Native Americans used various parts of Alnus rhombifolia medicinally for diarrhea, consumption, and burns, as a blood purifier, an emetic, and a wash for babies with skin diseases, and to facilitate childbirth (D. E. Moerman 1986).
Botanical description: This description covers characteristics that may be relevant to fire ecology and is not meant for identification. Keys for identification are available (e.g., [8,39,56,58]).
White alder is a small to medium-sized deciduous tree, usually ranging from 16 to 82 feet (5-25 m) tall [56,58]. The champion tree is 91 feet (28 m) tall and grows in Polk County, Oregon [3]. In California, white alders commonly reach 11 inches (28 cm) DBH but may reach 21 inches (53 cm) DBH [19]. The crown is conical, spreading, and open [39]; a mature tree may have one to several stems [42,63]. The bark is thin [58]. Leaves are broadly ovate; water-stressed trees produce smaller and fewer leaves than trees on moist sites [33]. The inflorescences are catkins. Staminate catkins grow in clusters of 3 to 7; pistillate catkins may be solitary or grow in racemose [58] clusters of 2 to 6 [39]. After fertilization, pistillate catkins become cone-shaped and woody [115]. The fruit is a leathery, irregularly-shaped samara [39,58] containing a small nut [53,58,157]. Samara wings are narrow, and the samara is light-weight [53].
White alder staminate catkins, pistillate catkins, and seeds. Photos by C. Stubler, M. Ritter, W. Mark, and J. Reimer, courtesy of selectree.calpoly.edu.Roots of mature white alders can withstand inundation by most large stream flows [72]. Scouring flows may expose the roots. In Humboldt County, California, white alders were rooted ≤1.3 feet (0.4 m) above the low-water line of streambanks. Roots of white alder, willows, and black cottonwood were intertwined and dense [72].
Exposed white alder roots.White alder's root nodules are infected with nitrogen-fixing bacteria in the genus Frankia [93,98], so white alder may increase soil nitrogen levels [21,53].
Stand structure: White alder communities vary from open woodlands to closed forests. In foothills of the Sierra Nevada, crown cover is typically continuous [91]. The canopy may thin toward white alder's southern distribution [57]; however, some southern locations support dense white alder forests. In the Eel River Basin of northwestern California, a white alder population showed normal distribution of size classes, so the authors suggested that most white alders established around the same time. Most trees (10%-38% frequency) were 10 to 20 inches (30-50 cm) DBH. Density averaged 0.15 tree/m of stream channel segment [134]. By the Nacimiento River of coastal central California, white alder ranged from 75% to 95% cover along narrow channels and had good representation in all size classes [118]. In Cleghorn Canyon on the San Bernardino National Forest, white alder forests had a closed canopy with all size classes well represented. The canopy was "well over" 9 feet (3 m) tall [31].
White alder is native to the western United States; it is the most common alder in the mediterranean region of the West. It is distributed from San Diego County, California, north to Chelan County, Washington, and east to Idaho County, Idaho [73,137]. Its core distribution lies in the foothills of northern California and southwestern Oregon [73]. Isolated populations occur in extreme western Nevada [65,73] and western Montana [39]. Some report that its southern distribution probably extends to northwestern Baja California Norte [42,63], but this is undocumented and refuted by others [84].
White alder may lose its dominant position in riparian communities after wildfire, and recovery may take decades. Among associated hardwood species in California, it is apparently slowest to recover after fire (see Plant response to fire).
Late October prescribed fires on the Blodgett Forest Research Station, California, burned at low to moderate severity and had little effect on riparian trees. Study sites were mixed-conifer forests with white alder and Pacific yew adjacent to first- and second-order streams. In postfire year 1, litter and woody debris loads were about 20% less than a year before burning. Overall mortality of canopy trees was low (4.4%), so the fires did not significantly reduce canopy cover. Data for white alder alone were not provided [14].
Riparian deciduous forests are important buffer strips. In postfire environments, they help protect aquatic zones by capturing sediment flows and nutrients from burned uplands [36]. To protect fisheries, Neary and others [90] recommended that prescribed fires in riparian zones be of low severity, with flame lengths ≤2 feet (0.6 m).
Moderate fire-return intervals (50-70 years) favor white alder [15,17]. Although white alder occurs in the riparian zones of many plant communities that have short (<50 years) fire-return intervals—such as palouse prairie [5], chaparral [96], oak woodlands [1], and Pacific ponderosa pine [5]—riparian areas with white alder have longer fire-return intervals than surrounding uplands in most years [102,120]. These riparian zones are often either too moist to burn ([68], review by [41]) or burn at lower severity than uplands. Some estimate that fire-return intervals in western riparian zones were historically several times longer than those in surrounding uplands ([119], review [41]). Riparian areas by first- and second-order streams may burn at intervals similar to those of surrounding uplands [30], although white alder is less prevalent by small streams (see Site Characteristics).
