Poa bulbosa



INTRODUCTORY


 

2006 Greta Murdoff, California Academy of Sciences

AUTHORSHIP AND CITATION:
Gucker, Corey L. 2007. Poa bulbosa. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: http://www.fs.fed.us/database/feis/ [].

FEIS ABBREVIATION:
POABUL

NRCS PLANT CODE [79]:
POBU

COMMON NAMES:
bulbous bluegrass
winter bluegrass

TAXONOMY:
The scientific name of bulbous bluegrass is Poa bulbosa L. (Poaceae) [2,6,21,31,37,76].

Infrataxa:
Poa bulbosa subsp. bulbosa
Poa bulbosa subsp. vivipara [1]

SYNONYMS:
Paneion bulbosum L. var. viviparum (Koch.) Lunell [48]=
    Poa bulbosa subsp. vivipara [1]

Poa bulbosa var. vivipara (Koel.) [26]=
    Poa bulbosa subsp. vivipara [1]

LIFE FORM:
Graminoid

FEDERAL LEGAL STATUS:
No special status

OTHER STATUS:
Information on state-level noxious weed status of plants in the United States is available at Plants Database.


DISTRIBUTION AND OCCURRENCE

SPECIES: Poa bulbosa
GENERAL DISTRIBUTION:
Bulbous bluegrass, a native species in southern Asia, Europe, and northern Africa, has established outside cultivation in North America. It is much more common in the western than in the eastern United States [21,24,47,86]. From North Dakota to Oklahoma and westward, bulbous bluegrass is common [10], but in the northeastern United States, bulbous bluegrass is generally restricted to fields, lawns, and meadows [21]. Bulbous bluegrass is also reported from parts of southern Canada [41].

Bulbous bluegrass was likely a contaminant in grass and/or clover seed imported by the United States [80]. Records indicate that in early 1900s bulbous bluegrass was intentionally planted on sites near Arlington, Virginia, and Pullman, Washington [47]. In the Pacific Northwest, bulbous bluegrass dispersal was tracked through the use of herbaria records. Bulbous bluegrass originated near Portland, Oregon, an active shipping port, spread into Washington and progressed eastward to northern Idaho and western Montana between 1891 and 1980 [17]. The distribution of bulbous bluegrass in the United States and southern Canada is available from the Grass Manual on the Web.

HABITAT TYPES AND PLANT COMMUNITIES:
Disturbed sites, roadsides, and abandoned fields are common bulbous bluegrass habitats. In the western United States, it occurs in perennial and annual grasslands, chaparral, pinyon-juniper (Pinus-Juniperus spp.) woodlands, riparian communities, sagebrush (Artemisia spp.)- steppe vegetation, and openings in coniferous forests. Bulbous bluegrass has been used to seed disturbed sites throughout the United States [54,61] and is possible in any disturbed area or opening near seeded sites. Species commonly associated with bulbous bluegrass include brome grasses (Bromus spp.), wheatgrasses (Agropyron spp.), bluebunch wheatgrass (Pseudoroegneria spicata), Kentucky bluegrass (Poa pratensis), big sagebrush (A. tridentata), low sagebrush (A. arbuscula), mountain-mahogany (Cercocarpus spp.), oaks (Quercus spp.), ponderosa pine (P. ponderosa), Douglas-fir (Pseudotsuga menziesii), and quaking aspen (Populus tremuloides).


BOTANICAL AND ECOLOGICAL CHARACTERISTICS

SPECIES: Poa bulbosa

 

  Charles Webber California Academy of Sciences

GENERAL BOTANICAL CHARACTERISTICS:
This description provides characteristics that may be relevant to fire ecology, and is not meant for identification. Keys for identification are available (e.g., [12,21,30,31,41,56,76]).

Aboveground description: Bulbous bluegrass is a nonnative tuft- or sod-forming grass [26,56]. It is described as a perennial [26,29,66], an annual [38], and as an annual with "perennial tendencies" [36]. Because bulbous bluegrass grows new roots each season [63,81] and has a relatively short life span [61,86], its life history is easily confused. Bulbous bluegrass stems and culms are erect, attach to bulbous bases, and measure 6 to 28 inches (15-70 cm) tall [29,31,76,86]. Culms are hollow and smooth [25]. Basal leaves are flat to involute, 1 to 3 mm wide, and 2 to 6 inches (5-15 cm) long [31,41,67].

The aboveground pear-shaped bulbs are approximately 1 cm long [86] and consist of a thick, solid internode that is surrounded by scale-like leaves [26]. Studies of bulbous bluegrass in its native habitats showed that bulbs were common in regions that experienced a periodic dry season but not in moist areas. When bulbous bluegrass was planted in moist areas, basal bulbs were not produced. The researcher indicated that although bulbous bluegrass basal bulbs were structurally similar to those of lilies and irises, they stored primarily water and not starch or sugar [32].

On its 1- to 4.5-inch (3-12 cm) long, one-sided panicle, bulbous bluegrass produces 2 types of spikelets: one typically has 3 to 6 normal florets; the other is a viviparous bulbil or bulblet without pistils or stamens [26,31,41,51]. Bulbils are more common than true florets in most US populations [28,29,31,56,86]. However, plants with both bulbils and true florets do occur [26,67]. Bulbils are transformed florets with all the necessary tissue to propagate a new plant, and measure 5 to 15 mm long [26,67].

Belowground description: Bulbous bluegrass develops a fairly limited root system. Five to twenty roots attach to the basal bulb. These roots range from 0.4 to 4 inches (1-10 cm) long and support numerous, short, thread-like rootlets about 1 mm long [26]. On disturbed sites in Utah, bulbous bluegrass roots formed mycorrhizal associations [62].

