Photo courtesy of Robert Soreng, Smithsonian Institution, Department of Systematic Biology-Botany.
AUTHORSHIP AND CITATION:
Innes, Robin J. 2012. Pleuraphis mutica. 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/ .
The scientific name of tobosa is Pleuraphis mutica Buckley (Poaceae) [71,79,81,95].
Hilaria mutica (Buckl.) Benth. [1,6,57,71,78,79,82,95]
|Distribution of tobosa. Green indicates that tobosa is present. Yellow indicates that it is present but rare.
Map courtesy of Kartesz, J. T.; The Biota of North America Program (BONAP). 2011. North American plant atlas, (Online). Chapel Hill, NC: The Biota of North America Program (Producer). Available: http://www.bonap.org/MapSwitchboard.html. (Maps generated from Kartesz, J. T. 2010. Floristic synthesis of North America, Version 1.0. Biota of North America Program (BONAP)). (In press).
Tobosa is native to the southwestern and south-central United States and northern Mexico. In the Southwest, it occurs in the Chihuahuan and Sonoran deserts and surrounding areas. In Texas, it occurs most commonly in the Trans-Pecos, High Plains, Rolling Plains, Edwards Plateau, and South Texas Plains ecoregions. It is rare in Oklahoma [6,80]. One publication stated that tobosa occurs in southeastern California  but this appears to be erroneous, as tobosa is not included in California floras.
United States: AZ, NM, OK, TX 
SITE CHARACTERISTICS AND PLANT COMMUNITIES:
Site characteristics: Throughout its range, tobosa is found on both lowland and upland sites. Most characteristically though, it is found on lowland sites, particularly in soils that have developed from basin fill material. These are "heavy" clay soils which are tight and relatively impervious . Tobosa attains its best development in depressions where surface run-off accumulates; in these locations it may occur in almost pure stands forming a dense, coarse sod. On dryer sites it grows in scattered stands of large, individual tufts [23,120,163].
Topography: Tobosa occurs on nearly level to steep sites (0-45%) on all aspects [45,47,128,135]. Tobosa communities occur in swales, on plains, piedmonts, and mesas, in basins, around playas, and on gentle foothill slopes [6,60,71,82,128].
Elevation: Across its distribution in the United States, tobosa occurs from 2,000 to 7,000 feet (600-2,100 m) [7,45,47,51,82,95,128,135,143,155].
Soils: Tobosa prefers moist, fine-textured soils .
Texture: Tobosa prefers fine-textured soils, including clays and loams [22,34,47,63,78,128,143,168]. It occasionally grows in soils with shallow sand on their surfaces [22,47,154], but it is intolerant of shifting sand . Soils in some tobosa communities, particularly those on steep slopes, may have abundant gravel, stones, or boulders [63,128]. For example, high-elevation stands in the tobosa shrub herbaceous alliance in the Sonoran Desert occur on steep (up to 45%), rocky slopes of mountains, hills, and mesas .
Tobosa characteristically occupies low, swale sites that are often flooded by run-off water from surrounding foothills. These sites typically have clay soils with slow infiltration rates, which results in moisture being maintained high in the rooting zone (i.e., the upper 20 inches (50 cm) of soil) [37,63,82,117,154,156]. On sites less subject to flooding, which typically have a coarser surface texture, tobosa may be the most important grass, but a combination of several grass species such as burrograss (Scleropogon brevifolius), alkali sacaton (Sporobolus airoides), and muhly (Muhlenbergia spp.), may make up a greater percentage of the composition . In the Animas Creek Valley, New Mexico, tobosa occurred only in soils with a high clay content in the rooting zone; thus, tobosa was considered a reliable indicator of clay soils . In contrast, Bestelmeyer and others  concluded that geomorphic processes rather than soil texture determined tobosa abundance and distribution, noting tobosa's abundance along an ecotone at the Corralitos Ranch in Doña Ana County, New Mexico. On one side of the ecotone—the "grass side"—tobosa was the dominant grass and its cover averaged 12%. On the "sparse side" of the ecotone, tobosa was also the dominant grass but its cover was only 2%. Soils on both sides of the ecotone were sandy loams, but the sparse side occupied a slightly lower landscape position (due to wind erosion from the sparse side to the grass side) on soils with higher surface carbonate and a less developed argillic horizon (due to bioturbation by kangaroo rats) compared with the grass side .
Parent materials and pH: Soils in tobosa communities may be derived from a variety of parent materials, including gravelly or silty alluvium [45,63,82,128,143], andesite , and basalt . Many researchers reported tobosa occurring in soils containing large amounts of calcium carbonate or gypsum [22,22,47,63,93,128,174].
Tobosa typically occurs on mildly alkaline to strongly alkaline soils (pH range: 7.4-8.8) [37,38,78,128,137,165]. In New Mexico and Arizona, pH values in a tobosa community were lower than those in tarbush (Flourensia cernua), creosotebush (Larrea tridentata), or burrograss communities .
Moisture: Tobosa occurs on dry to moist sites and on soils that are poorly drained to well drained [22,45,154]. Some authors suggested that tobosa prefers intermittently flooded areas [24,48,163]. However, tobosa may be killed when submersed in water for periods of "longer than a few days"  to "several months" [120,163]. At the Jornada Long Term Ecological Research Site in south-central New Mexico, Wondzell and others  concluded that tobosa was "intolerant of flooding" because of its absence from the lowest portions of playas. Instead, tobosa was most abundant in the soils surrounding the playas that absorbed water from sheet flow when the playas flooded . Many authors stated that tobosa reaches its best development in swales and depressions that receive run-off water, where water either slowly exits to an arroyo or enters the soil before standing very long (e.g., [25,39,48,128,159]).
|Tobosa grasslands with scattered mesquite (Prosopis spp.) on the northwest side of Flat Lake, New Mexico, looking east at the San Andres Mountains. Photo taken in February. Photo courtesy of Patrick Alexander.|
Tobosa abundance is positively related to rainfall [107,159,188]. On the Edwards Plateau region of Texas, tobosa's density was positively correlated with rainfall during the previous year (r=0.65) and during the previous 2 years (r=0.61) .
Tobosa grows quickly with increased moisture during the growing season . At the Mapimi Biosphere Reserve in Durango, Mexico, tobosa produced flowers and fruits after 3 inches (75 mm) of water was experimentally applied to vegetation arcs in mid-October during a relatively dry year. Thirty-two days after the simulated rainfall, green biomass production of watered tobosa plants was 4.73 g/m2/day, whereas unwatered individuals had produced no green biomass . Tobosa extracts a large portion of its moisture from shallow soil horizons through its dense network of shallow roots (see Tillers, rhizomes, and roots).
Although tobosa grows best in moist soils, it is drought tolerant [67,101,114,159]. Its growth may be reduced by drought, but it typically returns to predrought levels soon after the drought ends [46,52,67,107,188]. At the Jornada Experimental Range in south-central New Mexico, Moore  reported that tobosa was not readily killed by a "severe" 3-year drought during the 1910s, but its growth, which varied closely with rainfall, was reduced. In the same area during an "extreme" 5-year drought during the 1950s (when precipitation averaged 55% less than the predrought average) tobosa on lowland sites had almost no mortality. The authors concluded that tobosa was "little affected" by the drought . A subsequent study of the effects of the 1950s drought on tobosa in lowland sites indicated that tobosa cover decreased during the drought, but returned to predrought values within 10 years of the end of the drought .
Tobosa is often cited as being more drought tolerant than many cooccurring grasses [67,159]. Herbel  remarked that tobosa seemed to have a "much higher" drought resistance than black grama (Bouteloua eriopoda) at the Jornada Experimental Range . Tobosa was considered the most drought resistant of several grasses in shortgrass prairie on the Edwards Plateau because its density was the most stable during 16 years, despite less than average rainfall during 9 of those years . In contrast, a flora suggested that during extended drought periods, tobosa suffers more than its associates . In a greenhouse study of the water use and productivity of 4 desert grasses (tobosa, black grama, bush muhly (Muhlenbergia porteri), and mesa dropseed (Sporobolus flexuosus)), tobosa required the most water to produce 1 gram of shoot material. The water required for root production was intermediate among the grasses . For more information on this study, see Seedling establishment and plant growth.