Historical fire severity in riparian zones is not well known. Some suggest that typically, fires that burned at high or moderate severity in uplands burned at moderate or low severity in riparian zones [17,68]. Other suggest that when riparian areas burned, fires were probably of moderate to high severity [102]. A pattern of frequent fires of at least moderate severity is emerging in some riparian areas. Landsat data from Yosemite National Park showed that from 1984 to 2003, fire severity was moderate to high in 75% of black cottonwood-white alder-bigleaf maple riparian communities that burned; 904 acres (366 ha) or 5.1% of the Park's total riparian area burned during that time. Some riparian communities burned 2 or 3 times in 20 years. The authors placed median severity of riparian fires "at the high end of the moderate category" [130].
Riparian communities can have severe, stand-replacement fires in dry years [17]. Wind funneling down water channels can sustain and increase fire intensity in drought-stressed riparian zones [117]. During droughts, fire severity in riparian zones is strongly influenced by fire severity in upland zones [17]. In the Transverse Ranges, the 2002 Wolf Wildfire was stand-replacing in adjacent upland chamise-manzanita stands. The fire remained severe and stand-replacing when it burned into the white alder-coast live oak community on the floodplain of Piedra Blanca Creek [17].
Discussions and documentation of FIRE REGIMES by state follow. See the Fire Regime Table for further information on FIRE REGIMES of vegetation communities in which white alder may occur. Find further fire regime information for the plant communities in which this species may occur by entering the species name in the FEIS home page under "Find FIRE REGIMES".
California: Little is known of historical FIRE REGIMES of California's riparian areas [102,117,120], although historical fire-return intervals are thought to be longer in California's riparian zones than in upland zones [102]. Presently, fire-return intervals in riparian zones of the Klamath Mountains and the southern Cascade Range are twice those of adjacent uplands [117,120]. Fires miss riparian areas surrounded by highly flammable vegetation in some years. In the Santa Monica Mountains, the 1970 Maliby Wildfire and the 1973 Mugu and Topanga-Tuna Canyon wildfires were stand-replacing in coastal sage scrub and chaparral vegetation. However, the wildfires did not burn white alder stands in deep canyons [114].
In the Transverse Ranges, median fire-return interval of riparian communities was 60 years [15,17]. White alder communities had not burned for about 54 to 65 years. Willow communities with chaparral shrubs were associated with shorter times since fire (P≤0.05). Because white alder is fire-sensitive (see Plant response to fire), the author concluded that short fire-return intervals excluded white alder from willow-chaparral communities [15]. In chaparral areas near Santa Barbara, return intervals for large fires historically ranged from 30 to 65 years [28]. It is uncertain how often these large fires burned into riparian areas. In the Sespe Wilderness, 11 riparian sites on the Sespe Creek Watershed burned in large wildfires in 1932 [18] (Matilija Wildfire, 220,000 acres (89,100 ha)) [67] and 2002 (Wolf Wildfire, 21,600 acres (8,760 ha)). Two of the 11 sites also burned in a smaller fire in 1975 (in Potero John Creek watershed, ~100 acres (43 ha)). Because this study was conducted by small streams (< 35 km²) drainage area) 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 [18].
Skinner and others [120] stated that California's riparian areas were important topographic features affecting fire spread. In most years, riparian forests were probably barriers to spread of low-severity fires. Relatively moist conditions probably stopped spread of some moderate-severity fires as well. In many years, this barrier effect likely reduced fire severity or occurrence beyond the riparian zone [120].
Oregon: As of 2014, little documentation was available on FIRE REGIMES of Oregon's riparian zones containing white alder. A fire history study in in Rouge River Basin showed a regime of mixed-severity fires at moderate intervals. The mixed-conifer forests investigated were coastal Douglas-fir-white fir-incense-cedar/bigleaf maple, with white alder "a major overstory tree" on upland sites (67% frequency) and an important component of the forest in interior valley sites (23% frequency). Recruitment of overstory trees occurred in pulses, with cohorts establishing every 50 to 60 years. The oldest coastal Douglas-firs were 200+ years old. Fire scars on these trees recorded 4 fires on uplands from approximately 1750 to 1910 and 6 fires in interior valleys from 1850 to 1925 [81].
Idaho: Little documentation was available on FIRE REGIMES of Idaho's riparian zones containing white alder. In Hell's Canyon, fires are frequent in low-elevation bluebunch wheatgrass-Idaho fescue palouse prairies and adjacent riparian zones [86] where white alder may be present [82,86].
In the Sespe Wilderness, California, white alder established after a stand-replacement fire on the Sespe Wilderness, California, but not after another stand-replacement fire 70 years later. There was some white alder recruitment from 1935-1940, following the 1932 Matilija Wildfire. There was "continuous, but irregular" recruitment from 1966-2000. Whether these waves of recruitment were from sprouts or seeds could not be determined. The 2002 Wolf Wildfire killed white alders on 11 study sites, and white alder had not reestablished as of 2002. Peak flows on the 11 sites were low from 1933-1968, with 6 large floods from 1969-1983. The largest floods were in 1969, 1978, and 1983. White alder showed good recruitment in the 1970s, following the 1969 flood, and steady recruitment in the 1980s and 1990s. The authors suggested that white alder will likely reestablish on the burned sites in decades to come. See FIRE REGIMES for further information on this study [18].