RAUNKIAER [69] LIFE FORM:
Chamaephyte
Hemicryptophyte

REGENERATION PROCESSES:
Although bulbous bluegrass produces true florets as well as bulbils, seeds are rare [30,47,80]. Bulbous bluegrass regeneration is almost exclusively through asexual means including bulbil production [10,47], basal bulb production [47], and sometimes through culm rooting [12].

Reproduction through true floret and seed production does occur in bulbous bluegrass's native habitats. Some bulbous bluegrass populations reproduce primarily by seed, while in others reproduction is dominated by basal bulb growth and/or bulbil production. Researchers found a significant negative linear relationship (r=-0.596, P=0.02) between percentage of flowering plants and mean annual precipitation of habitats when plants were taken from field sites and grown in controlled conditions [59].

Pollination: When true florets are produced, they are wind pollinated [60].

Breeding system: Spikelets are perfect [53]. Although asexual regeneration is predominant, researchers found "surprisingly high levels of genetic diversity at the species and population level" when bulbous bluegrass bulbils were collected and grown from 10 populations in Idaho, Oregon, and Washington. An average of 26 plants were grown from each population. Researchers concluded that bulbous bluegrass was an autopolyploid and that the high levels of diversity resulted from multiple introductions and/or some past sexual reproduction [58].

Seed production: Through controlled experiments on bulbous bluegrass strains from Turkey and Afghanistan, Youngner [88] determined that floret and bulbil production were controlled by day length and temperature. Plants grown under high temperature (70-81 F (21-27 C)) and long day (16 hours of light) conditions produced normal florets. Plants produced bulbils when grown at lower temperatures (68 F (<20 C)) and exposed to shorter days (8 hours of light). Plants exposed to long day and low temperature or short day and high temperature conditions produced panicles with both florets and bulbils. The researcher recalled observations of bulbous bluegrass plants near Los Angeles, California, producing normal florets in late March, when day length exceeded 13 hours [88].

Seed dispersal: Animals, wind, and gravity are all possible dispersers of bulbous bluegrass. Small mammals cache and disperse bulbous bluegrass bulbils [24]. Because leaves and roots senesce with the dry season, it is possible that basal bulbs may be moved by wind (Sowerby cited in [26]). However, this method of dispersal is not described elsewhere, and Halperin [26] indicates he never observed this type of dispersal.

Roadsides are important dispersal corridors for bulbous bluegrass. The frequency of bulbous bluegrass was compared on sites with serpentine and nonsepentine soils at 30, 300, 3,000 feet (10 m, 100 m, and >1,000 m) from paved or improved roads. Bulbous bluegrass did not occur on serpentine soils but had 9.6% and 9.2% frequency on nonserpentine soil sites 30 feet (10 m) and 300 feet (100 m) from roads, respectively. Bulbous bluegrass was not found 3,000 feet (1,000 m) away from roads [20].

Seed banking: Studies regarding the longevity of bulbous bluegrass bulbs or bulbils are lacking. Bulbils attached to the plant that make contact with the soil can root and grow immediately without a dormancy period. Bulbils detached from the panicle lie dormant for 3 to 6 months before germinating [88]. Basal bulbs may live dormant for "several" years [24]. Basal bulbs can dry to below 10% moisture yet maintain growth potential for "extended periods" [83]. Depth of burial affects bulbous bluegrass emergence; for more on this topic, see the Depth of burial discussion.

Germination: Several studies of bulbous bluegrass germination under controlled conditions suggest that germination is best when temperatures are cool 41 to 64 F (5-18 C) for at least part of the day [36,58] and when seeds are uncovered or only shallowly covered with soil [63,81]. Bulbous bluegrass bulbils collected from populations in Oregon, Washington, and Idaho usually germinated within 24 hours after being chilled for 4 to 5 days at 41 F (5 C) [58]. Germination was 98% to 99% for bulbils collected from Moscow, Idaho, exposed to cool (50 to 64 F (10-18 C)) 8-hour days and cold (38 F (3.5 C)) nights. No bulbils germinated when exposed to 14-hour warm (79 F (26 C)) days and warm nights (68 F (20 C)). When day length was shortened to 11 hours under the warm day and night treatments germination was 18% [36].

Bulbous bluegrass bulbils from Intermountain seed suppliers germinated best under fluctuating greenhouse temperatures that ranged from 50 to 100 F (10-40 C), averaged 57 F (14 C), and closely approximated outdoor early spring conditions. Fluctuating greenhouse temperatures produced 98% germination in 14 days. Room temperature conditions that averaged 70 F (21C) and ranged from 66 to 73 F (19-23 C) produced 91% germination after 19 days. Germination was only 1% after 24 days at a constant temperature of 86 F (30 C) [63].

Depth of burial: Shoot emergence was delayed and reduced as bulbous bluegrass bulbil planting depths increased. Bulbils were kept moist in a greenhouse where temperatures averaged 64 F (18 C) and ranged daily from 54 to 90 F (12-32 C). Just 8% of seedlings emerged after 30 days when bulbils were planted 1.5 inches (3.8 cm) deep. Emergence was more rapid and more successful when seeds were left of the soil surface or shallowly planted. Researchers indicated that field conditions may not provide adequate moisture for successful germination of uncovered bulbils. A summary of the findings is provided below [63].