Depth: Soils with tobosa may be <1 foot to >5 feet (0.3-1.5 m) deep (e.g., [12,22,45,47,54,128,135,143,173]). Impermeable caliche and argillic horizons are common in areas with tobosa (e.g., [47,98]). These layers restrict deep percolation of soil-water and may favor the shallower rooted grasses like tobosa (see Tillers, rhizomes, and roots) over more deeply rooted shrubs like creosotebush or mesquite (Prosopis spp.) . Where tobosa occurs in Trans-Pecos shrub savanna, soils were usually <2 feet (0.6 m) deep above caliche .
Salinity: Tobosa tolerates mildly saline soils [22,128].
Climate: Tobosa grows in arid and semiarid regions of the Southwest . Within Texas, tobosa occurs in the semiarid climate of western Texas, the humid, subtropical climate of central Texas, and areas transitional to these climates [13,115,116]. Tobosa occurs in regions of the United States and Mexico with average annual rainfall ranging from <9 inches to >30 inches (230-762 mm) [47,84,92,124,125]. Precipitation may vary widely from year to year and drought is common [51,125]. Precipitation in regions with tobosa may be distributed either primarily in winter ; biseasonally with about 50% of it falling during the southwestern summer monsoon [92,125,128]; or primarily in summer, with about 75% of the total yearly precipitation falling during the monsoon [92,124,143]. In the Chihuahuan Desert, >50% of precipitation occurs in the summer months [47,92], whereas in the eastern Sonoran Desert, precipitation falls more biseasonally . Annual evaporation may be 2 to 10 times precipitation . Freezing temperatures may occur during winter. The average number of frost-free days ranges from <180 days to >300 days [47,84,157]. Strong winds from February to May may rapidly dry out the soil during a critical period for initiation of plant growth [84,143].
Plant communities: Tobosa occurs in southwestern shrub steppe, desert scrub, desert grassland, plains-mesa grassland, and Texas savanna ecosystems and their integrations. Tobosa grasslands and grama (Bouteloua spp.)-tobosa shrub steppe communities occur within these ecosystems [39,47,51,63]. See the Fire Regime Table for a list of plant communities in which tobosa may occur and information on the fire regimes associated with those communities.
Tobosa grasslands may occur on fine-textured, alluvial clays in enclosed basins with internal drainage ("tobosa swales"), on flats where salt accumulation is minimal ("tobosa flats"), or on upland mesas and hills where tobosa is typically mixed with other grasses [39,63,158]. Reid and others  described tobosa swales as shortgrass grasslands . A distinctive feature of swale vegetation is the consistent dominance of tobosa . Occasionally alkali sacaton or blue grama (Bouteloua gracilis) may be codominant with tobosa in swales [39,128,143]. Forbs are sparse. Scattered shrubs and succulents such as honey mesquite (Prosopis glandulosa), tarbush, and tulip pricklypear (Opuntia phaeacantha) may be present . Swale boundaries tend to be composed of deep sand that has accumulated as water flow slows and spreads as it enters the swale. These sandy borders support stands of soaptree yucca (Yucca elata). At these swale ecotones, tobosa may be codominant with soaptree yucca .
Tobosa flats are variously described as shortgrass or mixed-grass grasslands [128,158]. In these communities, tobosa may occur as nearly pure, monospecific stands or codominate with other perennial graminoids such as burrograss, alkali sacaton, buffalo grass, vine mesquite (Panicum obtusum), grama, or dropseed (Sporobolus spp.) [51,63,128,158]. For example, tobosa-blue grama, tobosa-alkali sacaton, and tobosa-burrograss communities occurred at the White Sands Missile Range, New Mexico . Perennial forbs are sparse in tobosa flats. Annual herbs are seasonally present and may be abundant in some stands . Shrubs (e.g., honey mesquite, creosotebush) and succulents (e.g., tree cholla (Opuntia imbricata), sacahuista (Nolina microcarpa)) may be scattered in tobosa flats (e.g., [13,47,78,109,128,158]).
Tobosa grassland communities often border and intergrade with desert shrublands. Tobosa is particularly common in desert shrublands with mesquite [13,33,34,89], creosotebush, and tarbush [47,51,54,66,78]. Tobosa grasslands in saline-influenced areas grade into alkali sacaton grasslands or saltbush (Atriplex spp.) shrublands [33,34,45,63,78,92,128,143,158]. For example, tobosa occurred in the cattle saltbush (Atriplex polycarpa) shrubland alliance in New Mexico 
Tobosa is a major grass species in grama-tobosa prairie [47,87] and grama-tobosa shrub steppe [47,88]. Grama-tobosa prairie is an open grassland of low growth and scattered succulents dominated by blue grama and tobosa . Succulents (e.g., sotol (Dasylirion spp.), beargrass (Nolina spp.), agave (Agave spp.), pricklypear (Opuntia spp.), and hedgehog cactus (Echinocereus spp.)) are well represented and characteristic of grama-tobosa prairie, particularly where soils are shallow and rocky.
Black grama and tobosa are diagnostic dominants in the grama-tobosa shrub steppe, although blue grama is often dominant at high elevations. Shrubs (e.g., mesquite, creosotebush, tarbush, snakeweed (Gutierrezia spp.), acacia (Acacia spp.), allthorn (Koeberlinia spinosa), saltbush (Atriplex spp.), jointfir (Ephedra spp.), and littleleaf sumac (Rhus microphylla)) may share or assume dominance in these communities [51,109,123]. Grama-tobosa shrub steppe communities at high elevations in Arizona and New Mexico often adjoin pinyon-juniper (Pinus spp.-Juniperus spp.) woodlands or mountain-mahogany-oak (Cercocarpus spp.-Quercus spp.) shrublands. In the Trans-Pecos region of Texas, high-elevation boundaries are with oak-juniper woodlands. Low-elevation boundaries of grama-tobosa shrub steppe are with creosotebush-bursage (Ambrosia spp.) communities in Arizona, creosotebush-tarbush communities in New Mexico, and Trans-Pecos shrub savanna in Texas .
GENERAL BOTANICAL CHARACTERISTICS:
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., [57,82,95,163]).
Form and architecture: Tobosa is a warm season, perennial midgrass [27,38,57,57,74,114,146,154,163]. It is a sod-forming grass with a tendency to form bunches (usually 6-12 inches (15-30 cm) in diameter) ) with bare spaces between . Some authors described tobosa as a bunchgrass [20,48,74] or a tussock grass [12,172]. Plants may be up to 3 feet (0.9 m) tall, but 1 to 2 feet (0.3-0.6 m) tall is more common [6,20,48,57,63,71,74,95,146,154,163]. The base of a tobosa plant is thick and hard [57,71,74,154,163].
Leaves and reproductive structures: Leaf blades are up to 6 inches (15 cm) long and are stiff, tough, and smooth [6,48,74,95,154,163]. They occur mainly as a mass of basal leaves, with only a few leaves located along the stem [25,26]. Tobosa can produce branches on culms from the previous year . The inflorescence consists of an erect spike 1.5 to 4 inches (4-10 cm) long [57,146,154,163].
Tillers, rhizomes, and roots: Ketchum  stated that tobosa produces tillers from reduced nodes at the bases of culms . Tobosa also produces tillers at the perimeter of the plant base from rhizomes . Tobosa rhizomes may be up to 3 inches (7.5 cm) long  and are stout, creeping, woody, and scaly [57,71,74,154,163]. A single plant may have up to 40 rhizomes . Because rhizomes have very short internodes, plants have a "tufted" growth habit . Ketchum  stated that because buds producing tillers are compressed at the bases of culms, they are protected from grazing, whereas buds producing branches on culms are exposed to grazing . See Ketchum  for detailed drawings of tobosa tillers, rhizomes, and roots.
