CNPS Alliance: Avena spp.

Avena spp. - Bromus spp. Herbaceous Semi-Natural Alliance
Wild oats and annual brome grasslands

USDA Ecological Section Map

Summary Information

Primary Life FormHerb
Elevation0-2200 m
State RaritySNA
Global RarityGNA
DistributionUSA: CA, OR?; Mexico (NatureServe)
Endemic to CaliforniaNo
Invasive species rank See remarks
Endemic to California Floristic Province and DesertsNo
Date Added2016/12/01

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Avena barbata, Avena fatua, Brachypodium distachyon, Briza maxima, Bromus diandrus, Bromus hordeaceus and/or Hordeum murinum is dominant or co-dominant with other non-natives in the herbaceous layer such as Atriplex semibaccata and Hordeum spp. Emergent trees and shrubs may be present at low cover.

Herbs < 1.2 m; cover is open to continuous.

Avena fatua > 50% relative cover, and native herbs relatively low in cover in the herbaceous layer (Keeler-Wolf and Evens 2006).
Avena spp. > 50% relative cover, and native herbs < 10% relative cover in the herbaceous layer (Evens and Kentner 2006, Klein et al. 2007).
Avena spp. > 75% relative cover; other non-native or native plants < 5% absolute cover, if present, in the herbaceous layer (Evens and San 2004).
Brachypodium distachyon > 60% relative cover in the herbaceous layer (Keeler-Wolf et al. 2003a).
Bromus diandrus > 60% relative cover with other non-natives in herbaceous layer and with a variety of annuals at low cover (Klein and Evens 2005).
Bromus diandrus, B. hordeaceus, and/or Brachypodium distachyon > 80% relative cover separately or co-dominant with non-natives; natives usually with low or insignificant cover (Klein et al. 2007).
Bromus hordeaceus > 50% relative cover in the herbaceous layer (Jimerson et al. 2000).
Avena, Brachypodium, Briza, Bromus diandrus, Bromus hordeaceus and/or Erodium > 50% relative cover individually or in combination (Klein et al. 2015).
Avena, Brachypodium, Briza, Bromus, Erodium and/or Hypochaeris > 30% relative cover individually, or share > 50% relative cover in the herbaceous layer (Buck-Diaz et al. 2021, Sikes et al. 2021).
Avena, Brachypodium, Briza, Bromus, Erodium and/or Hypochaeris > 30% relative cover individually, or share > 50% relative cover in the herbaceous layer; and overall non-native herbs > 80% relative cover (Sikes et al. 2023).
Avena, Brachypodium, Briza, Bromus, Erodium, Hypochaeris, Medicago spp., and/or Vulpia bromoides > 30% relative cover individually, or share > 50% relative cover in the herbaceous layer; and overall non-native herbs > 90% relative cover (Sikes et al. 2025).

All topographic settings in foothills, waste places, rangelands, openings in woodlands. The USFWS Wetland Inventory (2012 national list) recognizes Bromus hordeaceus as a FACU plant.

MCV (1995)	California annual grassland series
NVCS (2009)	Avena fatua herbaceous alliance, Bromus (diandrus, hordeaceus, madritensis) herbaceous alliance
Calveg	Annual grasses and forbs, Non-native/ornamental grass
Holland	Valley and foothill grassland, Non-native grassland
Munz	Valley grassland
WHR	Annual grassland
CDFW CA Code	42.027.00

Formation Class	Mesomorphic Shrub and Herb Vegetation (Shrubland and Grassland)
Formation Subclass	Mediterranean Scrub and Grassland
Formation	Mediterranean Grassland and Forb Meadow
Division	California Grassland and Meadow
Macro Group	California Annual and Perennial Grassland
Group	Mediterranean California naturalized annual and perennial grassland

Avena species are cool-season, annual grasses from Eurasia. Introduction of A. barbata and A. fatua dates back to the late 1770s in California. Adaptation to the state’s many environments has led to many ecotypes. Seeds form transitory or permanent seed banks depending on the ecotype. Most seeds germinate after the first significant rain in the fall (DiTomaso and Healy 2007).

Avena barbata, slender oat, has a Cal-IPC rank of Moderate. It has slender spikelets, long awns, and open, lax panicles that distinguish the species. A. fatua, wild oat, has a Cal-IPC rank of Moderate. It has round spikelets, long awns, and open, lax panicles that distinguish the species.