Percentage shoot emergence 5 to 30 days after planting at depths of 0 to 1.5 inches [63]
  Days after planting
Planting depth (inches) 5 7 9 11 13 15 30
surface 26 64 85 90 93 95 96
0.25 0 6 47 71 83 87 90
0.50 0 2 26 60 68 73 77
0.75 0 0 15 49 58 62 64
1.0 0 0 0 5 15 22 28
1.5 0 0 0 1 2 5 8

Seedling establishment/growth: Bulbous bluegrass plants produced the most growth in late April and May and senesced by 6 June in a mountain brush field nursery site in the Intermountain Region. The nursery occurred at 5,375 feet (1,638 m) on deep clay loam soils. Bulbils were sown in late fall, and root and shoot development was monitored periodically from 22 March to 6 June. Root and shoot growth was underway in early April. By mid-May there were 70 roots that penetrated 4.7 inches (12 cm) and spread 3.5 inches (9 cm), and shoot height reached 18 inches (46 cm). By 6 June, the approximate start of summer drought conditions, bulbous bluegrass was senescing [63].

Bulbous bluegrass root and shoot growth over the growing season [63]

Sampling date

April 5 April 12 April 18 April 25 May 2 May 9 June 6
Root length (cm) 10 10 14 30 41 73 9
Shoot length (cm) 5 6 10 16 21 46 0
Number of roots 9 9 28 35 37 70 6
Number of leaves 2 3 3 7 9 18 0
Root penetration (cm) 4 4 5 6 8 12 0
Root spread (cm) 2 2 3 7 6 9 0

Vegetative regeneration: Asexual or vegetative regeneration is the primary means of reproduction in US bulbous bluegrass populations. Bulbils produced in the panicle through the transformation of flowers and lemmas germinate to produce new individuals [10,22,47]. Basal bulbs that multiply and grow laterally lead to vegetative spread [47,64]. Vegetative regeneration is also possible through the rooting of decumbent culms [12].

SITE CHARACTERISTICS:
Bulbous bluegrass is primarily described in disturbed, dry to moist habitats throughout its range. Low- to mid-elevation pastures, abandoned fields, and roadsides near low elevation chaparral, oak, sagebrush, pinyon-juniper, mixed conifer, meadow, and streambank communities are typical bulbous bluegrass habitat [25,28,56,66,86]. Growth is considered best on sites with moderately fertile soils that experience cool, moist growing seasons [81].

Climate: Bulbous bluegrass tolerates drought and cold [82] and grows best in climates with dry summers and mild, moist winters. Big sagebrush-bluebunch wheatgrass communities in Grant County, Washington, where bulbous bluegrass occurs, experience a semiarid climate. Annual precipitation averages 8 inches (203 mm), and most precipitation comes in the fall and winter [46]. Near Pocatello, Idaho, bulbous bluegrass occurs in big sagebrush/mixed grasslands that average 10.9 inches (276 mm) of precipitation annually, most of which is delivered as snow in the winter and rain in the spring [68]. In bulbous bluegrass habitats in Tooele County, Utah, average annual precipitation was 12.9 inches (328 mm) for a 30-year period. In the same period, the maximum annual precipitation was 19 inches (483 mm), and the minimum was 6.8 inches (173 mm). Temperature extremes for the 30-year period were 102 F (39 C) and -22 F (-30 C) [19].

Available moisture can affect bulbous bluegrass abundance. In big sagebrush- and low sagebrush-dominated communities in Washington County, Idaho, bulbous bluegrass cover averaged 23% to 37% during a cool, wet spring and summer. When conditions were hot and dry in the spring and summer months, bulbous bluegrass cover averaged 13% to 22% [71].

Summer dormancy/drought avoidance: Bulbous bluegrass avoids drought conditions by senescing roots and green tissue. Tissues regrow from the bulb when growing conditions are favorable. Experiments conducted in Davis, California, revealed that bulbous bluegrass exhibited summer dormancy even when supplied with summer moisture [45]. Volaire and Norton [83] determined that summer dormancy was triggered by increasing day lengths and temperatures and that mature tissues within the bulbs remain dormant for at least 4 weeks. Regrowth from bulbs was rapid.

Elevation: Based on plantings of bulbous bluegrass in the Intermountain Region, Plummer [63] indicates that bulbous bluegrass establishes well in the lower foothills but typically does not survive at "high" elevation sites. Elevational tolerances in other western US habitats are provided below.

Elevational tolerances for bulbous bluegrass by state or region
State/region Elevation
California below 6,600 feet [29]
southern California below 5,000 feet [56]
northern Colorado 5,000-8,000 feet [28]
Nevada 4,300-7,200 feet [38]
Utah 3,000-7,000 feet [61]
Intermountain West low to middle elevations [10]

Soils: Bulbous bluegrass occurs on sandy, gravelly, silty, loamy, and clayey soils and tolerates salty, shallow, and low-fertility conditions [61,82]. Bulbous bluegrass is adapted to shallow, coarse-textured soils in northeastern Oregon [87]. Bulbous bluegrass is also common on shallow scabland soils with cheatgrass (Bromus tectorum) [24]. In a 320,000-acre (130,000 ha) study area that included parts of Napa, Lake, and Colusa counties in California, bulbous bluegrass occurred on nonserpentine but not on serpentine soils [20]. On Fire Island in Suffolk County, New York, bulbous bluegrass was rare and occurred on dry, ruderal sands [13].

SUCCESSIONAL STATUS:
In its native habitats, bulbous bluegrass occurs in both early- and late-seral habitats, but in the United States, it is most typical of disturbed sites and early-seral communities.

Native habitats: Bulbous bluegrass dominated late-seral pastures within open woodland habitats north of Madrid, Spain, and was not listed as a common species in early-successional pastures dominated by annual grasses [49]. In Israel, bulbous bluegrass was associated with closed and semi-open maquis vegetation and oak woodlands. In western Spain, bulbous bluegrass occurred on ploughed dehesas and persisted in later-seral shrub-dominated communities [34].