Tobosa roots are dense, coarse, and fibrous [30,120,163]. Most roots are shallow (<2 feet (0.6 m)), but some may extend to 6 feet (1.8 m) deep [30,54,120,163]. Lateral spread of most roots is <2 feet [30,54]. At the toe of a bajada slope at the Jornada Experimental Range, tobosa roots were 16 inches (40 cm) deep and extended horizontally only 16 inches in fine loams where the calcic horizon (the extremely compact B horizon) was about 30 inches (80 cm) deep. The authors suggested that the calcic horizon may have contributed to the relatively shallow penetration of tobosa roots at this site and further suggested that tobosa at this site may not be able to withstand drought as well as more deeply rooted species. At another site in a tarbush community where run-off water accumulated, tobosa roots had well-developed 1st-order branches that penetrated into the calcic horizon to 43 inches (110 cm) deep and spread laterally 24 inches (60 cm). In a tobosa-burrograss ephemeral lake basin, most tobosa roots extended to 24 inches deep, but sparse roots extended to 55 inches (140 cm) deep . At the Mapimi Biosphere Reserve, 70% of tobosa roots on a bajada slope in Chihuahuan Desert scrub occurred between 2 and 12 inches (5-30 cm) deep where clay layers, found below 10 inches (25 cm), did not allow deep percolation of soil water. Characteristics of tobosa roots in clay loam soils were as follows :
|Table 1. Characteristics of tobosa plants and roots in clay-loam soils on a bajada slope at the Mapimi Biosphere Reserve, Durango, Mexico |
|Plant density||0.19 plants/m²|
|Maximum root depth||0.19 m|
|Mean root length||0.39 m|
|Total root length||14.31 m|
|Influence area||0.51 m²|
|Influence volume||0.09 m³|
Tobosa root branches are sparse (only 2 to 3 roots/inch) and short (only 1.0 to 1.5 inches (2.5-3.8 cm) long) . Ketchum  described tobosa root branching as "meager" and primarily in the upper 1 foot (0.3 m) of soil. Campbell  stated that tobosa roots are inefficient for absorbing moisture from soil, whereas Cottle  stated that the high degree of 3rd-order branching indicated that tobosa's absorbing system was efficient.
Stand and age class structure: Tobosa stand structure varies with site characteristics. On bottomland sites, tobosa often grows in pure stands, forming a sod [58,125]. In the Chihuahuan Desert, it may occur in monospecific stands that are "hundreds of hectares" in size . Because tobosa plants primarily reproduce by rhizomes, Henrickson  suggested that a 0.2-mile² (0.5 km²) stand may represent a single clone. On drier sites, tobosa typically grows in scattered stands of large, tufted individuals [57,120] with abundant rock and bare ground cover (e.g., [55,78,128]). In shrublands, tobosa tends to occur in intershrub spaces rather than under shrubs [4,126]. Thus, as shrub abundance increases, tobosa abundance typically decreases (see Successional Status).
Tobosa may be highly productive. Biomass of tobosa in communities ranged from <2 pounds/acre (2 kg/ha) in a year in moderately grazed Pinchot juniper (Juniperus pinchottii) communities on the Texas Rolling Plains  to >3,700 pounds/acre (4,200 kg/ha) in a year in lightly grazed tobosa grasslands in the Trans-Pecos . Without periodic removal of dead plant material, such as via burning or grazing, tobosa litter may accumulate and decrease biomass production . For more information on tobosa production after fire, see Plant response to fire.
The maximum life span of a tobosa plant at the Jornada Experimental Range was 7 years; mean life span was 2 years .
Tobosa's growing season depends upon temperature and the timing of precipitation [24,26,154,186]. Tobosa plants become dormant when the surface soil layer dries out [31,67] and are dormant in winter [38,113,138]. Growth may occur any time during the frost-free season as long as sufficient moisture is available [23,24]. Generally about 1 inch (25.4 mm) of rain, concentrated within 1 week, is needed to initiate growth of tobosa and other desert grasses . In the Chihuahuan Desert in the Trans-Pecos, where most precipitation occurs in summer months, the growing season of tobosa began in mid-July and ended in early October . In the Chihuahuan Desert in Durango, Mexico, the growing season started in June and ended in September . At the Jornada Experimental Range, where precipitation occurs during winter storms and summer monsoons, green-up typically occurs biseasonally in early spring and again in mid- to late summer. During the wettest of 6 years, when winter-spring precipitation was 2.9 inches (74 mm), tobosa was green and growing in March and April. When winter-spring precipitation was only 1.3 inches (32 mm), "a little green-up" of tobosa occurred in mid-March. Following the driest winter and spring, when winter-spring precipitation was only 0.7 inches (19 mm), tobosa green-up occurred in March, apparently because the previous fall precipitation was high and temperatures were warm in February . During a year when 10.0 inches (255 mm) of rainfall was received during 17 rainfall events between 11 July and 1 December, tobosa plants were actively growing in August. During a year when 11.2 inches (285 mm) of rainfall was received during 24 rainfall events from 6 June to 7 December, plants were actively growing in July. Most tobosa tillers senesced in early August because of lack of rainfall, but 1.9 inches (48 mm) of rainfall on 23 August initiated new growth and tobosa remained actively growing into October . Another study at the Jornada Experimental Range reported that tobosa plants had new growth in July after heavy rainfall in June and that plants did not respond to precipitation after September .
According to Arizona, New Mexico, and Texas floras, tobosa flowers and sets seed from April to October [57,82,95]. In Durango, Mexico, tobosa flowers and fruits towards the last 2 months of the rainy season, which starts in June and ends in September .
Tobosa reproduces by seeds and rhizomes. Seed production and viability are typically low, and seedling survival may also be low [57,114,154]. Generally, vegetative sprouting from rhizomes is the major mode of reproduction in established plants [23,25,57].
Pollination and breeding system: Tobosa plants are bisexual. They can produce seeds both by outbreeding and by inbreeding (Watson and Dallwitz 1992 cited in ).
Seed production: Floras reported "low" or "poor" seed production in tobosa plants [57,154] but did not provide more information. A study of a wild population in a tobosa-honey mesquite community on the Rolling Plains reported seed stalk production ranged from 0 to 92 seed stalks/m² 4 months after spring prescribed fires . For more information on this study, see Fire intensity. Mean seed yields of tobosa ranged from 7.3 to 11.1 grams/4 foot² in plants that were transplanted to field plots and irrigated .
Tobosa seed production may depend upon tobosa density. At the Mapimi Biological Reserve, where tobosa occurs in vegetation arcs, tobosa plants at low density in the "colonization zone" of vegetation arcs produced more seeds (35.2 seeds/spike) than plants in dense clumps in the "central zone" of vegetation arcs (28.5 seeds/spike) . For more information on vegetation arcs, see Vegetative regeneration.
Few tobosa seeds are viable. Average viability of tobosa seeds collected from 8 populations in central and western Texas from May to August was 4.7%. Eighty-five percent of the sterility of tobosa seeds was the result of abortion before pollination . Heavy parasitism of the inflorescence by fungi may also contribute to low viability in tobosa seeds . Ketchum  reported "very few" viable seeds after "many hours" of inspecting tobosa spikelets on the Edwards Plateau. Amount and timing of rainfall may also affect seed viability (see Germination).
Seed dispersal: Tobosa seeds drop from parent plants at maturity [48,74] and are dispersed by wind and water . Ants  and likely other animals disperse tobosa seeds.
Seed banking: As of 2012, little information regarding tobosa seed banking was available in published literature. A flora stated that tobosa seeds are "short-lived" but did not provide additional details . Given its lack of viable seed production, it is unlikely that tobosa produces a persistent seed bank.
Germination: Limited evidence suggests that some tobosa seeds have a dormant embryo. In a laboratory, 1 of 6 tobosa seeds collected on the Edwards Plateau germinated in July. Two of the 5 remaining seeds germinated in a subsequent emergence test in February . Mechanical scarification with sandpaper increased tobosa seed germination at the Mapimi Biosphere Reserve. Seeds were collected in September and dry-stored for 8 to 9 months at 39 °F (4 °C). In a growth chamber, scarified seeds had 88% germination, whereas unscarified seeds had 60% germination (P<0.05) .
Percent germination of tobosa seeds varies. Germination of seeds collected at the Jornada Experimental Range and germinated in a laboratory ranged from 0% to 87% . A flora reported seed germination as 55% but did provide germination conditions .
Germination is best under moist, warm conditions, and light does not appear to affect germination [86,147]. Tobosa germination in a laboratory germinator was <10% under low moisture conditions and >95% under high moisture conditions . In a light chamber, viable seeds from Las Cruces, New Mexico, had 73% emergence at 102 °F (39 °C) and 24% emergence at 127 °F (53 °C). The difference was not statistically significant . In a growth chamber, scarified seeds had similar germination (70%-80%) under 3 light treatments: white light (simulating open habitat), far-red light (simulating canopy cover), and complete darkness (simulating buried seeds) .
The amount of rainfall during the year, especially during the growing season and at the time of harvesting, may affect tobosa seed germination rates. Tobosa plants in an area of New Mexico that received 18.3 inches (465 mm) of rain annually, 41% of which fell during July, August, and September, was substantially greater (91% germination) than seeds collected from an area of New Mexico which received 14.7 inches (373 mm) of rain annually, 33% of which fell during July, August, and September (34% germination) .