The Regeneration Processes section provides detailed information and documentation on white alder regeneration.
Plant response to fire: White alder plants are not fire resistant [35]. They apparently have sparse postfire seedling establishment [139] and a weak postfire sprouting response. White alder may require decades to recover after severe fire [17,18,35], especially if fire has killed parent trees [35]. In 3 riparian communities on the Lassen National Forest, California, white alder dominance was positively associated with time since fire (P=0.009) [108]. However, white alder escapes many fires because it is restricted to stream- and riverbanks that burn less frequently than surrounding uplands [128]. Along the San Gabriel River in California, Brothers [26] observed no evidence of fire damage on large white alder trees, which were older than the last major fire in the area.
One author stated that white alder is top-killed or survives fire, although no examples or studies were cited [66].
Limited studies show that associated hardwoods have stronger ability to sprout after fire [12,17,18,35], outcompeting white alder for space and light on streambanks. In the Transverse Ranges, the 2002 Wolf Wildfire crowned in 2 white alder-coast live oak/Eastwood's manzanita galleries [16,17]. Overstory trees included white alder, coastal sage scrub oak, and Fremont cottonwood. Most white alders died; the other hardwoods were mostly top-killed. Surveys in postfire year 3 found few white alders sprouting, and coast live oak sprouts dominated the postfire community. White alder had the weakest sprouting response of all overstory trees, and the authors suggested there was little opportunity for white alder to establish by seed due to prolific postfire sprouting of associated hardwoods [17].
Relative abundance of dominant overstory trees before and 3 years after the 2002 Wolf Wildfire in the Transverse Ranges [17] Species Prefire* Postfire Relative basal area (%) Relative density (%) Relative frequency (%) Importance value** Relative basal area (%) Relative density (%) Relative frequency (%) Importance value White alder 56.5 29.4 29.9 38.6 0.2 4.4 10.0 4.8 Coast live oak 18.2 3.0 5.7 9.0 91.2 26.1 30.0 49.1 *Prefire stand structure determined from postfire snags.Three years after the high-severity Wheeler Gorge Wildfire in summer 1985, surveys found that about 97% of white alder trees on the upper Santa Ynez River basin had died. The prefire plant community was white alder-California sycamore-Fremont cottonwood-coast live oak/currant-rose. There was no correlation between white alder diameter and fire-caused mortality [12].
Effects of the 1985 Wheeler Gorge Wildfire on white alders >10 cm in diameter as of postfire year 3 [11] Canyon Total # trees sampled # aldersIn contrast to white alder's poor sprouting response, most associated oaks, California sycamores, and Fremont cottonwoods had only been top-killed and were sprouting. The few white alders that were only top-killed sprouted several months after the other hardwoods ([12], Barro 1989 personal communication [11]). By spring of postfire year two, 7% percent of burned white alders were sprouting compared to 83% of California sycamores and 70% of coast live oaks [35]. Abundance of white alder and other hardwoods in postfire year 3 was [12]:
Abundance of overstory tree species on the upper Santa Ynez River basin, 3 years after the 1985 Wheeler Gorge Wildfire [12].Coring of white alder snags on the Wheeler Gorge Burn showed a cohort of white alders that established after a 1969 flood, but none that established after fire (Barro 1989 personal communication [11]). White alder seed rain was low through postfire year 3. There were no white alder seedlings in burned plots by then, and <1% of white alder seeds collected on burned plots were viable. However, there were white alder seedlings in unburned plots just downstream. Some of those seedlings were 10 feet (3 m) tall by postfire year 3, but others had been uprooted by postfire winter storms. The authors suggested that due to the scarcity of viable seed, white alder will likely take decades to fully recover from the wildfire. They also suggested that the postfire storms were typical, not severe, and that the amount of channel erosion was typical of early postfire chaparral ecosystems. A storm on 30-31 January 1986 had produced a streamflow of 2.1 m³/second in the study reach. A second storm on 13 to 15 February 1986 produced a stream flow of 2.5 m³/second. These storms widened the stream channel by 1.6 feet (0.5 m) and deepened it by 2.3 feet (0.7 m) [35].
As that study shows [35], fire can have indirect effects on white alder and other riparian species by altering fluvial processes on watersheds. Increased water flow, erosion, debris, sediment, and nutrient loads are common after fire above or within riparian zones ([141], review by [36]), and these effects may last from several postfire months to 4 years in mediterranean climates [141]. Postfire erosion of stream channels can remove white alders that survived a fire. The prefire white alder-California sycamore-coast live oak riparian forest that burned in the Wheeler Gorge Fire was surrounded by chaparral. After the 2 storms in January and February 1986, 8 of 23 surviving white alders were uprooted, and 2 more were uprooted in the next 2 years [35].