Shade tolerance: Open sites and canopy openings are preferred bulbous bluegrass habitat. Bulbous bluegrass cover and herbage production were greater in the openings than in the shrub understory in seeded sites within shrub/bunchgrass communities in south-central Idaho [54].

Disturbance-related succession: Most research and studies suggest that bulbous bluegrass establishment requires some type of disturbance. In Oregon and Washington, bulbous bluegrass is characteristic of overgrazed or disturbed western juniper/curlleaf mountain-mahogany (Juniperus occidentalis/Cercocarpus ledifolius) and riparian habitats [8,9]. In southeastern Oregon, bulbous bluegrass occurs on "severely degraded" riparian sites [15]. On the Fitzner-Eberhardt Arid Lands Ecology Reserve in Washington, bulbous bluegrass frequency was 40% to 60% in 57-year-old abandoned fields but was absent from adjacent uncultivated big sagebrush/bluebunch wheatgrass vegetation [73]. However, in bluebunch wheatgrass-dominated canyon grasslands near Lewiston, Idaho, bulbous bluegrass occurred in communities classified as early-, mid-, and late-seral. Bulbous bluegrass abundance increased after fire in all seral stages [23].

While the introduction of bulbous bluegrass is typically associated with disturbances, it established in never tilled, native shrub-steppe communities in Methow Valley, Washington. A comparison of tilled and untilled sites revealed that bulbous bluegrass abundance increased with increased time since abandonment, and cover increased over time in untilled, native shrub-steppe communities. Tilled sites spanned a 52-year chronosequence. Bulbous bluegrass cover was significantly greater on tilled than untilled (P<0.05) communities, averaging 6.6% on old fields and 1.3% in never tilled communities. Bulbous bluegrass cover was greater in untilled areas adjacent to older tilled fields than those adjacent to younger tilled fields [39].

Grazing: Bulbous bluegrass often increases on grazed sites. Bulbous bluegrass is classified as an invader of grazed sites, as its total annual production typically increases as range site conditions deteriorate, or as site dominance changes from late-seral to early-seral species [40]. In the Trout Creek Mountains of southeastern Oregon, bulbous bluegrass was typical of "severely degraded" riparian sites that received high animal use. Researchers considered bulbous bluegrass an "aggressive colonizer" [15]. Bulbous bluegrass cover typically increased as bunchgrass cover decreased with trampling and use by domestic and native grazers in Idaho fescue (Festuca idahoensis)-bluebunch wheatgrass-lupine (Lupinus spp.) and oatgrass-Wasatch desertparsley (Danthonia spp.-Lomatium bicolor var. leptocarpum) communities in eastern Oregon's Blue and Ochoco mountains [35]. In crested wheatgrass (Agropyron cristatum) communities in Tooele County, Utah, bulbous bluegrass frequency increased after sites were grazed for 10 years. Before grazing, bulbous bluegrass occurred on 16% of the study plots. After grazing, bulbous bluegrass occurred on 39% of plots. Bulbous bluegrass often grew adjacent to or within crested wheatgrass crowns. Cattle grazing treatments included all possible combinations of 3 intensities (50%, 56%, and 80% utilization) and 4 systems (continuous, continuous delayed start, continuous early removal, and rotation) [19].

Cover and aboveground production of bulbous bluegrass were not significantly (P<0.05) different between an exclosure and a grazed site in the Case Creek Drainage of south-central Idaho. Before a fire in the area, sites were dominated by mountain big sagebrush (Artemisia tridentata subsp. vaseyana), antelope bitterbrush (Purshia tridentata) and mixed bunchgrasses. After the fire, the area was seeded with desert wheatgrass (Agropyron desertorum), tall oatgrass (Arrhenatherum elatius), smooth brome (Bromus inermis), bulbous bluegrass, yellow sweetclover (Melilotus officinalis), and antelope bitterbrush. In 1981, cover and production were compared on unprotected sites and sites within the exclosure, constructed soon after seeding in 1942. Bulbous bluegrass cover and production were slightly greater in the exclosure [54]. Bulbous bluegrass "production and vigor" were not affected by domestic sheep grazing at an average utilization intensity of 61% in crested wheatgrass pastures near Ephraim, Utah. Grazing occurred for 7 years [3].

SEASONAL DEVELOPMENT:
Bulbous bluegrass begins growth in the fall, winter, or early spring, produces bulbils by spring or early summer, and senesces before the summer drought period. Development and maturation are rapid [47,80]. Bulbous bluegrass is typically dormant through the summer and resumes growth in the fall or winter before or after freezing conditions persist [24]. Typical timing of flower and/or bulbil production is provided below.

Timing of bulbous bluegrass bulbil and/or flower production by state or region
State/region Date of bulbil/flower production
California spring [29]
southern California April-July [56]
Carolinas April [67]
Nevada April-July [38]
New York, Fire Island, Suffolk County June [13]
Intermountain West April-July [10]

FIRE ECOLOGY

SPECIES: Poa bulbosa
FIRE ECOLOGY OR ADAPTATIONS:
Fire adaptations: A review of studies and literature from the Mediterranean Region suggests that bulbous bluegrass may sprout vegetatively after fire [57]. The postfire regeneration methods for North American bulbous bluegrass populations have not been described, but inferences based on its reproductive biology are possible. Postfire sprouting may depend on fire season since roots shrivel and disappear during the summer dormant period. Survival of basal bulbs and bulbils on the soil surface is likely dependent on fire severity. Buried bulbils may survive fire, and consumption of litter and humus layers may uncover bulbils enough for successful germination. (See discussion on Depth of burial). Dispersal of bulbils from adjacent unburned sites or unburned refugia into the burned area is probable.