Seedling establishment and plant growth: Tobosa seedling survival is generally low , and few seedlings are found in wild populations [23,38]. Low seedling survival has been attributed to fluctuating climatic conditions during late spring and early summer . A 2-year study in northwestern Texas  and a 6-year study on the Edwards Plateau  reported no tobosa seedlings. At the Mapimi Biological Reserve, no seedlings established during a 2-year study, possibly due to drought during those years . For information on the effects of drought on established plants, see Moisture.
Tobosa seedling survival and growth are best under warm, moist conditions. After 156 hours in the laboratory, mean radicle growth of tobosa seedlings was 1 inch (2.6 cm) under high moisture conditions, but only <0.04 inch (0.1 cm) under low moisture conditions . Germination may be reduced by high temperatures. When viable seeds were placed 0.7-inch (1.3 cm) deep in soil in a laboratory, mean survival, shoot height, and root length of emerging seedlings 21 days later were significantly higher at a soil temperature of 102 °F (39 °C) than at a soil temperature of 127 °F (53 °C) :
|Table 2. Tobosa seedling survival and growth after 21 days in a laboratory at 2 soil temperatures |
|Shoot height (cm)||11.7||3.4|
|Root length (cm)||6.3||9.9|
Growth of tobosa seedlings may be rapid. In a germination chamber, the primary leaf of tobosa plants was 0.6 inches (15 mm) long at 15 days. The 2nd leaf appeared at 18 days and the 3rd leaf at 51 days. At 100 days, the seedling was 4.7 inches (12 cm) tall and consisted of 2 large branches and 3 tillers. Tillering was extensive as the seedling got older, but no rhizomes were produced by the 259th day. Root growth of seedlings may also be rapid. Forty hours after being put in a germination chamber, the length of the primary root of a tobosa plant was 0.4 inches (10 mm) and 65 hours later, the primary root was 0.7 inches (17 mm) .
Established tobosa plants green-up rapidly when water is available and become dry and dormant during drought . After a 5-inch (127 mm) May rain in western Texas, tobosa grew "several inches" in a few days . Experimental irrigation of soils with tobosa at the Mapimi Biosphere Reserve, indicated that tobosa growth is most strongly influenced by water present in the upper 100 inches (40 cm) of soil . Tobosa attains best growth in depressions where surface run-off accumulates [82,163]. For more information, see Moisture.
Vegetative regeneration: Tobosa reproduces vegetatively through a system of large, well-developed rhizomes connected to a coarse root system . From these rhizomes, tobosa produces tillers at the perimeter of the plant base. Thus, plants tend to expand at their periphery. With advancing maturity the centers of plants die, leaving decadent openings . One publication stated that tobosa propagates via stolons , but this appears to be erroneous.
At the Mapimi Biological Reserve, tobosa was dominant in vegetation arcs, which are bands of vegetation separated by bare areas. The bands are oriented perpendicular to the main slope and have upslope and downslope boundaries. The upslope boundary is dominated by herbs and is considered the "colonization zone", while the middle and downslope boundaries have both herbs and shrubs and are considered the "mature" and "senescent" zones. Over time, the vegetation arc is displaced upward in the direction of colonization, as new vegetation growth occurs upward and plants in the downslope boundary die. Because most seed dispersal is downslope (with the flow of water) and seedling survival is low [97,172], most of the upward movement of tobosa in vegetation arcs is probably the result of vegetative sprouting.
According to a review, most tobosa growth begins from terminal nodes on solid perennial stems, with only a small amount of growth occurring from tillers . Thirty-four years after a drought in shortgrass prairie on the Edwards Plateau, tobosa had not yet spread into bare areas created by the drought-caused death of other plants, suggesting that clonal expansion of tobosa was either relatively slow even under favorable conditions and/or that tobosa was a "poor competitor" for resources with other grasses . Other researchers have also described tobosa vegetative spread as relatively slow [23,29]. According to Brown and Coe , its vegetative spread is limited to a "few centimeters" each year.
Tobosa tillering increases with water availability. In a greenhouse, tobosa plants produced short, leafy culms that became better distributed through tillering as moisture increased . Tillering and rhizome production are influenced by disturbances, such as mowing, that remove herbage. See Mowing for more information.
Frequent clipping stimulates vegetative reproduction . Canfield  postulated that the removal of old growth stimulated rhizome growth by activating buds that otherwise may have remained dormant, but this stimulus dissipated if plants were clipped too closely. He found that clipping plants weekly to a 4-inch (10 cm) stubble height produced 110% more total herbage than clipping plants to 2 inches (5 cm) .
Tobosa and black grama are 2 of the most common grasses in desert grasslands (which are often referred to in the literature as semidesert grasslands) [16,39]. In this ecosystem, tobosa is considered a "climax" species of lowland sites with clay soils that receive some surface run-off from upland sites, whereas black grama is considered characteristic of gravelly upland sites [23,39]. On lowland sites with clay soils, tobosa often forms dense, almost pure stands. Campbell  stated that succession to a tobosa swale starts with pioneers such as lichen (Lecidea spp.) and bluegreen algae (Nostoc commune), followed by establishment of alkali sacaton, burrograss, and various forbs such as hog potato (Pomaria jamesii), collegeflower (Hymenopappus flavescens var. canotomentosus), Mexican drymary (Drymaria holosteoides), and pigweed (Amaranthus spp.). Ear muhly (Muhlenbergia arenacea) dominates the community before tobosa becomes dominant.
Grazing and drought may shift dominance of plants in tobosa swales. In some areas, burrograss and ear muhly, which generally preceded tobosa as dominants in swales, increased during drought years when tobosa swales did not receive surface run-off. When sufficient moisture returned, tobosa typically recovered and regained dominance . Campbell  classified the burrograss vegetation type as a seral stage leading to a tobosa climax. However, Gibbens and Beck , noting that tobosa grasslands were relatively stable during a 66-year study at the Jornada Experimental Range despite drought and grazing, concluded that "although this may be the case on a few sites", persistence of burrograss on other sites indicates it is a very stable vegetation type and may occupy a climax position of its own .
In heavily grazed desert grasslands, many native grasses have been replaced by introduced annuals. Brown  noted that in these areas tobosa may be the only native grass still remaining, but the reasons for this were not stated. Other authors reported that tobosa had decreased historically due to livestock grazing and fire exclusion, and that vast areas of tobosa grasslands were subsequently dominated by creosotebush, tarbush, and honey mesquite [22,54,66]. In western Texas, southern New Mexico, southeastern Arizona, and northern Chihuahua, Herbel  considered the creosotebush-tarbush vegetation type to be a "persistent subclimax" of the grama-tobosa grassland shrub steppe vegetation type. He concluded that honey mesquite or tarbush typically invaded lowland areas dominated by tobosa, while creosotebush invaded drier sites . At the Jornada Experimental Range, tarbush died during drought years and its invasion into tobosa grasslands was "set back" .
In general, tobosa cover decreases as shrub cover increases. At the Jornada Experimental Range, in lowland areas on a desert grassland range, tobosa was slowly reduced as tarbush established in mixed-grass and burrograss-tobosa communities during 105 years. After tarbush established, mesquite established . At the Mapimi Biosphere Reserve, dense patches of tarbush generally corresponded to low or absent tobosa cover . In pinyon-juniper communities in Arizona, tobosa production doubled after removal of the overstory via cutting, girdling, and/or herbicide spraying, and livestock exclusion; production was 30 kg/ha before treatment and 62 kg/ha 3 years after treatment .
Tobosa often occurs in postfire successional communities. See Plant response to fire for more information on this topic.
Literature reviews of fire effects in tobosa communities used in this review included these sources: [15,114,162,167,181,183,185].
Immediate fire effect on plant: Tobosa is likely to survive most fires and sprout from extensive rhizomes and/or protected buds located at the bases of culms (see Tillers, rhizomes, and roots and Plant response to fire). Rhizomes are short, typically <3 inches (8 cm) long . As of this writing, no published literature reported the depth of tobosa rhizomes in the soil. However, even the most severe fires rarely damage plant tissues below 2 inches (10 cm) in the soil , so many tobosa rhizomes are probably insulated by soil from heat damage and are likely survive most fires. As of this writing (2012), no information was available in the published literature regarding the immediate effects of fire on tobosa seeds.