Postfire browsing can reduce white alder seedling establishment and sprout growth (see Mule deer browsing).
Fuel loads vary in riparian forests with white alder due to variable stand structure and species composition. Because they are moist, riparian forests are firebreaks in many years [128].
On riparian rangelands, livestock grazing removes many fine fuels [102]. Oak woodlands with annual grass ground layers are often used for cattle grazing, and cattle often seek out white alder communities surrounded by oak woodlands. On an oak rangeland in Tehama County, California, most surface fuels were grasses. By Dye Creek, crews measured litter loads, woody debris loads, and stand structure of an interior live oak-valley oak-white alder/California wild grape-Pacific poison oak-Himalayan blackberry riparian forest. The area was used for cattle grazing; mule deer, feral pigs, and black-tailed jackrabbits were other important foragers. Results of the survey were [132]:
Fuel and stand structure characteristics of a riparian forest in Tehama County, California. Data were collected in spring (SD) [132]. Litter depth (cm) 1.3 (1.3) Canopy height (m) 4.1 (3.8) Canopy closure (%) 25.2 (18.7) Basal area (m²/ha) 7.8 (7.4) Vertical strata cover (%) Ground level (0.0-0.5 m) water 0.7 (1.6) rock 16.9 (11.4) bare soil 10.3 (6.4) litter 18.5 (11.2) dead wood 1.4 (1.8) moss 1.2 (2.8) grass 26.3 (18.1) forbs & ferns 18.5 (9.8) live wood 0.6 (0.7) evergreen broadleaf foliage 0.6 (0.9) deciduous broadleaf foliage 5.2 (7.2) Low strata (2-5 m above ground) dead wood 0.1 (0.3) live wood 1.1 (1.2) evergreen broadleaf foliage 3.4 (4.5) deciduous broadleaf foliage 4.8 (4.4) Mid strata (10-15 m above ground) dead wood <0.1 (0.2) live wood 0.5 (1.0) evergreen broadleaf foliage 0.2 (0.5) deciduous broadleaf foliage 3.0 (4.5) Subcanopy strata (20-30 m above ground) dead wood 0.0 (0.0) live wood 0.1 (0.4) evergreen broadleaf foliage 0.0 (0.0) deciduous broadleaf foliage 0.2 (1.1) Total live plant cover 83.9 (27.5)Brooks and others [25] report that in desert riparian communities, surface fuel loads and fuel continuity can by "very high". This facilitates spread of surface fire; however, ladder and crown fuels are often discontinuous, impeding spread of crown fire [25].
Several fuel metrics are available for white alder. Fuelwood characteristics—including energy output, specific gravity, and moisture content—are provided in Wilson and others (review [155]). Specific leaf area can be used to calculate foliar fuel loads [116]. Studies in California found the specific leaf area of white alder averaged 10.33 m²/kg [143].
Seeds germinate rapidly on sunny, wet mineral soils exposed or deposited by receding floodwaters [26]. Fresh seed is immediately germinable [37,157], and seeds may remain viable for a few months [37]. Three-month stratification increases germination rates in the laboratory [37,53], but seed viability drops from fall through spring in the field [53].
When damming and flood control disrupt natural seed dispersal and time of flooding, so tamarisk, tree-of-heaven, and Russian-olive often germinate more successfully than white alder and cottonwoods [91].
Riparian zones with white alder are highly important habitats for a variety of wildlife (for example, [24,55,61,80]), and white alder provides forage for many wildlife species. Mule deer eat white alder twigs, leaves, and buds [95,105,110]. Maximum consumption is probably in fall, winter, and early spring [110,136]. Domestic livestock lightly browse the leaves and young twigs [110]. American beavers use alder bark for food and for building dams and lodges [110]. Desert woodrats on the San Dimas Experimental Forest collected white alder as a minor but frequent component of their diet [60].
As an important member of riparian woodland communities, white alder contributes to structural diversity and provides cover and food—all important habitat requirements for many bird species. In California, deciduous riparian woodlands support a higher diversity and density of breeding birds than other habitats [43,49,55,71]. For example, an 8-year survey on the Sierra National Forest, California, found that 13 bird species, including Cooper's hawk and several woodpecker, hummingbird, and flycatcher species, used white alder as nest trees [100]. Redpolls, siskins, and goldfinches eat alder seeds [136].
Riparian forests with white alder are important habitat for threatened and rare species [23] including the least Bells' vireo [49] and mountain yellow-legged frog [123]. In Anza-Borrego Desert State Park, riparian forests provide critical habitat for the least Bell's vireo and peninsular bighorn sheep [140]. Foothill riparian hardwoods are a minor habitat type for California spotted owls [142]. Southwestern willow flycatchers use riparian areas as habitat and white alders for nesting [139]. The valley elderberry longhorn beetle is an obligate feeder on elderberries, which are common in riparian zones. The beetle is most abundant in diverse riparian communities with mature overstories and dense understories; white alder is often a component of such communities [138].