Fire regimes: In native bulbous bluegrass habitats, the prevailing fire regime in which the species evolved is unknown. It is also unclear how the presence of bulbous bluegrass may affect fire regimes in its nonnative US habitats. Kartesz [38] suggests that bulbous bluegrass may pose a fire risk when dry; however, Plummer and others [65] suggest that bulbous bluegrass establishment in cheatgrass-dominated areas may decrease fire risk. Because stems and culms often shrivel and detach from basal bulbs during drought conditions, bulbous bluegrass establishment in cheatgrass-dominated areas can break up the continuity of available fine fuels. Decreased fuel availability could lead to decreased ignition and/or fire spread potential.

The distribution of bulbous bluegrass in the western United States is described, while its distribution throughout the rest of the United States is not. Below is a list of habitats containing species that bulbous bluegrass is associated with in the West. For information on communities not listed below, see the complete FEIS Fire Regime Table.

Fire regime information on vegetation communities in which bulbous bluegrass may occur. For each community, fire regime characteristics are taken from the LANDFIRE Rapid Assessment Vegetation Models [43]. These vegetation models were developed by local experts using available literature, local data, and/or expert opinion as documented in the .pdf file linked from each Potential Natural Vegetation Group listed below. Cells are blank where information is not available in the Rapid Assessment Vegetation Model.
Pacific Northwest California Southwest Great Basin Northern Rockies
Pacific Northwest
Vegetation Community (Potential Natural Vegetation Group) Fire severity* Fire regime characteristics
Percent of fires Mean interval
(years)
Minimum interval
(years)
Maximum interval
(years)
Northwest Grassland
Bluebunch wheatgrass Replacement 47% 18 5 20
Mixed 53% 16 5 20
Idaho fescue grasslands Replacement 76% 40    
Mixed 24% 125    
Northwest Shrubland
Wyoming big sagebrush semidesert Replacement 86% 200 30 200
Mixed 9% >1,000 20  
Surface or low 5% >1,000 20  
Wyoming sagebrush steppe Replacement 89% 92 30 120
Mixed 11% 714 120  
Low sagebrush Replacement 41% 180    
Mixed 59% 125    
Mountain big sagebrush (cool sagebrush) Replacement 100% 20 10 40
Northwest Woodland
Western juniper (pumice) Replacement 33% >1,000    
Mixed 67% 500    
Oregon white oak-ponderosa pine Replacement 16% 125 100 300
Mixed 2% 900 50  
Surface or low 81% 25 5 30
Ponderosa pine Replacement 5% 200    
Mixed 17% 60    
Surface or low 78% 13    
Oregon white oak Replacement 3% 275    
Mixed 19% 50    
Surface or low 78% 12.5    
Northwest Forested
Douglas-fir (Willamette Valley foothills) Replacement 18% 150 100 400
Mixed 29% 90 40 150
Surface or low 53% 50 20 80
Ponderosa pine (xeric) Replacement 37% 130    
Mixed 48% 100    
Surface or low 16% 300    
Dry ponderosa pine (mesic) Replacement 5% 125    
Mixed 13% 50    
Surface or low 82% 8    
Douglas-fir-western hemlock (dry mesic) Replacement 25% 300 250 500
Mixed 75% 100 50 150
Douglas-fir-western hemlock (wet mesic) Replacement 71% 400    
Mixed 29% >1,000    
Mixed conifer (southwestern Oregon) Replacement 4% 400    
Mixed 29% 50    
Surface or low 67% 22    
Mixed conifer (eastside dry) Replacement 14% 115 70 200
Mixed 21% 75 70 175
Surface or low 64% 25 20 25
Mixed conifer (eastside mesic) Replacement 35% 200    
Mixed 47% 150    
Surface or low 18% 400    
California
Vegetation Community (Potential Natural Vegetation Group) Fire severity* Fire regime characteristics
Percent of fires Mean interval
(years)
Minimum interval
(years)
Maximum interval
(years)
California Grassland
California grassland Replacement 100% 2 1 3
California Shrubland
Chaparral Replacement 100% 50 30 125
Montane chaparral Replacement 34% 95    
Mixed 66% 50    
California Woodland
California oak woodlands Replacement 8% 120    
Mixed 2% 500    
Surface or low 91% 10    
Ponderosa pine Replacement 5% 200    
Mixed 17% 60    
Surface or low 78% 13    
California Forested
Mixed conifer (North Slopes) Replacement 5% 250    
Mixed 7% 200    
Surface or low 88% 15 10 40
Mixed conifer (South Slopes) Replacement 4% 200    
Mixed 16% 50    
Surface or low 80% 10    
Aspen with conifer Replacement 24% 155 50 300
Mixed 15% 240    
Surface or low 61% 60    
Southwest
Vegetation Community (Potential Natural Vegetation Group) Fire severity* Fire regime characteristics
Percent of fires Mean interval
(years)
Minimum interval
(years)
Maximum interval
(years)
Southwest Grassland
Desert grassland with shrubs and trees Replacement 85% 12    
Mixed 15% 70    
Plains mesa grassland with shrubs or trees Replacement 76% 20    
Mixed 24% 65    
Montane and subalpine grasslands Replacement 55% 18 10 100
Surface or low 45% 22    
Montane and subalpine grasslands with shrubs or trees Replacement 30% 70 10 100
Surface or low 70% 30    
Southwest Shrubland
Low sagebrush shrubland Replacement 100% 125 60 150
Mountain sagebrush (cool sage) Replacement 75% 100    
Mixed 25% 300    
Gambel oak Replacement 75% 50    