Three other Avena species grow in California: A. sterilis, a federal noxious species, grows uncommonly in California, as do A. occidentalis and A. sativa. Reports of A. strigosa in the state are misidentifications (Baum 2007). The easiest oat to recognize is A. sativa, the cultivated oat; the glumes are generally awnless. DiTomaso and Healy (2007) offer a comparison table to help in identification.

Bromus diandrus, ripgut brome, has a Cal-IPC rank of Moderate. It is a cool-season, annual grass from Eurasia with greenish-tan dangling inflorescences of many spikelets, sharp florets, and long awns. Most of its seeds germinate after the first significant rain in the fall. Germination is best on a seedbed of thatch. Most germinating seeds survive the sporadic wetting and drying cycles that occur though the growing season. Plants set seed and die by the end of the growing season in the spring (DiTomaso and Healy 2007).

Bromus hordeaceus, soft brome, has a Cal-IPC rank of Limited. It is a cool-season, annual grass from Eurasia with greenish-tan plump inflorescences of many spikelets and short awns. Most of its seeds germinate after stratification and only few accumulate in a persistent seed bank. Germination is best on a seedbed of moderate mulch at warm (10-30 ºC) temperatures. Most germinating seeds survive the sporadic wetting and drying cycles that occur though the growing season. Survival is highest on clay substrates. Plants set seed and die by the end of the growing season in the spring (DiTomaso and Healy 2007).

Brachypodium distachyon, false brome, has a Cal-IPC rank of Moderate. It is a cool-season, annual from Europe with inflorescences, often tinged purplish, of 1-6 spikelets and short awns. Plants are identified easily by their shorthairy stem nodes. Most of its seeds germinate after the first significant rain in the fall. Plants tolerate shade and thin, rocky soils (DiTomaso and Healy 2007).

Wild oats, annual bromes, and annual natives occur in similar habitats. Natives in the genera Clarkia, Dichelostemma, Galium, Lupinus, Plagiobothrys, and Trifolium are often present in these stands. However, Avena stands have some ecological distinctions from those dominated by the annual bromes. Tall stands of Avena spp. dominate grasslands with years of accumulated thatch (Reiner 2007). The oats grow more slowly and produce less seed under high-frequency clipping (or grazing) regimes than do the shorter, non-native Bromus rubens and Vulpia spp. (Savelle and Heady 1970).

Fall temperatures and precipitation, light intensity affected by shading from plants and litter, and differences in microtopography determine annual grassland composition the following spring (Bartolome et al. 2007a, b, Evans and Young 1989). Bartolome (1989) suggested that the fine-scale variation in temporal and spatial structure found in the California annual grasslands are unimportant to recognize and define associations. However, we find associations useful for conservation. The presence of native plants appears sufficient to identify certain alliances when they have regular constancy and cover. This is our approach here, and we need further study in ways to increase native plant populations in the California annual grassland dominated by wild oats and annual bromes.

This alliance was formerly treated as two separate alliances in the 2009 publication A Manual of California Vegetation, second edition which cited both Avena (barbata, fatua) and Bromus (diandrus, hordeaceus) - Brachypodium distachyon Semi-natural Alliances. These two alliances were merged due to their frequent overlap in species composition. The Avena spp. - Bromus spp. Alliance accounts for the largest acreage of non-native grassland vegetation in cismontane California. We should have more interest in the native plants with low cover in studies of annual grasslands. For example, despite the widely held notion that serpentine substrates are resistant to many non-native annual grasses, Harrison and Viers (2007) found that many serpentine stands often have high Bromus hordeaceus cover. The native herb dominance depended on amount of rainfall and rockiness of soil. Stands on rocky serpentine soils in areas with high rainfall include that of Lasthenia californica - Plantago erecta - Vulpia microstachys alliance and occur with this Avena spp. - Bromus spp. type.