Caddisfly larvae feed on white alder litter in streams [45].
Palatability and nutritional value: Ungulate use of white alder browse is rated poor to fair for mule deer and domestic sheep, unused to poor for cattle, and unused for horses [110].
Cover value: When growing with other riparian trees, white alder contributes to structural diversity for many wildlife species and provides cover important for numerous perching birds [49]. Livestock seek riparian zones for shade and resting [127].
Nesting birds use living and snag white alders. Cavity nesters such as pileated woodpeckers [54] and red-breasted nuthatches [7] sometimes select white alders. On 54- to ~70-year-old burns on the Sierra National Forest, woodpecker, nuthatch, and owl species apparently nested in white alder snags as much as expected based on availability [7].
White alder provides shade for fish and other aquatic animals [36,121].
Riparian deciduous forests with white alder are substantially reduced from their historical extend [153]. Dams and levees limit flooding and riparian forest development [153]. The Central Valley of California had an estimated 1 [153] to 3 million acres (0.4-1.2 million ha) of riparian lands in the mid-1800s; 90% of that is gone [147]. Very little riparian forest remains today, and what is left has mostly lost its presettlement structure [153]. About 90% of riparian ecosystems of southern coastal California have been lost. Historically, the Los Angeles coastal plain had the most extensive riparian habitat in the region (review by [66]).
Many extant riparian systems have been greatly altered, especially where placer mining was conducted. Stream channeling and reduction of American beaver populations have also altered water courses. Historically, stream channels were broader and more braided. Shaffer and others [117] suggest 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.
Invasive nonnative
plant species may reduce cover of white alder and other native riparian species and lower overall diversity of native riparian plant species. Nonnative plants common in many riparian communities with white alder include giant reed [158], Himalayan blackberry [125], tamarisk, Russian-olive, tree-of-heaven [91,103], and black locust [99]. In Hell's Canyon, some white alder/Lewis' mockorange communities are infested with Himalayan blackberry and/or bittersweet nightshade [86]. In the Santa Monica Mountains of southern California, nonnative invasive annual and perennial herbs such as black mustard, ripgut brome, and horehound dominated the ground layer of a white alder-California bay woodland in Solstice Canyon. Residual native annuals included caterpillar phacelia and hollowleaf annual lupine [97].
White alder can be used as a landscape tree if watered sufficiently [110].
California Indians used white alder medicinally and to make dyes [34,129,133]. They used the wood for making utensils and as firewood [133].
Alder seeds have small cotyledons and lack endosperm [53], so germination and establishment must proceed quickly or germinants will die.
Seedling establishment appears restricted to sites with a continuously moist substrate. White alder seedlings apparently cannot survive on sites that dry out during the summer [26]. White alder usually establishes on banks after floodwaters recede, typically just above active bars [72]. For stands to develop, white alder requires a stable substrate above the scour zone and a shallow rooting depth to year-round moisture. Once established, stands can survive most spring inundations [72]. On 4 sites across western Oregon, most (10% frequency) white alder seedling establishment was on streamside plots, and almost none (~1% frequency) was on upland plots. Density on streamside plots ranged from 109 to 11,227 seedlings/ha. Scouring was the most important factor deterring establishment, with presence of white alder seedlings positively associated with low-scour channels (R²=0.08) [113]. In a common garden study, nearly all seedlings subject to moderate water stress (stomal closure and loss of turgor after 5 days without water) survived, while over half of those subject to severe water stress (leaf loss after 10 days without water) died [33].
Drought [112] and plant removal by channel scouring [75] are the main causes of seedling and sapling mortality. Although mature white alders can survive inundation [112], high flows may remove white alders of any age [75,109]. In the channel of Dry Creek in Sonoma County, California, high flows from storms in winter 1981-1982 eroded a gravelbar into an island, flushing white alder saplings downstream. A few adult trees on the channel margin also fell into the stream. On intact gravelbars, however, survivorship of 2- to 5-year old and 5+-year-old saplings was 100% from June 1981 to June 1982 [75]. In the San Bernardino Mountains, winter floods in 1969 removed entire white alder stands [83].
Mule deer browsing can reduce growth or cause mortality in all age classes. On second- and third-order streams in Mendocino County, California, white alder saplings had greater mean density in deer exclosure plots (0.5 sapling/m²) than in open control plots (0.05 sapling/m²). Saplings in exclosures were also taller (65% were >3 feet (1 m)) than saplings in controls (97% were <3 feet) [95].
White alder generally grows quickly. However, growth is slow and plants are small on sites with fluctuating soil moisture in summer. In Long Canyon on the Sequoia National Forest, white alders by streams with sparse summer flows grew no more than 20 feet (6 m) tall [31].