Mixed 25% 150    
Mountain-mahogany shrubland Replacement 73% 75    
Mixed 27% 200    
Southwest Woodland
Pinyon-juniper (mixed fire regime) Replacement 29% 430    
Mixed 65% 192    
Surface or low 6% >1,000    
Ponderosa pine/grassland (Southwest) Replacement 3% 300    
Surface or low 97% 10    
Southwest Forested
Riparian deciduous woodland Replacement 50% 110 15 200
Mixed 20% 275 25  
Surface or low 30% 180 10  
Southwest mixed conifer (warm, dry with aspen) Replacement 7% 300    
Mixed 13% 150 80 200
Surface or low 80% 25 2 70
Southwest mixed conifer (cool, moist with aspen) Replacement 29% 200 80 200
Mixed 35% 165 35  
Surface or low 36% 160 10  
Stable aspen without conifers Replacement 81% 150 50 300
Surface or low 19% 650 600 >1,000
Great Basin
Vegetation Community (Potential Natural Vegetation Group) Fire severity* Fire regime characteristics
Percent of fires Mean interval
(years)
Minimum interval
(years)
Maximum interval
(years)
Great Basin Grassland
Great Basin grassland Replacement 33% 75 40 110
Mixed 67% 37 20 54
Mountain meadow (mesic to dry) Replacement 66% 31 15 45
Mixed 34% 59 30 90
Great Basin Shrubland
Basin big sagebrush Replacement 80% 50 10 100
Mixed 20% 200 50 300
Wyoming big sagebrush semidesert Replacement 86% 200 30 200
Mixed 9% >1,000 20 >1,000
Surface or low 5% >1,000 20 >1,000
Wyoming big sagebrush semidesert with trees Replacement 84% 137 30 200
Mixed 11% >1,000 20 >1,000
Surface or low 5% >1,000 20 >1,000
Wyoming sagebrush steppe Replacement 89% 92 30 120
Mixed 11% 714 120  
Mountain big sagebrush Replacement 100% 48 15 100
Mountain big sagebrush with conifers Replacement 100% 49 15 100
Mountain sagebrush (cool sage) Replacement 75% 100    
Mixed 25% 300    
Mountain shrubland with trees Replacement 22% 105 100 200
Mixed 78% 29 25 100
Black and low sagebrushes Replacement 33% 243 100  
Mixed 67% 119 75 140
Black and low sagebrushes with trees Replacement 37% 227 150 290
Mixed 63% 136 50 190
Curlleaf mountain-mahogany Replacement 31% 250 100 500
Mixed 37% 212 50  
Surface or low 31% 250 50  
Great Basin Woodland
Juniper and pinyon-juniper steppe woodland Replacement 20% 333 100 >1,000
Mixed 31% 217 100 >1,000
Surface or low 49% 135 100  
Ponderosa pine Replacement 5% 200    
Mixed 17% 60    
Surface or low 78% 13    
Great Basin Forested
Interior ponderosa pine Replacement 5% 161   800
Mixed 10% 80 50 80
Surface or low 86% 9 8 10
Ponderosa pine-Douglas-fir Replacement 10% 250   >1,000
Mixed 51% 50 50 130
Surface or low 39% 65 15  
Great Basin Douglas-fir (dry) Replacement 12% 90   600
Mixed 14% 76 45  
Surface or low 75% 14 10 50
Aspen with conifer (low to midelevation) Replacement 53% 61 20  
Mixed 24% 137 10  
Surface or low 23% 143 10  
Douglas-fir (warm mesic interior) Replacement 28% 170 80 400
Mixed 72% 65 50 250
Aspen with conifer (high elevation) Replacement 47% 76 40  
Mixed 18% 196 10  
Surface or low 35% 100 10  
Stable aspen without conifers Replacement 81% 150 50 300
Surface or low 19% 650 600 >1,000
Northern Rockies
Vegetation Community (Potential Natural Vegetation Group) Fire severity* Fire regime characteristics
Percent of fires Mean interval
(years)
Minimum interval
(years)
Maximum interval
(years)
Northern Rockies Grassland
Northern prairie grassland Replacement 55% 22 2 40
Mixed 45% 27 10 50
Mountain grassland Replacement 60% 20 10  
Mixed 40% 30    
Northern Rockies Shrubland
Riparian (Wyoming)
Mixed 100% 100 25 500
Wyoming big sagebrush Replacement 63% 145 80 240
Mixed 37% 250    
Basin big sagebrush Replacement 60% 100 10 150
Mixed 40% 150    
Low sagebrush shrubland Replacement 100% 125 60 150
Mountain shrub, nonsagebrush Replacement 80% 100 20 150
Mixed 20% 400    
Mountain big sagebrush steppe and shrubland Replacement 100% 70 30 200
Northern Rockies Forested
Ponderosa pine (Northern Great Plains) Replacement 5% 300    
Mixed 20% 75    
Surface or low 75% 20 10 40
Ponderosa pine (Northern and Central Rockies) Replacement 4% 300 100 >1,000
Mixed 19% 60 50 200
Surface or low 77% 15 3 30
Ponderosa pine (Black Hills, low elevation) Replacement 7% 300 200 400
Mixed 21% 100 50 400
Surface or low 71% 30 5 50
Ponderosa pine-Douglas-fir Replacement 10% 250   >1,000
Mixed 51% 50 50 130
Surface or low 39% 65 15  
Western redcedar Replacement 87% 385 75 >1,000
Mixed 13% >1,000 25  
Douglas-fir (xeric interior) Replacement 12% 165 100 300
Mixed 19% 100 30 100
Surface or low 69% 28 15 40
Douglas-fir (warm mesic interior) Replacement 28% 170 80 400
Mixed 72% 65 50 250
Douglas-fir (cold) Replacement 31% 145 75 250
Mixed 69% 65 35 150
*Fire Severities: Replacement=Any fire that causes greater than 75% top removal of a vegetation-fuel type, resulting in general replacement of existing vegetation; may or may not cause a lethal effect on the plants.
Surface or low=Any fire that causes less than 25% upper layer replacement and/or removal in a vegetation-fuel class but burns 5% or more of the area.
Mixed=Any fire burning more than 5% of an area that does not qualify as a replacement, surface, or low-severity fire; includes mosaic and other fires that are intermediate in effects [27,42].

