valley, where most floodplains have been secured with levees, dredged, drained, or otherwise drastically altered (see Contemporary Changes in Hydrology, Fuels, and Fire Regimes).

The Central Valley was a large population center for California Indians in presettlement times, and Indians lived and used fire in riparian areas of the Central Valley. Accounts of Indian use of fire are based on limited historical documentation [26,51] and interviews with tribal elders [4,32]. Jepson [26] reported such use in his original California flora, and Anderson [3,5] details tribal uses of fire to encourage growth of plants used for basketry, fiber, food, and medicine. California Indians of the Central Valley also used fire to control insects, benefit game animals, clear vegetation, and reduce fire hazard in riparian areas [10]. They burned many riparian plant species at regular intervals to produce stems suitable for making baskets and other implements (for example, willows, red-osier dogwood, and blue elderberry stems) or to stimulate fruit production (for example, blue elderberry, California blackberry, California wild grape, and California wildrose fruits) [4,11,32].

Historically, most ignitions in the riparian zone were anthropogenic. Lightning strikes are rare in the Central Valley; lightning strike density increases with elevation in the Sierra Nevada [28,54]. Strike denisty averages 6-12 strikes/year/ 100 km² in the valley compared to 41-47 strikes/year/100 km² in high elevations of the Sierra Nevada [55]. Many authors [37,46] suggested that the largest impacts of Indian-set fires in California were probably in the largest population centers: the Central Valley and coastal regions. Most fires were probably set in summer and fall [41,60], but since they were anthropogenic, the fire season could extend from spring to late fall or even year-round, depending on the year, fuel conditions, and management goals [23,60].

In summer and fall, fires ignited and burning in the mountains may have eventually spread onto the valley floor [60]. There is no documentation of how often such fires burned into riparian zones in presettlement times, however. Wills [60] suggested that wildfires were historically uncommon in riparian communities of the Central Valley; most often, these communities were fuelbreaks. When fires did occur, they were probably during the fire season (summer-fall) and of limited extent, but of varying severity and type (low-high; surface, mixed, and crown). Wills wrote that "it is fair to assume that during periods of prolonged drought, riparian forests did burn readily" [60].

In riparian zones, flooding is often a more important driver of succession than fire [42,60]. In the Central Valley, annual flooding was the primary natural disturbance affecting riparian vegetation [60]. Periodic flooding and sediment relocation were critical for the reproduction and growth of many riparian species [38] and to set back succession in riparian zones. Scouring likely helped control growth of understory shrubs such as California wildrose and lianas such as Pacific poison-oak and California wild grape [53].

Most riparian vegetation is resilient to fire [40]. Adaptations of most riparian species to flooding and scouring (thick bark, sprouting, masting, and/or water- and wind-dispersed seeds) also enable them to recover quickly from fire [17,29,44]. For details on the fire ecology of riparian plant species that occur in the Central Valley, follow the links in Appendix B to FEIS Species Reviews.

Prescribed fire may provide a surrogate disturbance to set back vegetation succession where historical hydrological functions are no longer operating [53]. To date (2015) , however, prescribed fire studies in riparian zones of the Central Valley are so few that they can lend little insight into whether prescribed fire may benefit riparian vegetation or help restore historical plant species composition and stand structure. Along Cache Creek near Woodland and the Mokelumne River near Lockeford, low-severity surface prescribed fire applied in spring, summer, or fall increased the proportion of native:nonnative plant species. Cover of shrubs and lianas was greater with spring than with summer or fall burning, while cover of overstory trees was not affected by prescribed fire in spring, summer, or fall [23]. Wills [60] recommended prescribed fire for specific targets: controlling particular nonnative invasive species, promoting particular native species, or for site preparation. Strict air-quality regulations in the Central Valley limit when managers can apply fire to their landscapes [60].

CONTEMPORARY CHANGES IN HYDROLOGY, FUELS, AND FIRE REGIMES
Hydrological function of Central Valley riparian systems is compromised due to many human-caused factors [39]. Land use and management have altered physical and biological characteristics of these systems. Alterations include lowering of surface water, groundwater, and biotic diversity; and changes in floodplain topography, stand structure, and species composition [17].

Greatest alterations in hydrology are due to water diversion for agriculture, dams, decline of American beaver populations, man-made levees, and mining. Consequently, historical seasonal flow patterns on floodplains no longer occur. Water diversion has lowered water tables across the valley and reduced the extent of wetland and riparian systems. For example, water from the Tule River historically fed Tulare Lake in southern San Joaquin County. Based on surface area, Tulare Lake was the 2nd largest freshwater lake lying entirely in the United States. It dried completely after water from Tule River was diverted for agriculture in the 1920s [41]. Dams reduce the magnitude and frequency of downstream floods [9,39,58], which may result in more homogeneous vegetation below dams [9]. Dams have greatly reduced the extent and changed the composition and structure of riparian communities [17], as well as reducing the volume of water delivered to valley riparian systems [60].

The decline of American beaver populations has also altered flow rates [17,18,20,39]. Historically, beaver dams were far more numerous than they are now, 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 [47]). Narrow channels increased water velocity and tended to separate channels from floodplains [47]. The lateral extent of riparian vegetation likely decreased as the zone of saturation decreased [17].

Man-made levees, construction of which began before the Gold Rush, similarly constrain the channels of most large streams and nearly all rivers of the Central Valley [48]. Hydraulic mining, placer mining, and dredging were also destructive to stream channel function [18,39,47]. In the Central Valley, mining debris flushed and deposited downstream—particularly sediment from hydraulic mining—apparently raised riverbanks and separated riparian corridors from floodplains [48].