Site characteristics: White alder grows along permanent streams [8,118] and adjacent slopes [39,58]. On 4 sites across western Oregon, its presence was positively correlated with streamsides (X²=23.8, P<0.001) [112], and on the Lassen National Forest, California, white alder was negatively correlated with distance from streams (P=0.001) [108]. It is mostly restricted to flood zones and becomes infrequent farther upland [26,32,35,76,118]. The most well-developed white alder forests are along rapid, well-aerated perennial streams with high spring runoffs [33,51]. In dry years, white alder is a better indicator of moist soils than either cottonwoods or willows [6,59].
White alder is most common along rivers and third-order streams [52,151]. In the Siskiyou Mountains of California and Oregon, it was most frequent (90%) on the most mesic sites by large streams [151]. It grows along small streams in the Coast Ranges and the Sierra Nevada [130], however. For example, white alder is most common on upper stream reaches in the Central Coast Ranges above Carmel Valley [50].
White alder is classified as a facultative riparian species [137], although it may be an obligate riparian species [51,88] in dry locations such Hell's Canyon, Idaho [82]. Anatomy of its wood vessels makes water transpiration difficult on seasonally dry sites [29]. Mature white alders are classified as very tolerant of flooding [145]. White alder also tolerates coastal fog [57].
Soils: White alder grows in sands [57], gravels [57,134], and other alluvial soils [125]. Soils are often rocky [58]. In Sequoia National Park, for example, white alder grows on alluvial soils over granodiorite bedrock [4].
White alder grows in soils derived from various parent materials. In Oregon and California, it grows in mafic soils and tolerates ultramafic soils. For example, it grows in gabbro soils in the Sierra Nevada [154]. In the Siskiyou Mountains, it grows in soils derived from olivine gabbro and diorite [151]. In the Klamath Mountains, it occurs in Port-Orford-cedar forests on ultramafics [92]. White alder is also common on nonmafic soils. On Matilija Creek in Ventura County, California, a white alder-California sycamore-coast live oak/willow-hollyleaf cherry riparian forest occurs on marine sandstones and shales [35]. In the No Business Creek Research Natural Area, Idaho, white alder grows in limestone soils. The area is a watershed of the lower Salmon River. On drainages of the Snake River, Idaho, white alder grows in soils derived from basalt and other volcanics [62].
Topography: White alder generally occupies low positions in riparian zones. It is more common on banks just above floodplains than on floodplains or upper terraces [23,118]. In the Eel River Basin of northwestern California, white alder was common on banks and low terraces but uncommon on upper terraces. White alders on upper terraces were associated with springs or tributaries [134]. In 1900, Sudworth [126] reported white alders in the Lake Tahoe area grew in "wet boggy creek bottoms", occasionally in stands as large as 0.25 acre (0.1 ha). In the Sacramento and San Joaquin deltas, white alder grows above the tidal zone [149,152].
White alder communities often occupy steep canyons, so the riparian corridor is narrow [57,62,86]. These canyons are subject to frequent slippage and erosion [86].
In southwestern Oregon, white alder is most common in riparian zones of interior valleys, especially the Willamette Valley [94]. In the Umpqua River valley, white alder is codominant in willow/field horsetail communities. These are tall-shrub, seral communities on low floodplains [131].
Elevation: White alder is most common in low-elevation montane zones [63,112,158]. At its northernmost limits, it is reported from 1,200 feet (370 m) along the Clearwater River of Idaho [63]. In southwestern Oregon, surveys on the Biscuit Burn (postfire year 4) and Bear Butte and Booth Complex Burn (postfire year 2) found white alder presence was positively associated with low elevations, low-gradient topography, large streams, well-drained organic soils, and low prefire cover of deciduous trees (P<0.05 for all variables) [52]. A study in Oregon's North Coast Ranges found white alder was positively associated with relatively low elevations, summer moisture stress, and high temperatures (P<0.01 for all variables). Elevational ranges across white alder's distribution are as follows:
Region Elevation (feet) California 0-8,000 [39,51]; mostly below 5,000 [57,87] Idaho 900-2,800 [62,86] Nevada 5,000-6,000 [65] Oregon 100-4,500 [63]; most common around 1,600 [112] Intermountain region typically 2,460-5,090 but up to 7,870 [58] Pacific Northwest 0-5,000 [146] Siskiyou Mountains 1,500-5,500; most common from 1,500-2,500 [151]Plant communities: White alder grows primarily in riparian conifer forests [48,63] and riparian oak woodlands along major streams and rivers [48]. It also grows in riparian zones of bunchgrass, sagebrush-grass [63], and chaparral communities [63,145].
Riparian communities with white alder are often diverse and historically, they were sometimes dense [9,31,91] and difficult to navigate. In Castle Crags State Park, the white alder-coastal Douglas-fir-incense-cedar/Himalayan blackberry association had highest plant species richness (70 species) and second-highest basal area (48 m²/ha) among 17 plant associations. It occurred on warm alluvial soils along Castle Creek and the Sacramento River. The white alder-bigleaf maple/vine maple association occurred along small streams; these sites were cooler and had lower species richness (37 species) [125]. Historical accounts describe California's pristine riparian forests as "jungle-like", with "impenetrable walls of vegetation" near the water [9]. Riparian associations with white alder are habitat for a wide variety of wildlife (see IMPORTANCE TO LIVESTOCK AND WILDLIFE).