POSTFIRE REGENERATION STRATEGY [75]:
Tussock graminoid
Ground residual colonizer (on site, initial community)
Initial off-site colonizer (off site, initial community)
Secondary colonizer (on-site or off-site seed sources)


FIRE EFFECTS

SPECIES: Poa bulbosa
IMMEDIATE FIRE EFFECT ON PLANT:
Although immediate fire effects are not described in the literature, bulbous bluegrass is likely top-killed or killed by fire.

DISCUSSION AND QUALIFICATION OF FIRE EFFECT:
No additional information is available on this topic.

PLANT RESPONSE TO FIRE:
Based on studies and literature from the Mediterranean Region, bulbous bluegrass may sprout vegetatively after fire [57], but regeneration methods in nonnative US habitats have not been described (2007). Postfire sprouting may depend on fire season since roots shrivel and disappear during the summer dormant period. Survival of basal bulbs and bulbils on the soil surface is likely dependent on fire severity. Buried bulbils may survive fire, and consumption of litter and humus layers may uncover bulbils enough for successful germination. (See discussion on Depth of burial). Dispersal of bulbils from adjacent unburned sites or unburned refugia into burned areas is probable.

DISCUSSION AND QUALIFICATION OF PLANT RESPONSE:
There are very few studies that compare bulbous bluegrass abundance on unburned and burned sites. Of the few fire studies available, most fail to provide prefire or unburned comparison data and/or fire season or severity information. While bulbous bluegrass is often present on burned sites, without more information on postfire recovery, inferences regarding the effects of fire season and severity on bulbous bluegrass abundance are difficult to make. Discussion of bulbous bluegrass's response to fire is also made difficult because of its often low frequency and/or cover and patchy distribution relative to disturbed sites. Without prefire data and land use history on burned and unburned sites, the validity of comparisons is unknown.

Bulbous bluegrass is often present on recently burned sites [44,52,62]. Three to four years after a wildfire in a forage kochia (Bassia prostrata)-dominated community near Ephraim Canyon in central Utah, researchers indicated that bulbous bluegrass was "not materially affected by fire". The assessment was based on transect data from burned and adjacent unburned sites, but abundance of bulbous bluegrass was not reported [52]. In the fourth postfire growing season after a severe sagebrush-mixed grass fire on West Mountain in Utah, bulbous bluegrass cover averaged 4.5% on the burned site but was absent from the unburned site [62]. Bulbous bluegrass was present only on early-spring grazed and burned sites when burned and unburned, summer and spring grazed purple needlegrass (Nassella pulchra) grasslands at the Jepson Prairie Preserve in Solano, California, were compared. "High intensity, short duration" grazing by domestic sheep occurred in March to early April or in August from 1988 to 1990. The fire burned in September when temperatures were 75 to 82 F (24-28 C), relative humidity was 47% to 61%, and wind speeds were 10 to 25 km/hour with gusts of up to 40 km/h [44].

While some report increased bulbous bluegrass abundance with time since fire [68], others report that time since fire may not affect abundance [11,23,46]. It is likely that postfire growing conditions and/or fire severity would affect recovery over time. Bulbous bluegrass cover averaged 6.7% on burned sagebrush-grasslands southwest of Pocatello, Idaho, and cover was significantly (P=0.001) greater in the 2nd postfire growing season than in the 1st. The fire burned in August in an area free of domestic grazing for more than 35 years. No unburned or prefire data were reported, and neither burning conditions nor fire behavior was described [68]. While bulbous bluegrass cover greatly increased after a fire in bluebunch wheatgrass/Sandberg bluegrass (Poa secunda) grasslands near Lewiston, Idaho, cover in the 2nd and 3rd postfire years was relatively unchanged. The fire was low severity and burned in August. Average bulbous bluegrass cover before the fire was 0.09%. In the 2nd and 3rd postfire years, bulbous bluegrass cover was 1.11% and 1.04%, respectively [23]. Bulbous bluegrass occurred on 1 of 6 plots before a prescribed fire and 1 of 6 plots 1 and 2 years after a prescribed fire in big sagebrush-bluebunch wheatgrass vegetation on the Columbia National Wildlife Refuge in Grant County, Washington. Sites burned previously in 1993 before the early October 2002 prescribed fire. The 2002 fire was a flanking fire although some back and head fire behavior occurred [46]. When 2- to 3-year-old and 5- to 6-year-old burned Wyoming big sagebrush (Artemisia tridentata ssp. wyomingensis)-dominated vegetation in southern Idaho were compared, bulbous bluegrass cover averaged 0.16/m on both sites. Both sites burned in August. Prefire or unburned bulbous bluegrass cover was not reported [11].