	Avena barbata	Avena fatua	Brachypodium distachyon	Briza maxima	Bromus diandrus	Bromus hordeaceus	Hordeum murinum
Life forms	Annual; herb	Annual; herb	Annual/perennial; herb	Annual; herb		Annual; herb	Annual; herb
Seed storage	soil; transient	soil; transient	null		null	Soil-stored
Seed longevity	Short; up to 3 years	Short; up to 3 years	null		null	Long
Mode of dispersal	Animal	Animal	null		null	Gravity
Germination agents	Melatonin	Melatonin	null		null	Stratification
Mode of sprouting	None	None	null		null	None
Survivability after fire/disturbance	No	No	null		null	Fire-hardy
Disturbance-stimulated flowering	Unknown	Unknown	null		null	No
Reproductive range	1 year	1 year	null		null	1 year
Recruitment	High	High	null		null	High
Regional variation	Moderate	Moderate	null		null	By subspecies

Recently, ecologists (Keeley 2006, Minnich 2008, Wills 2006) have argued that plants of the California annual grassland tolerate a wide variety of fire regimes. Prior to the European settlement, herbs probably dominated the more xeric, herbaceous landscapes of central and southern California. Areas burned at high frequencies close to Native American habitation, but they may have burned only once or twice a century elsewhere as a result of infrequent lightning strikes. Today the dominance of wild oats and annual bromes has changed the ecology of California’s grassland landscapes. Summer and fall fires appear to have little direct effect on these grasses. Fast-burning and relatively cool fires do not kill seed (DiTomaso and Healy 2007). Some brome seeds germinate after a fire in open light conditions, but best germination occurs within a thatch layer. Avena has awns and bristles that “unscrew” as the seeds dry, tending to bury the seeds relatively quickly following seed maturation, allowing seeds to remain insulated from fire (Reiner 2007).

Fire return interval	—
Seasonality	—
Size/extent	—
Complexity	—
Intensity	—
Severity	—
Type	—
Regional knowledge	—

The range of Avena barbata and A. fatua includes low elevations across California, except the Sonoran Desert (322B). The range of these bromes includes cismontane California at low elevations.

Central California Coast (261Aa-l). Stands of Brachypodium distachyon in the Santa Cruz Mountains also contain trace cover of Nassella pulchra and other natives (CNPS 2005).
Central California Coast Ranges (M262Aa-k). Stands dominated by Bromus hordeaceus in the western Diablo Range at Coyote Ridge (Evens and San 2004) include Amsinckia menziesii, Calystegia collina, Castilleja affinis ssp. neglecta (a CNPS list 1B plant), Chlorogalum pomeridianum, Cryptantha flaccida, Eschscholzia californica, Hordeum leporinum, and Lolium perenne. Stands dominated by Avena spp. with Bromus hordeaceus, Centaurea spp., Erodium spp., Nassella pulchra, Dichelostemma capitatum, and Sisyrinchium bellum occur on deep, loamy soils where haying and cattle grazing continue (Evens and San 2004).
Great Valley (262Aa-z). Stands exist throughout the section adjacent to other grassland types, including the Amsinckia (menziesii, tessellata), Brassica (nigra) and other mustards, Leymus triticoides and Quercus lobata alliances. Stands are common at Great Valley Grasslands State Park (Solomeshch and Barbour 2006).
Klamath Mountains (M261Aa-f, Ah-n, Ap-r, Au). Stands in the section (Jimerson et al. 2000) contain Aira caryophyllea, Dichelostemma multiflorum, Erodium botrys, Limnanthes douglasii, and Taeniatherum caput-medusae.
Northern California Coast (263Aa-m). Stands in the Point Reyes National Seashore and environs (Keeler-Wolf et al. 2003a) contain Avena fatua, Brassica rapa, Carpobrotus edulis, Cynosurus echinatus, Hypochaeris glabra, and emergent Baccharis pilularis shrubs.
Northern California Coast Ranges (M261Ba-f). Stands throughout the section (Jimerson et al. 2000) contain Aira caryophyllea, Dichelostemma multiflorum, Erodium botrys, Limnanthes douglasii, and Taeniatherum caput-medusae.
Northern California Interior Coast Ranges (M261Ca-c). Stands occur throughout the section (Jimerson et al. 2000).
Sierra Nevada (M261Ea-n, Ep-u). Stands have been sampled in Yosemite National Park (Keeler-Wolf et al. 2003b).
Sierra Nevada Foothills (M261Fa-e). Bromus hordeaceus stands at Peoria Wildlife Area (Evens et al. 2004) contain Clarkia purpurea, Holocarpha virgata, Lolium perenne, Lupinus nanus, Plagiobothrys nothofulvus, Taeniatherum caput-medusae, Trifolium hirtum, Vicia villosa, and Vulpia microstachys. Brachypodium distachyon stands include Amsinckia menziesii, Centaurea spp., P. nothofulvus, Vicia villosa, emergent Toxicodendron diversilobum shrubs and Quercus douglasii trees. Bromus hordeaceus stands in the northern subsection (Klein et al. 2007) have Daucus pusillus, Erodium botrys, Holocarpha virgata, Leontodon taraxacoides, Lupinus nanus, Plagiobothrys fulvus, P. nothofulvus, Taeniatherum caput-medusae, and Trifolium hirtum. Stands dominated by A. barbata also had Bromus hordeaceus, B. rubens, Clarkia purpurea, Croton setigerus, Filago gallica, Galium murale, G. parisiense, Lactuca serriola, Lupinus nanus, and Trifolium microcephalum (Klein et al. 2007).
Southern California Coast (261Ba-j). Stands at Santa Monica Mountains (Keeler-Wolf and Evens 2006) include Avena barbata, A. fatua, Asclepias fascicularis, Brassica nigra, Bromus diandrus, B. rubens, Centaurea melitensis, Dichelostemma capitatum, Hirschfeldia incana, and Vicia villosa.
Southern California Mountains and Valleys (M262Ba-p). Bromus diandrus stands in western Riverside Co. (Klein and Evens 2005) contain Croton setigerus, Hirschfeldia incana, Hordeum marinum, Lactuca serriola, and Salsola tragus; those in central San Diego Co. (Evens and San 2004) contain the following natives: Ambrosia psilostachya, Elymus glaucus,Croton setigerus, Lotus spp., Lupinus bicolor, and Trichostema lanceolatum.