While some observations indicate that tobosa may be killed by fire , most studies indicate that tobosa may be top-killed by fire but is unlikely to be killed. Even high-intensity prescribed fires are unlikely to kill tobosa (see Fire intensity) (e.g., [132,133]). After a spring prescribed fire on the Rolling Plains, mean tobosa cover was reduced from 74% before the fire to 0% immediately after the fire. By the middle of the 1st growing season, however, tobosa cover averaged 16% . In Truth or Consequences, New Mexico, during a year of average precipitation, tobosa cover on black grama grasslands was significantly lower 1 month following a patchy July prescribed fire than prior to the fire. However, 1 year after the fire, tobosa cover was similar between burned and unburned plots, which were similar to preburn values . At the Jornada Experimental Range, Bill McCall (1963 personal communication cited in ) observed that fire killed tobosa plants in swales. Another researcher at the Jornada Experimental Range reported that a fire in the early 1940s on tobosa range killed tobosa plants (K. A. Valentine 1964 personal communication cited in ). In 1965, the site remained almost bare and no tobosa occurred there .
|A 675-acre November prescribed fire in a tobosa grassland, Carlsbad Caverns National Park, New Mexico. Photo courtesy of Travis Neppl, National Park Service.|
Postfire regeneration strategy :
Surface rhizome and/or a chamaephytic root crown in organic soil or on soil surface
Rhizomatous herb, rhizome in soil
Ground residual colonizer (on site, initial community)
Initial off-site colonizer (off site, initial community)
Secondary colonizer (on- or off-site seed sources)
Fire adaptations and plant response to fire:
Fire adaptations: Tobosa is very resistant to fire mortality. It can sprout and grow quickly after top-kill by fire . Tobosa seedling establishment is generally low, and no studies reported postfire seedling establishment either from on-site or off-site seed sources. Tobosa seeds have the potential for long-distance dispersal by wind, water, or animals after fire, but tobosa is unlikely to establish from the soil seed bank after fire.
Plant response to fire: Tobosa may increase, decrease, or remain unaffected by fire depending upon soil moisture and plant phenology at the time of the fire, precipitation in the months following the fire, and site characteristics that influence soil moisture availability (see Fire timing) [40,116,180]. In general, tobosa is most likely to benefit from fire during a year of average or above average precipitation (a "wet" year) if the fire occurs when the soil is moist and plants are dormant, such as in winter or early spring. Tobosa palatability and nutrition may increase following fire, and livestock often prefer to graze on tobosa soon after fire (see Postfire palatability and nutrition and Livestock grazing after fire).
Fire timing: Tobosa response to fire depends in part on season, due to plant phenology at the time of burning and soil moisture levels that vary with time of year. Tobosa plants burned during dormancy may increase more during the 1st postfire growing season than plants burned when green. In tobosa-honey mesquite grasslands in Justiceburg, Texas, plots burned under prescription in February had 400 kg/ha greater tobosa yield than March-burned plots during the 1st growing season. Plants were dormant at the time of the February fire, whereas in March they were initiating growth. The March burns consumed the new growth; this loss in production, plus the stress of reinitiating growth, probably caused much of the reduced yield .
Soil moisture at the time of fire greatly influences tobosa production after fire. In a chained honey mesquite-mixed-grass rangeland on the Rolling Plains, tobosa production increased 71% the 1st growing season after a March prescribed fire that was conducted when soil moisture was "good", but after a prescribed fire that occurred a month earlier when soil moisture was "low", tobosa production did not increase . In northwestern Texas, tobosa production (pounds/acre) increased 3-fold the 1st growing season after a late March prescribed fire in chained and unchained tobosa-honey mesquite communities compared with unburned and unchained controls. The fire was conducted during a year with a "mild drought", but the site received >3 inches (8 mm) of rain just prior to and immediately following the fire. For more information on this study, see this Research Project Summary [61,62]. In tobosa grasslands on a clay bottomland site in south-central New Mexico, tobosa production after prescribed fires in June was reduced more and for 1 year longer than production after prescribed fires in July, August, or December (Table 3). The author suggested this was due to lower soil and fuel moistures, higher soil and air temperatures, and lower relative humidity during the June fire than that during the other fires .
With sufficient soil moisture, tobosa can sprout and grow quickly after top-kill by fire . In western Texas, tobosa was top-killed by a "cool" prescribed fire conducted in a tobosa-honey mesquite community soon after green-up in April. From the time of the fire until the first substantial rain in May, there was almost no grass cover. After a 5-inch (127 mm) May rain, tobosa grew "several inches" in a few days . During a year of above-average rainfall on the Rolling Plains, dormant tobosa plants burned by a prescribed headfire in February had new growth about 2 weeks later . Two months after a "hot" March prescribed fire in a tobosa-honey mesquite community in Mitchell County, Texas, tobosa was ≤1 inch (3 cm) tall; after rains at the end of May, tobosa grew to 3 to 5 inches (8-13 cm) tall .
Postfire response of tobosa is greatly influenced by precipitation during the year of the fire. Several researchers reported increased tobosa production after prescribed fires during wet years and reduced tobosa production after fires during years of below average precipitation ("dry" years) (e.g., [13,40,62,116,181]). In tobosa-mesquite communities in northwestern Texas where mesquite had been variously treated with herbicides and/or masticated, tobosa production was 2- to 3-fold higher (approximately 2,000-3,500 pounds/acre) on burned areas the 1st growing season following prescribed fire than on unburned controls. Fires were conducted when plants were dormant, in either early spring or late winter during wet years. Production subsequently decreased until it was similar to that in unburned areas during approximately the 5th postfire year. Average annual precipitation in the study areas was approximately 19 inches (48 cm) [15,62,116,140,141,180,181]. In tobosa grasslands on a clay bottomland site in south-central New Mexico, tobosa production did not increase the 1st year after prescribed fire regardless of fire timing, probably due to below-average precipitation that year. Plots were burned either in June, July, August, or December. During the 1st postfire growing season (2-10 months after fire), precipitation was 26% below average and all burned plots had significantly less herb production than control plots. During postfire years 1 to 4 (14-58 months after fire), when precipitation ranged from 30% below average to 10% above average, herbaceous abundance on most burned plots were similar to unburned controls (Table 3) . In tobosa grasslands in Texas, recovery of total yield (live plus dead materials) was slower following high-intensity summer fires than after either high-intensity or low-intensity winter fires. However, total yield never exceeded the unburned control in any fire treatment up to 5 years after fire. The authors attributed this relatively poor postfire productivity in part to a severe drought during the 2nd postfire year. For more information, see this Research Paper .
|Table 3. Average herbaceous plant production (total production, pounds/acre) on tobosa grasslands burned under prescription on various dates |
|Time since fire||
Herbaceous plant production*
|15 June||15 July||1 Aug**||1 Dec|
|2-10 months||1,062ab***||335d||473cd||589cd||519cd||26% below average|
|14-22 months||1,803ab||1,110c||1,253bc||1,722ab||1,984ab||near average|
|26-34 months||1,711c||1,800c||1,857c||1,893c||1,890c||10% above average|
|38-46 months||930b||696c||792bc||976b||901b||10% above average|
|50-58 months||560c||435c||502c||579c||651c||30% below average|
|*Plant production measured in mid-October.|
|**Growth had begun and basal leaves were 1-inch long at the time of the fire.|
|***Means having different postscripts were significantly different at P<0.05 for that sample date.|
Tobosa production may have increased on burned areas following fire in years with above average precipitation because high soil temperatures (due to litter removal and/or heat absorption by black ash) accelerated bacterial activity on burned areas and thus increased nitrate production. During dry years and years when rainfall is delayed until fall, soil moisture, rather than soil temperature and nitrate production, appeared to limit tobosa production. Ash appeared to have little if any fertilizing effects [140,141,167,183], and most ash was gone within 3 months after fire .
Because of its influence on soil moisture, topography may influence postfire response of tobosa, with bottomland sites having greater tobosa production after fire than upland sites. During a wet year in a lightly grazed tobosa grassland with honey mesquite in west-central Texas, tobosa production on bottomland sites the 1st postfire growing season (1,646 kg/ha) was greater than that on upland sites (1,005 kg/ha). Mid-slopes had intermediate tobosa production (1,596 kg/ha). However, the average yield of tobosa increased after fire compared to prefire levels by a greater percentage on upland sites (232%) than on bottomland sites (104%). Tobosa production on upland sites was similar to unburned controls in 3 growing seasons, whereas bottomland sites had increased production for a longer period following fire. This was likely because bottomland sites were moist due to the accumulation of surface run-off. During a "dry" year, tobosa production during the 1st postfire growing season on bottomland sites (697 kg/ha) was still greater than that on upland sites (190 kg/ha) but much less than during the wet year. As in the wet year, mid-slopes had intermediate tobosa production (230 kg/ha) .