Many white alder communities are infested with nonnative plant species. See Other Management Considerations for further details.
Discussions of white alder communities by state follow. See Appendix B for a complete list of common and scientific names of plant species discussed in this Species Review and links to other FEIS Species Reviews. See the Fire Regime Table for a list of plant communities in which white alder may occur and information on the FIRE REGIMES associated with those communities.
California: White alder is most common in chaparral, oak woodland, and Pacific ponderosa pine zones [87]; it typically dominates riparian vegetation in these zones. In the Santa Monica Mountains, white alder-California sycamore riparian associations occur within coastal sage scrub [13]. Roberts and others [104] list white alder as uncommon in riparian forests of the North and Central Coast ranges but common in the South Coast Ranges.
White alder is an understory dominant or important component of riparian conifer and mixed-evergreen forests [14,31], including Pacific ponderosa pine, coastal Douglas-fir, mixed-conifer, and forests with more restricted geographical ranges [31]. In the southern Cascade Range and northern Sierra Nevada, it dominates the midstory of coastal Douglas-fir/white alder/Indian rhubarb forests [38]. White alder riparian forests intergrade into redwood forests in the North Coast Ranges [22] and giant sequoia forests in the Sierra Nevada [4,106,107]. In Monterey County, white alder is a subcanopy component of redwood/western bracken fern-giant chainfern streamside forests [20]. It is sometimes important in bigcone Douglas-fir-canyon live oak woodlands [78]. In late succession, incense-cedar may dominate many riparian forests with white alder [14,31].
White alder dominates or codominates many riparian hardwood forests [27,51,59,111] on the Coast [33,51,57], Transverse, and Peninsular ranges [57] and the western slope of the Sierra Nevada [33,51]. Across white alder's range in California, codominant or commonly associated tree species include Fremont cottonwood, valley oak, California sycamore, Pacific willow [9,12], bigleaf maple, California sycamore, and/or Oregon ash. Shrub and herb layers of white alder communities are sparse to well-developed [91]. California wildrose, snowberry, and Pacific poison-oak typically grow in shrub layers. White alder is common or codominant California bay woodlands and forests; these are rare communities in the Coast Ranges [19,50,97,109]. In the Santa Ana and Santa Monica mountains, white alder codominates with California sycamore and red willow in large-stream canyon bottoms. These communities occur on west slopes at midelevations (2,000-7,000 feet (600-2,000 m)) [83,144]. In the Transverse Ranges, Fremont cottonwood/white alder communities were associated with high stream flows, high elevations (~3,300 feet (1,000 m)), and relatively long times since fire (54-65 years). Willow communities with chaparral shrubs were associated with low elevations and shorter times since fire (P≤0.05) [15] (see Plant response to fire).
Extent of riparian hardwood communities generally thins in arid southern California. In the South Coast [57], Transverse, and Peninsular ranges, white alder, Fremont cottonwood, and California sycamore form narrow gallery forests along streams [85]. At low elevations in the South Coast Ranges, these riparian woodlands intergrade to arroyo willow types; white alder is characteristic in arroyo willow riparian forests [57]. In Mojave Desert riparian communities, white alder may codominate with blue palo verde and honey mesquite as well as Fremont cottonwood and California sycamore [145]. In Anza-Borrego Desert State Park, San Diego County, white alder is a component of narrowleaf willow-Fremont cottonwood-California sycamore riparian forests. These forest tracts run along Coyote Creek, which is the only perennial stream in that part of the Colorado Desert [140].
Although it prefers streambanks and riverbanks, white alder also grows on springs and seeps [31]. On the East Bay Hills, it grows in spring communities dominated by coast live oak, interior live oak, valley oak, and blue oak [2].
Pacific Northwest: Red alder is more common than white alder in most of the Pacific Northwest. White alder tends to replace red alder in southerly valleys between the North Coast and Cascade ranges in Oregon and Washington [40]. Where the 2 species cooccur in Oregon, white alder tends to grow at lower elevations [77]. In southern Oregon, white alder is a common component of mixed-conifer forests of coastal Douglas-fir, white fir, and incense-cedar [81,150]. In the Willamette Valley, white alder codominates forested floodplains with bigleaf maple, and it is characteristic in black cottonwood/scouringrush horsetail and Oregon ash/common snowberry/stinging nettle floodplain communities [77].