The role of grazing or other disturbances in the reduction of native plants in California’s annual grasslands is varied and complex. It appears that the original soil conditions, climate, fire frequency, the presence of roads, and the intensity and duration of grazing all play a role (D’Antonio et al. 2007, Gelbard and Harrison 2003, Harrison 1999, Harrison et al. 2002, Harrison and Viers 2007, Safford and Harrison 2004). Stands dominated by these non-native annual grasses often have conservation value as wildlife habitat and contain seasonal presence of native plants, many with limited ranges. The non-native grasses attain strong dominance in years of high precipitation (Bartolome et al. 2007a, Harrison and Viers 2007), and the natives are often overlooked. Restoration plans should take this variability into account. The most successful programs use integrated approaches (DiTomaso et al. 2007). Stands with a history of deep tillage and crop cultivation lack native plants (D’Antonio et al. 2007), and they are less amenable to restoration. Depending on geographic location and soil condition, fire and grazing have variable effects on annual grasslands. In the northern Coast Ranges, Harrison et al. (2003) found that A. fatua increased in frequency with response to fire on non-serpentine soil but decreased after fire on serpentine soil. On both soils, however, the oats decreased with grazing. In the central Sierra Nevada foothills, Parsons and Stohlgren (1989) found that forb cover, diversity, and overall productivity increased with the removal of litter by fire, thereby enhancing the availability of nutrients, light, and space.

Atriplex semibaccata - Hordeum (murinum) [14]
Avena barbata - Avena fatua [1], [5], [8], [12], [13], [14], [15], [16], [17], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28]
Brachypodium distachyon [6], [12], [14], [15], [16], [23], [24], [25], [27], [28]
Brachypodium distachyon - Bromus diandrus / Quercus douglasii [8]
Briza maxima [15], [23]
Bromus diandrus [5], [11], [12], [13], [14], [15], [17], [18], [19], [23], [25], [28]
Bromus diandrus - Avena spp. [5], [14], [18], [25], [28]
Bromus diandrus - Mixed herbs [2], [5], [7], [20], [26], [27]
Bromus hordeaceus - Aira caryophyllea [4]
Bromus hordeaceus - Amsinckia menziesii - Hordeum murinum [1]
Bromus hordeaceus - Bromus tectorum [4]
Bromus hordeaceus - Dichelostemma multiflorum [4]
Bromus hordeaceus - Erodium botrys [4], [10], [14], [15], [23], [24], [25], [26], [27], [28]
Bromus hordeaceus - Erodium botrys - Plagiobothrys fulvus [8], [13], [17], [19]
Bromus hordeaceus - Hordeum spp. - Medicago polymorpha [13], [17], [19], [26], [27], [28]
Bromus hordeaceus - Leontodon saxatilis [8], [13], [17], [19]
Bromus hordeaceus - Limnanthes douglasii [4]
Bromus hordeaceus - Taeniatherum caput-medusae [4], [13], [27]
Bromus hordeaceus - (Vicia villosa - Lolium perenne) - Trifolium hirtum [3], [8], [13], [27]
Bromus hordeaceus - Vulpia myuros var. hirsuta [9]
Hypochaeris glabra - Vulpia bromoides [13], [14], [17], [19], [23]