Fire frequency: Too frequent burning in tobosa grasslands may deplete soil nitrogen reserves and reduce plant growth. Observations in tobosa communities in Texas suggested that areas that were burned once produced more herbage than areas reburned within 2 or 3 years; thus, burning at <3-year intervals may reduce tobosa productivity . Based on the amount of time it took for litter nitrogen in tobosa grasslands to recover to prefire levels, a study of 5 prescribed fires on the Rolling Plains concluded that tobosa should not be burned more frequently than every 5 to 8 years, depending upon topography. Convex (upland) sites were dryer and required an estimated 8 years to reach prefire litter nitrogen levels, whereas moister concave (bottomland) sites reached prefire litter nitrogen levels 5 years after the fires .
Fire intensity: Tobosa production appears unaffected by fireline intensity . In a tobosa-honey mesquite community on the Rolling Plains, tobosa height, seed production, and yield 4 months after spring prescribed fires were not related to fireline intensity, nor did they differ between headfires and backfires. The fires burned 82% of the study areas, on average [132,133]. In tobosa grasslands in Texas, tobosa total yield and live yield were similar following low-intensity and high-intensity winter prescribed headfires. The low-intensity fire had a fireline intensity of 483 kW/m, while the high-intensity fire had a fireline intensity of 1,437 kW/m . For more information, see the Research Paper by Ansley and others .
Postfire palatability and nutrition: Burning tobosa may increase its palatability and nutritive quality by reducing litter and stimulating green and succulent new growth . New growth may have higher protein content than unburned plants for a few months after fire ([40,148,149], Huston and Uekert 1980 cited in ). On tobosa grasslands in the Chihuahuan Desert, crude protein content increased to 10.2% 14 months after a winter prescribed fire and to 8.6% 10 months after a summer prescribed fire compared to unburned control levels, although biomass was only 180 pounds/acre on the winter-burned areas and only 300 pounds/acre on the summer-burned areas compared with 2,850 pounds/acre prior to the fires. Digestibility increased to 63.8% on the winter-burned area and to 60.7% on the summer-burned area compared to 51.4% on unburned controls [148,149]. In contrast, in Justiceburg, Texas, crude protein concentrations were similar on burned and unburned tobosa-honey mesquite grasslands .
Postfire increases in tobosa palatability and nutritive quality may be short lived, sometimes lasting less than 1 growing season. On the southern Rolling Plains, digestible organic matter (burned sites: 62.1%, unburned controls: 57.1%) of tobosa in domestic sheep diets on mesquite-tobosa rangelands were higher the spring following a March prescribed fire than on control sites. By mid-June, however, tobosa had matured and digestible organic matter was similar on burned and control sites (Huston and Uekert 1980 cited in ). Wright  recommended grazing tobosa immediately after spring burning because, at that time, it is young and tender. If it is ungrazed for 3 to 4 months after fire, cattle will eat very little of it. See Livestock grazing after fire and Importance to Wildlife and Livestock for more information.
FUELS AND FIRE REGIMES:
Fuels: Tobosa may accumulate a large amount of litter that decays slowly [106,138], which provides "excellent fuel for fires" . Fire consumes tobosa litter, which is replaced by new growth. The previous year's new growth becomes litter and standing dead material for the current growing season . Culms produced following fire stand erect for several years before they die, break off, and contribute to the litter layer. During the 1st postfire year, litter consists mostly of dead leaf blades, but by the end of the 4th or 5th growing season after fire tobosa litter typically consists of large amounts of standing dead material and litter from stems that have died and fallen . Litter build-up is typically "very fast" during the first 3 growing seasons after fire and then slows. It may take >4 years for tobosa litter cover on burned areas to approach that on unburned controls . In a lightly grazed tobosa grassland with honey mesquite (a "tobosa flat") in west-central Texas, tobosa standing litter accumulation was most rapid the 1st and 2nd growing seasons after prescribed fires and then declined. Rate of standing litter accumulation was influenced by topography: litter accumulation was faster on concave (bottomland) sites than convex (upland) sites .
When found in pure stands, tobosa may accumulate large amounts of litter, up to 7,000 pounds/acre of total fuels (live, standing dead, and litter) in a year [15,186], which carries fire easily. According to Stinson and Wright  the compact growth form of tobosa "usually ensures a complete burn". A spring prescribed fire in tobosa grasslands in Colorado City, Texas, consumed "virtually all" of the 5,451 kg/ha of litter present; an April prescribed fire in Post, Texas, consumed 97% of the 6,800 kg/ha of litter present . However, in some tobosa communities, tobosa may be sparse, and rock and bare ground cover may be high (see Stand and age class structure). In these communities, fire may be "patchy" or "spotty" [15,56,62,132,133]. For example, near Guthrie, Texas, a March prescribed fire in a tobosa community was "spotty" and the fire required frequent relighting due to the abundant bare ground and rock at the site, which served as natural fire breaks .
Tobosa burns "hot" when in pure stands, reaching temperatures up to 1,260 °F (682 °C) with temperatures >150 °F (66 °C) lasting for more than 8 minutes. At 6 sites in the southern mixed prairie of Texas, where total fuels in tobosa grasslands ranged from 3,758 to 7,025 pounds/acre, average maximum temperatures reached during late March headfires ranged from 593 to 984 °F (312-529 °C) and were sustained from 2.0 to 5.4 minutes on average . In honey mesquite/mixed-grass savanna on the northern Rolling Plains, a prescribed headfire was conducted in early March when tobosa production was 4,968 kg/ha and its fuel moisture was 12.9%. Fuel loads, composed mostly of tobosa, were "moderate" and the fire was of high intensity: flame length was >20 feet (6 m) and fireline intensity was >15,000 kW/m, driven by high winds (16 miles/hr (26 km/hr), high atmospheric temperature (85 °F (29 °C)), and low relative humidity (11%) . In north-central Texas, prescribed headfires were used to reduce honey mesquite cover in tobosa-buffalo grass grasslands. In these grasslands, fine fuels were nearly continuous and bare ground cover was <10%. Fine fuels (litter plus standing crop of herbaceous plants) ranged from 2,484 to 3,803 kg/ha in all plots. Prescribed fire was applied to plots either in winter or in late summer. Fireline intensity in winter-burned plots was 402 kW/m and in summer-burned plots ranged from 981 to 3,723 kW/m .
The amount of fine fuels in tobosa communities affects maximum temperatures reached during fire. Using prescribed fire in an herbicide-sprayed tobosa-honey mesquite community, Wright and others  found that the greater the fuel load the longer the duration of elevated temperatures.
Total fuel load and total fuel moisture content best determined the success of 29 spring prescribed headfires in tobosa stands in northwestern Texas (as measured by fire spread and accomplishment of burning objectives). Success of 16 backfires was best determined by total fuel load, wind speed, and relative humidity. Fireline intensity of the 45 headfires and backfires ranged from 46 to 5,504 btu/ft/s . In the spring, a minimum total fuel load of 2,791 kg/ha was necessary for successful fire spread on level plots. Maximum total fuel moisture in a successful fire was 57%. Successful fires spread at 0.02 to 2.06 miles (0.03-3.32 km)/hour .
Fire regimes: Tobosa occurs in a variety of plant communities in New Mexico, Arizona, Texas, and Oklahoma. It is most common in desert grasslands, southwestern shrub steppe, and shortgrass and mixed-grass prairies that have predominantly replacement fire regimes with mean fire-return intervals ranging from <10 years to >80 years. Tobosa is also abundant in mesquite savannas, where surface fires predominated historically at about 6-year intervals. See the Fire Regime Table for further information on fire regimes of vegetation communities in which tobosa may occur.
Humphrey  stated that historical records indicating the occurrence of fires in the Sonoran or Chihuahuan deserts are rare. He suggested that vegetation was too sparse to carry fire in most areas, with the exception of tobosa swales. He hypothesized that fires must at one time have been the primary factor that maintained tobosa swales as pure grasslands rather than as grasslands with shrubs. He noted that although tobosa grows on both upland and lowland sites, only in tobosa swales is the growth dense enough to carry fire. During the late 1800s and early 1900s, the vegetation composition of many tobosa swales and tobosa flats changed from pure grasses to a grass-shrub mixture. By 1963, fire in tobosa swales was "largely nonexistent" and mesquite and acacia were establishing within them. Because of the small areas of tobosa swales and their juxtaposition with "nonflammable vegetation types", Humphrey concluded that tobosa swales must have burned less frequently historically than more extensive grasslands. As of this writing (2012), no wildfires were reported in tobosa communities in published literature.