The following Research Project Summaries provide information on prescribed fire and postfire response of many plant species including bulbous bluegrass:

FIRE MANAGEMENT CONSIDERATIONS:
There is little information regarding the effects of fire on bulbous bluegrass. Additional studies of fire in bulbous bluegrass habitats are needed before recommendations regarding fire effects on this species are warranted.

MANAGEMENT CONSIDERATIONS

SPECIES: Poa bulbosa
IMPORTANCE TO LIVESTOCK AND WILDLIFE:
Elk, deer, domestic sheep, game birds, deer mice, and ground squirrels consume bulbous bluegrass. Due to the rapid development and senescence of bulbous bluegrass, it is only available for part of the growing season. Bulbous bluegrass bulbils are said to contain "appreciable amounts" of starch and some fat, making them valuable when available [47].

Large mammals: Bulbous bluegrass stems and bulbils are consumed by large mammals in the winter and spring. Deer consumed bulbil-filled panicles in early spring and green growth in the late winter in Utah rangelands [65]. Bulbous bluegrass made up 20% and 31%, respectively, of the relative densities of elk and deer feces collected from January through March on northern Utah winter range [55]. Bulbous bluegrass bulbils were "avidly" consumed by domestic sheep in crested wheatgrass pastures near Ephraim, Utah [3].

Birds: Bulbous bluegrass was important in the fall and winter diets of chukars and gray partridge in the Hells Canyon area of Idaho and Oregon. Crop analyses revealed that bulbous bluegrass made up 23.3% of the volume of gray partridge diets in 1999 and 5.8% in 2000. Chukar diets were 3.5% bulbous bluegrass by volume in 1999 and 4% in 2000 [7].

Small mammals: Deer mice and ground squirrels utilize bulbous bluegrass as a food source. Bulbous bluegrass bulbs were common in deer mouse caches in the Benmore Experimental Area of Tooele County, Utah [18], but bulbous bluegrass was not a preferred food of native deer mice fed bulbils in captivity. Bulbous bluegrass preference ranked from 12th to 15th in the list of 18 presented seeds [16].

Both Idaho and Columbian ground squirrels fed on bulbous bluegrass leaves and flowers (likely bulbils) in areas where these species overlap northwest of Council, Idaho. Feces were collected from 2 meadows. In the Huckleberry site, bulbous bluegrass made up 1.9% and 0.9% of Idaho and Columbian ground squirrel diets, respectively. In the Squirrel Valley site, diets of Idaho and Columbian ground squirrels were 21.4% and 19.6% bulbous bluegrass, respectively. Bulbous bluegrass was more abundant in Squirrel Valley than in Huckleberry. Researchers reported that bulbous bluegrass leaves were also consumed [14].

Palatability/nutritional value: Most report that bulbous bluegrass is unavailable to grazers for a large part of the growing season; however, it is considered palatable when green in the early spring and often in the fall and winter [10,50,81,82]. Bulbils are considered "highly nutritious" [24].

OTHER USES:
Potential for use in revegetation: Bulbous bluegrass has many characteristics that make it attractive for use in revegetation efforts, such as ease of establishment and tolerance of stressful growing conditions [24,61,65,82]. However, often these same characteristics can cause a species to become invasive. Generally, bulbous bluegrass is not considered highly invasive. However, Hull [33] reports that bulbous bluegrass is "highly competitive" and that it can be difficult to get other species established in a "good" stand of bulbous bluegrass. Bulbous bluegrass is also a nonnative species, and the use of nonnative species in revegetation and restoration projects is often cautioned against (e.g., [5]).

The early seasonal development of bulbous bluegrass has lead to its use in "depleted rangelands" in the hopes that it may compete successfully and eventually replace less desirable, early developing weeds and annual grasses [24,50]. Bulbous bluegrass "competes successfully with cheatgrass" [81]. Seeding of bulbous bluegrass in closed and open cheatgrass communities showed that bulbous bluegrass can successfully invade but that invasion is slow [70]. In 27 of 28 unmanipulated cheatgrass-dominated plots in southern Idaho, broadcast seeding of bulbous bluegrass began to reduce cheatgrass abundance. Researchers suggested that heavy bulbous bluegrass bulbils were able to work through the litter to mineral soil and grow rapidly [74]. Establishment of bulbous bluegrass in cheatgrass-dominated sites may also serve to decrease the available late spring, summer, and early fall fuels since bulbous bluegrass biomass disappears during drought conditions [24].

Indicator species: Presence of bulbous bluegrass in grand-fir, Douglas-fir, and/or western redcedar forests in Gifford Pinchot National Forest indicates disturbance [77]. It is likely that these are not the only communities in which the presence of bulbous bluegrass indicates past disturbance.

IMPACTS AND CONTROL:
Impacts: There was little literature devoted to the impacts or control of bulbous bluegrass in native plant communities, suggesting that to date (2007) bulbous bluegrass impacts are small and/or unnoticed. In a ranking of nonnative plants' "invasiveness" in Eastern Region National Forests, bulbous bluegrass was categorized as a "common" nonnative species that is restricted to disturbed sites. It was considered "not especially invasive in undisturbed natural habitats" [78].

Control: There are few recommendations for bulbous bluegrass control in the available literature. Prevention of dispersal by cleaning equipment when moving between infested and uninfested sites, planting seed free of bulbous bluegrass, and maintaining perennial and native species were suggested as control methods by Locke and Burrill [47]. On Curlew National Grasslands, fall burning, deep plowing, disking, and drill seeding were used to control bulbous bluegrass, but when treatment areas were visited 9 years later, the researcher suggested that control treatments were "questionable at best" [85].


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