[1] Evens, J.;San, S. 2004
[2] Evens, J.;San, S. 2005
[3] Evens, J.M.;San, S.;Taylor, J. 2004
[4] Jimerson, T.M.;Menke, J.W.;Carothers, S.K.;Murray, M.P.;VanSickle, V.;McClellan, K.H. 2000
[5] Keeler-Wolf, T.;Evens, J. 2006
[6] Keeler-Wolf, T .;Schirokauer, D.;Meinke, J.;van derLeeden, P. 2003a
[7] Klein, A.;Evens, J. 2006
[8] Klein, A.;Crawford, J.;Evens, J.;Keeler-Wolf, T.;Hickson, D. 2007
[9] Kopecko, K.J.;Lathrop, E.W. 1975
[10] Schlising, R.A.;Sanders, E.L. 1982
[11] Solomeshch, A.;Barbour, M. 2006
[12] Sproul, F.;Keeler-Wolf, T.;Gordon-Reedy, P.;Dunn, J.;Klein, A.;Harper, K. 2011
[13] Buck-Diaz, J.;Batiuk, S.;Evens, J.M. 2012
[14] Rodriguez, D.;Sikes, K.;Keeler-Wolf, T.;Kittel, G.;Curtis, J.;Curley, C.;Evens, J. 2017
[15] Klein, A.;Keeler-Wolf, T.;Evens, J. 2015
[16] Verdone, L.;Evens, J. 2010
[17] Buck-Diaz, J.;Harbert, B.;Evens, J. 2011
[18] AECOM, 2013
[19] Buck-Diaz, J.;Ratchford, J.;Evens, J. 2013
[20] Buck, J.;Evens, J. 2010
[21] Rodriguez, D. 2015
[22] Parsons, D.J. ;Stohlgren, T.J. 1989
[23] Buck-Diaz, J.;Sikes, K.;Evens, J.M. 2021a
[24] Sikes, K.;Buck-Diaz, J.;Evens, J. 2021
[25] Sikes, K.;Buck-Diaz, J.;Vu, S.;Evens, J. 2023
[26] Buck-Diaz, J.;Sikes, K.;Vu, S.;LaFever-Jackson, A.;Evens, J. 2023
[27] Ratchford, J.;Harbert, B;Boul, R.;Keeler-Wolf, T.;Evens, J. 2024a
[28] Sikes, K.;Buck-Diaz, J.;Vu, S.;Bibbo, M.;Evens, J. 2025
Bartolome, J.W.;Barry, W.J.;Griggs, T.;Hopkinson, P. 2007a
Bartolome, J.W.;Hayes, G.F.;Ford, L.D. 2007b
Baum, B.R. 2007
D'Antonio, C.M.;Malmstrom, C.;Reynolds, S.A.;Gerlach, J. 2007
DiTomaso, J.M.;Healy, E.A. 2007
HDR, 2014b
Heady, H.F. 1977
Keeler-Wolf, T.;Evens, J.M.;Solomeshch, A.I.;Holland, V.L.;Barbour, M.G. 2007
Keeler-Wolf, T.;Schindel, M.;San, S.;Moore, P.;Hickson, D. 2003b
Keeley, J.E. 2006
Kittel, G.;Reyes, E.;Evens, J.;Buck, J.;Johnson, D. 2012
Stromberg, M.R.;Corbin, J.D.;D’Antonio, C.M. 2007
Wills, R. 2006

USDA Ecological Section Map

Summary Information

Related Links

Characteristic Species

Vegetation Layers

Membership Rules

Habitats

Other Habitat, Alliance and Community Groupings

National Vegetation Classification Hierarchy

Remarks

Life History Traits of the Principal Species

Fire Characteristics

Regional Status

Management Considerations

Associations

References