FIRE MANAGEMENT CONSIDERATIONS:
Prescribed fire is commonly used in tobosa grasslands with one or more of the following objectives: 1) removing accumulated litter; 2) increasing tobosa and other grass production; 3) increasing accessibility and palatability of tobosa and other grasses for livestock and wildlife; 4) reducing shrub, succulent, and tree (especially honey mesquite) cover; and 5) reducing "undesirable", cool-season herbaceous annuals, particularly prairie broomweed (Xanthocephalum dracunculoides) [167,181,182,183,184].
Whether prescribed fire is beneficial to tobosa depends predominantly upon precipitation during the year of the fire, and soil moisture and plant phenology at the time of the fire (see Plant response to fire). Although wildfires generally occurred historically during the growing season during dry years , Wright [181,182] recommended burning tobosa communities in spring, when plants are dormant and the soil is wet, but only during years with adequate precipitation. Fires during dry years may be harmful to tobosa because they magnify drought stress on plants, whereas fires during wet years are generally beneficial (see Fire timing) . Based on data from late winter and spring prescribed fires in Texas, he concluded that tobosa could be burned throughout the year without reducing the following year's production; only the current season's production would be reduced if a site was burned between 10 April and 15 November . However, one study in Texas suggested that tobosa production may be greater the 1st growing season after winter prescribed fire than the 1st growing season after summer prescribed fire. During the 1st postfire growing season in a mesquite community in Foard County, Texas, tobosa production on sites burned in winter was about 1,000 pounds/acre greater than that on sites burned the previous September . Based on observations made during a series of prescribed fires in north-central Texas, Neuenschwander and others  concluded that biomass changes in tobosa grasslands were dependent on moisture and topography, and that good winter-spring precipitation was the "key" to a successful prescribed burn. In contrast, Dwyer , based upon prescribed fires conducted in June, July, August, and December in New Mexico, recommended burning tobosa to remove old growth in August, shortly after initiation of new growth. He also suggested that burning from late October to December was unlikely to damage tobosa plants. For more information on this study, see Fire timing.
Fuels in tobosa communities range from sparse to abundant (see Fuels). In order for a prescribed fire to carry in tobosa communities, fine fuel loads need to be sufficient and continuous. In a review, Wright  suggested that prescribed fire not be attempted in grasslands in Texas where fine fuels (grasses and forbs) are <1,000 kg/ha, and fine fuels should generally be >2,500 kg/ha. Because many southwestern grasslands with tobosa have been heavily grazed, fine fuels may be too sparse to carry prescribed fire . Ansley and Jacoby  suggested that grazing should be deferred on tobosa and other rangelands on a regular basis to allow sufficient accumulation of fine fuel to carry fires. Based on the amount of time it took for tobosa fuels to build-up after prescribed fire in Texas, Wright  suggested burning tobosa grasslands on a 4- to 7-year rotation. Based on the amount of time it took for nitrogen in tobosa stands to return to prefire levels, a study on the Rolling Plains suggested burning on a 5- to 8-year rotation to maintain productivity [15,142].
Specific guidelines for burning tobosa range in Texas to increase tobosa productivity for livestock use were as follows :
First, create a fireline by bulldozing a line out of mineral soil around the area to be burned. A 2nd bulldozed line should be cut 100 feet (30 m) inside the perimeter line on the north and east sides. This 100-foot strip should be burned between late January and early March when the following weather conditions prevail:
1. 40% to 60% relative humidity
2. 40 to 60 °F (4-16 °C) air temperature
3. <10 mile/hour (16 km/hour) wind speeds
The main area should be burned in March before green-up of tobosa. Weather conditions for the headfires should be as follows:
1. 25% to 40% relative humidity
2. 70 to 80 °F (21-27 °C) air temperature
3. 8 to 15 mile/hour (13-24 km/hour) wind speeds 
Where fuels in tobosa grasslands are continuous, high winds are not necessary to move the fire across an area . In Texas, tobosa and other grasses burned satisfactorily at moisture contents between 12% and 57% [14,27,61,62,153].
High-intensity prescribed fires intended to damage or kill shrubs encroaching into tobosa grasslands are not likely to adversely impact tobosa during years of normal or higher precipitation (see Fire intensity). Roberts  suggests that methods of quantifying fire behavior other than fire intensity, such as those that consider the area around the base of the plant (e.g., residence time), may be more appropriate for determining postfire response of herbaceous vegetation.
Broomweeds and firewhirls are primary concerns when conducting prescribed fires in tobosa stands. Broomweeds can burn off at their bases and tumble into other areas, potentially igniting nontarget areas. Firewhirls generally develop where wind shears occur, such as when a headfire runs into a backfire or when burning across ridges [181,184,185]. Wright  mentioned that firewhirls caused escapes during prescribed burns in tobosa-honey mesquite communities in Texas.
For an analysis of the economic feasibility of prescribed fire in tobosa-honey mesquite communities on the Rolling Plains, see Ethridge and others .
Soils: Prescribed fire may increase run-off and soil loss in some communities. However, March prescribed fire had little effect on rainfall infiltration of honey mesquite-tobosa communities in clay soils in northern Texas. In addition, most of the soil physical properties that affect infiltration on these soils were not altered significantly by burning. Prescribed fire in these communities had little influence on sediment load in run-off from slopes of less than 1%. Although sediment loss in overland flow increased following the fire, total soil loss was not significant, and within 2 to 3 years the sediment load stabilized. The authors suggested that potential sediment loss may be minimized by burning when soils are relatively moist .
Litter: Sharrow and Wright  noted that litter and standing old growth often play an important role in regulating the productivity of grassland communities. Together, they form an insulating layer which can effectively reduce the rates of spring soil warm up, delay fall soil cooling, reduce evaporative water loss from the soil surface, and protect the soil surface from erosion. In addition, they may serve as an important nitrogen reserve which can become available for plant growth through the processes of organic decay, nitrogen mineralization, and nitrification . Too frequent burning in tobosa grasslands may deplete soil nitrogen reserves and thus reduce plant growth. For more information on this topic, see Fire frequency.
Wildlife: Prescribed fires in tobosa grasslands that occur prior to the nesting season of rangeland birds may benefit some birds by reducing cover of old, matted, and dense stands of tobosa (e.g., lark sparrow; see Cover value) . However, fire in tobosa communities with trees and shrubs may reduce important cover for other bird species, such as bobwhite quail . Renwald and Wright  suggested that firelines be bulldozed to protect clumps of lotebushes and stands of large mesquite trees from fire to provide cover for bobwhite quail. For more information on fire management considerations for wildlife species in tobosa communities, see FEIS reviews.
Livestock grazing after fire: Several studies reported increased use of tobosa by livestock after fire [32,61,62]. Near Post, Texas, cattle grazed tobosa on plots from 15 April to 15 June after a late March prescribed fire, consuming 1,852 pounds/acre of tobosa in the burned area and only 122 pounds/acre on an unburned control. However, during summer when tobosa was dry, cattle avoided tobosa on both burned and unburned areas. Cattle returned to burned tobosa stands in fall when new growth appeared. They avoided tobosa again in winter when it was dormant. For more information on this study, see this Research Project Summary [61,62]. According to Wright , cattle typically eat very little tobosa the 2nd postfire spring unless it was grazed heavily the 1st postfire year. For a review of livestock response to prescribed fire in tobosa rangelands, see Britton and others . For information on postfire changes in tobosa palatability and nutrition see Postfire palatability and nutrition.
Other plants: Prescribed fire may be detrimental to other plant species in communities with tobosa. At the John Cargile Ranch near San Angelo, Texas, tobosa production was 65% greater on burned mesquite savanna during the 1st postfire growing season than on unburned savanna . However, Texas wintergrass (Nassella leucatricha) production was 76% less and buffalo grass production was 52% less on burned areas than unburned areas. Some burned and unburned areas were herbicide-sprayed 4 years prior to the study . For more information on fire management considerations for other plant species, see FEIS reviews.
FEDERAL LEGAL STATUS:
Information on state-level protection status of plants in the United States is available at NatureServe.