Figure 2. Photo of the lower Sacramento River, courtesy of the California Department of Water Resources. Few native plant communities remain along the lower reaches of the San Joaquin and Sacramento rivers; most land has been cleared for agriculture or development. Fuels are usually discontinuous as a result.

Fuel loads have been drastically reduced in most woody riparian systems of the Central Valley. Throughout most of the Central Valley, loss of riparian wildlands to agriculture and urbanization has greatly reduced and disrupted fuel loads [60]. Shaffer and others [45] suggested that although fire-return intervals 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. Some authors note that lack of flooding on regulated watercourses has contributed to higher fuel accumulations in some riparian areas of the West [19,30]. However, heavy fuels are spotty in riparian areas of the Central Valley. Nonnative, flammable plant species have contributed to heavy fuel loads in some wildland areas.

Nonnative invasive plants: Riparian ecosystems are among the most invaded ecosystems globally [15,43], and this is likely to continue [15]. Many of California's riparian communities have undergone dramatic changes in species composition due to nonnatives [12,16]. Klinger and others [31] identified giant reed and saltcedar as 2 highly invasive nonnative plants in California's riparian areas. However, the total list of nonnative invasive plant species is long: About 20% of California's flora is nonnative [12]. Other common invasives in riparian systems of the Central Valley include tree-of-heaven [34,53], northern California black walnut × English walnut hybrids (Paradox hybrid walnut) [34,53], common fig [12,25], and Himalayan blackberry [34,59].

Holland [24] identified a saltcedar cover type, dominated by Tamarix chinensis or T. ramosissima, that occurs in the Central Valley. Giant reed grows in low-gradient riparian systems of California [13,30]. Both giant reed [8,31] and saltcedar [31] are implicated in increasing fire frequency and intensity in some riparian zones in the West. Giant reed and other flammable invasives can increase the amount and continuity of riparian fuels, which may alter fire behavior [8,30] and result in more severe and intense fires [30]. On some riparian sites in the West, giant reed stands form continuous, dense fuels, potentially converting these riparian zones from fire barriers to areas of fire spread [56]. In the Central Valley, giant reed is especially invasive on point and gravel bars [34]. Giant reed has fueled wildfires in riparian areas of southern California [33]. It has the potential to do so in the Central Valley, where it may facilitate fire spread into upland wildlands or rangelands.

Figure 3. A giant reed infestation along a valley oak riparian woodland along Stoney Creek in Glenn County, California. Photo © 2009 Neal Kramer.

LIMITATIONS OF INFORMATION
It is probably impossible to determine presettlement fire regimes of the Central Valley's riparian communities. There are too few written historical records, and presettlement fire data are apparently unobtainable.

Historical accounts (for example, [26,51]) of the Central Valley's vegetation during the settlement period may not accurately reflect presettlement stand structure of riparian communities of the Central Valley. At least by Indian villages, presettlement riparian vegetation may not have been as dense as settlers' accounts suggest. By the early 1900s, California Indians had experienced disease outbreaks, skirmishes, wars, and relocations that greatly reduced tribal populations [41,53]. Decline in California Indian populations likely resulted in fewer Indian-set fires in riparian zones. Given the extraordinary productivity of these riparian systems, succession to denser, more forested communities would have proceeded rapidly when Indian-set fires became infrequent.

Vaghti and others [53] caution that there is little information on the response of California's riparian plant species to fire and call for "investment in fire effects research".

Considerations for LANDFIRE: LANDFIRE modelers did not consider fire an important disturbance in these systems (fire regime group of "NA"). This review found no information indicating otherwise, so further modeling does not seem necessary. Although California Indians used fire in riparian areas of the Central Valley in presettlement times (see Historical Fire Regimes), fire is now rarely used for managing these systems, and wildfires are quickly suppressed. Altered land use and fragmentation, discontinuous fuels, and effective fire suppression mean that fire can only be used on preserves, and on small scales.
APPENDIX A: Summary of fire regime information for Biophysical Settings covered in this synthesis

APPENDIX B: Common and scientific names of plant species.
Follow the links to FEIS Species Reviews.
Common name Scientific name
Trees
blue oak Quercus douglasii
boxelder Acer negundo
California sycamore Platanus racemosa
coast live oak Quercus agrifolia
common fig Ficus carica
English walnut Juglans regia
Fremont cottonwood Populus fremontii
interior live oak Quercus wislizeni
northern California black walnut Juglans hindsii
oaks Querucus spp.
Oregon ash Fraxinus latifolia
Paradox hybrid walnut Juglans hindsii × J. regia
saltcedar Tamarix chinensis, T. ramosissima
tree-of-heaven Ailanthus altissima
valley oak Quercus lobata
white alder Alnus rhombifolia
Shrubs
blue elderberry Sambucus nigra subsp. cerulea
buttonbush Cephalanthus occidentalis
California wildrose Rosa californica
California blackberry Rubus ursinus
dusky willow Salix melanopsis
Himalayan blackberry Rubus armeniacus
narrowleaf willow Salix exigua
red-osier dogwood Cornus sericea
willows Salix spp.
Lianas
California wild grape Vitis californica
Pacific poison-oak Toxicodendron diversilobum
Forbs
black mustard Brassica nigra
yellow starthistle Centaurea solstitialis
Graminoids
beardless wildrye Leymus triticoides
blue wildrye Elymus glaucus
bulrushes Schenoplectus spp.
giant reed Arundo donax
Italian ryegrass Loluim multiflorum
ripgut brome Bromus diandrus

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