Idaho: In southwestern Idaho, white alder codominates riparian forests with water birch and shrubs, including Lewis' mockorange and redosier dogwood. These forests are particularly common along the lower Salmon and Snake rivers and their tributaries. They are surrounded by sagebrush steppe, mountain shrubland, and/or bluebunch wheatgrass-Idaho fescue palouse prairie [62,86]. White alder is sometimes common or codominant with netleaf hackberry in shrublands by the lower South Fork of the Salmon River [135]. It is also common or dominant along the Snake River and its tributaries. In Hell's Canyon, it grows in gallery forests that have wide gradients of elevation, channel width, and channel stability. Pure white alder stands occur on unstable channels [82]. In 1917, Weaver [148] described white alder as a "numerous secondary tree" in willow-Douglas hawthorn associations of west-central Idaho and eastern Washington.White alder occurs in early succession [26,53,76,145,157] but also establishes decades after stand-replacing floods or fires on some sites [18]. It prefers open sites with direct light [39,53,112] and exposed mineral soils [53]. Riparian zones, especially low-bank positions where white alder grows, are constantly set back successionally due to flood, streambank failure, and less often, fire. Riparian systems seem resilient to these disturbances [91].
White alder establishes during both primary [26,72,76] and secondary [157] succession. In primary succession, it colonizes sandbars and other fresh alluvium exposed or deposited by receding floodwaters [26,72,76]. Such succession typically happens on only small patches of a watercourse [91]. White alder's ability to fix nitrogen helps it establish on young, nutrient-poor soils [53].
In secondary succession, white alder is noted on slumps [31] and burns (see Plant response to fire). However, it is sensitive to interference from other sprouting hardwood trees and may not establish well after fire if other hardwoods were present before fire [17]. On the Biscuit Burn and the Bear Butte and Booth Complex Burn, white alder presence in early postfire communities (postfire years 4 and 2, respectively, for each burn) was positively associated with low prefire cover of deciduous trees (P<0.05) [52].The scientific name of white alder is Alnus rhombifolia Nutt. (Betulaceae) [8,39,47,56,58,64,74]. White alder hybridizes with thinleaf alder in southwestern Idaho [86].
Common names are used throughout this Species Review. See Appendix B
for a complete list of common and scientific names of plant species discussed in this review and links to other FEIS Species Reviews.
White alder is used for wildfire rehabilitation [53], riparian restoration [53,156], and to stabilize streambanks [65,72], flood-control channels [46], and levees [149]. Its nitrogen-fixing ability improves soil fertility [53].
For information on artificial regeneration and seedling care of white alder, see these sources: [53,122,157]. Commercial plant sources are available [44].
White alder sprouts from the root crown [42,110], and/or roots [110]. It is apparently better able to sprout after flooding [17] or top-killing frost [42] than after fire [17,35] (see Plant response to fire).
White alder also layers, although layering is rarely documented. A study by Dry Creek found that white alders growing on alluvial sediments reproduced mostly by layering. Trees on terraces did not layer [76].
Alnus rhombifolia, the white alder, is an alder tree native to western North America, from British Columbia and Washington east to western Montana, southeast to the Sierra Nevada, and south through the Peninsular Ranges and Colorado Desert oases in Southern California.[2] It occurs in riparian zone habitats at an altitudes range of 100–2,400 metres (330–7,870 ft).[3][4] While not reported in northern Baja California, it has been predicted on the basis of its climatic adaptation to occur there also.[3] Alnus rhombifolia is primarily found in the chaparral and woodlands, montane, and temperate forests ecoregions.[2]
Alnus rhombifolia is a medium-sized deciduous tree growing to 15–25 metres (49–82 ft) (rarely to 35 metres (115 ft)) tall, with pale gray bark, smooth on young trees, becoming scaly on old trees. The leaves are alternate, rhombic to narrow elliptic, 4–10 centimetres (1.6–3.9 in) long and 2–5 centimetres (0.79–1.97 in) broad, with a finely serrated margin and a rounded to acute apex; they are thinly hairy below.[5]
The flowers are produced in catkins. The male catkins are pendulous, slender, 3–10 centimetres (1.2–3.9 in) long, yellowish, and produced in clusters of two to seven; pollination is in early spring, before the leaves emerge. The female catkins are ovoid, when mature in autumn 10–22 millimetres (0.39–0.87 in) long and 7–10 millimetres (0.28–0.39 in) broad, on a 1–10 millimetres (0.039–0.394 in) stem, superficially resembling a small conifer cone.[5] The small winged seeds disperse through the winter, leaving the old woody, blackish 'cones' on the tree for up to a year after.[2][3][6]
The white alder is closely related to the red alder (Alnus rubra), differing in the leaf margins being flat, not curled under. Like other alders, it is able to fix nitrogen, and tolerates infertile soils.[6]
Some Plateau Indian tribes use white alder for female health treatment needs.[7]
Alnus rhombifolia, the white alder, is an alder tree native to western North America, from British Columbia and Washington east to western Montana, southeast to the Sierra Nevada, and south through the Peninsular Ranges and Colorado Desert oases in Southern California. It occurs in riparian zone habitats at an altitudes range of 100–2,400 metres (330–7,870 ft). While not reported in northern Baja California, it has been predicted on the basis of its climatic adaptation to occur there also. Alnus rhombifolia is primarily found in the chaparral and woodlands, montane, and temperate forests ecoregions.