IMPORTANCE TO WILDLIFE AND LIVESTOCK:
Tobosa is a productive species across rangelands that cover large areas of the southwestern and south-central United States and northern Mexico [26,125,163]. For example, on bottomland sites where tobosa occurs in nearly pure stands it may produce >3,700 pounds/acre (4,200 kg/ha) in a year (see Stand and age class structure). Thus, tobosa is an important forage resource for many wildlife species and livestock in the Southwest [26,125,163]. Numerous mammals, including coyotes, white-tailed deer, pronghorn, prairie dogs, jackrabbits, and kangaroo rats [9,21,96,99,102,104,144], tortoises [108,170], and probably birds eat the seeds, leaves, and stems of tobosa. Forage collected by ants in tobosa grasslands in the Chihuahuan Desert consisted of 70% tobosa seeds .
Tobosa may be eaten by cattle, horses, and occasionally domestic sheep [48,74]. The forage value of tobosa for wildlife and livestock ranges from good during the growing season, when new growth is green and succulent, to poor at maturity when it is coarse and dry (e.g., [21,25,26,57,60,74,112,120,154,185]). Forage value during the growing season may be reduced if abundant old growth remains on plants .
In some areas, tobosa may be the most preferred forage species by wildlife and livestock. In Nuevo Leon, Mexico, when tobosa was actively growing it was the most preferred species by white-tailed deer and cattle on a shrub mixed-grass savanna . In other areas, other grasses are preferred over tobosa . On the Edwards Plateau, assessments based on expert opinion of year-round preference of cattle, domestic goats, and domestic sheep for 167 plant species found that tobosa ranked 56th for cattle, 113th for domestic goats, and 89th for domestic sheep . Grazing may increase or decrease tobosa abundance. See Livestock grazing for more information.
Palatability: Tobosa is most palatable during the growing season when new growth is green and succulent (for about 90 days/year); it is relatively unpalatable at maturity when it is dry and coarse (e.g., [21,25,26,57,60,74,112,120,154,185]). Dormant plants are typically refused by livestock if other forage is available .
Large quantities of standing dead material accumulate on tobosa plants, decreasing their palatability. Tobosa palatability often increases the 1st growing season after disturbances that reduce litter and stimulate new growth. For example, in southwestern New Mexico, livestock only consumed tobosa near prairie dog colonies, where clipping by prairie dogs apparently increased tobosa palatability . Burning tobosa may increase its palatability to livestock during the 1st growing season after fire. For more information on this topic, see the Postfire palatability and nutrition and Livestock grazing after fire sections.
Nutritional value: Nutritional value of tobosa is highest when it is young and decreases with maturity . At the Jornada Experimental Range, tobosa crude protein content ranged from 7.9% to 10.2% in summer and fall but decreased to 4.8% in February (Table 5) . In November, crude protein in tobosa old growth from previous growing seasons averaged 4.7% . Another study at the Jornada Experimental Range reported low crude protein levels (5.9%) and high fiber content (80.7%) in August when plants were mature and contained considerable portions of dead material . In another study in the Trans-Pecos, crude protein level of immature leaves and stems was 6.9%; that of mature leaves and stems was 4.8% . Near Justiceburg in the Texas Rolling Plains, crude protein content in tobosa peaked around 1 May (range: 9.0%-10.5% during 3 years) and generally declined throughout the summer (range: 7.0%-8.3% during 3 years) . Another study in the same location reported that crude protein content in current year's growth declined from 16% in April to 5% in July, while in vitro digestible organic matter declined from 55% to 35% . For more information on this study, see Postfire palatability and nutrition. In contrast, in the Trans-Pecos, crude protein content of tobosa was similarly low in spring (4.5%), summer (4.5%), fall (3.9%), and winter (3.6%) .
|Table 5. Chemical composition (% of dry matter) of tobosa at different stages of maturity at the Jornada Experimental Range, New Mexico |
Month (stage of maturity)
|July (mature)||August (overripe)||September (mature)||October (dough)||February (dormant)|
|Acid detergent fiber||47.2||42.0||43.2||48.6||50.3|
Tobosa's nutritional composition is often below that required by livestock [17,44,112,114]. A review reported that the percentage of crude protein in tobosa during the growing season was less than the minimum protein requirement for cattle; phosphorus levels in tobosa met cattle requirements for only a short period at the peak of the growing season; and calcium levels were usually below cattle needs in winter and spring . In the Trans-Pecos, crude protein was below that required by cattle year-round . In northwestern Texas, tobosa protein was "fair" during the young stages of growth but deficient when mature; phosphorus was deficient in all stages . At the Jornada Experimental Range, crude protein, calcium, and phosphorus in tobosa were adequate to meet the minimal nutritional requirements of livestock during all phenological stages except dormancy. In the dormant stage, protein and phosphorus were deficient [112,114]. For additional information on tobosa chemical composition, see the following studies: [72,76,112].
According to a review, most nutrients in tobosa can be increased by fertilization . For example, at the Jornada Experimental Range, nitrogen fertilization increased crude protein in dormant tobosa during 4 of 5 years . For more information about fertilizing tobosa grasslands, see Dwyer  and Herbel .
Cover value: Many small mammals, including deer mice, pocket mice, harvest mice, grasshopper mice, voles, cotton rats, woodrats, kangaroo rats, ground squirrels, jack rabbits, and skunks, use tobosa communities as cover (e.g., [9,37,41,43,70,179]). White-sided jackrabbits in New Mexico were flushed only from tobosa stands and appeared to flee to other tobosa stands when disturbed, where they created shelter forms exclusively in tobosa . In the Chihuahuan Desert in southern New Mexico, hispid cotton rats and western harvest mice apparently preferred tobosa and vine mesquite communities because of these communities' dense undergrowth and protective overstory . Denyes  stated that many small mammals occurred in tobosa-mesquite associations in the Big Bend region of Texas but that small mammals were scarce in tobosa-burrograss associations near the Glass Mountains. Although there was a good ground cover in tobosa-burrograss associations, soils were hard and there was little variety of food . Large mammals also use tobosa communities as cover. Tobosa was the 2nd most frequent plant providing cover around pronghorn fawn bed sites in the Trans-Pecos 
Many birds, particularly ground-nesting birds, use tobosa communities as cover. Tobosa and false grama (Cathestecum brevifolium) were considered essential components of optimum habitat for rufous-winged sparrows in the Southwest . Mourning dove ground nests were only located in tobosa in a tobosa-honey mesquite community in western Texas where honey mesquite had sprouted after being top-killed by herbicide 6 years prior. Mourning doves always used dried tobosa litter for nest materials . Gambel's quail , Montezuma quail , and bobwhite quail  commonly use tobosa stands as cover in the Southwest and Texas. In Mexico, tobosa communities provide important cover for Bolson tortoises . Ant mounds are commonly found in tobosa communities , and ants commonly consume and disperse tobosa seeds .
Fire effects on wildlife in tobosa communities: Because tobosa cover is important for many wildlife species, wildlife may be affected by fire in communities with tobosa. Mourning dove nesting success increased following April prescribed fire in western Texas . On the Rolling Plains, breeding densities of lark sparrows were highest in the most recently burned honey mesquite-tobosa communities and declined in older burns. The 1st growing season after a "cool" prescribed fire that occurred prior to the nesting season, cover of tobosa was 33% and appeared "ideal" for nesting and foraging lark sparrows because of the interspersion of burned and unburned areas after the fire. In a 6-year-old burn tobosa cover was 53% and was thick and matted down, with few openings available for foraging and too much cover for nesting . Another study on the Rolling Plains found that bobwhite quail coveys were absent from burned tobosa grasslands, apparently because of lack of woody cover such as lotebush (Ziziphus obtusifolia), and because the dense stand lacked bare spaces for travel lanes and dusting sites. The authors suggested that prescribed fire in tobosa grasslands may benefit bobwhite quail if some lotebushes are protected from fire . After >10 years of prescribed burning in semidesert grasslands, pronghorn populations in the Prescott National Forest, Arizona, increased because of the decreased tobosa litter, decreased shrub cover, and increased forb growth . For more information on fire effects on wildlife species in tobosa communities, see FEIS reviews.
VALUE FOR REHABILITATION OF DISTURBED SITES:
Because of its sod-forming growth habit, tobosa may be useful for revegetation of disturbed areas .
Tobosa has been cut for hay. The hay is eaten by cattle if it is cut when green, and cattle fed on it will remain in good condition throughout the winter .
OTHER MANAGEMENT CONSIDERATIONS:
Status: Since about 1880 a combination of livestock grazing, fire exclusion, recurrent drought, and soil erosion have led to increased cover of trees and shrubs and decreased cover of grasses in grasslands of the Southwest [51,53,66,73,128,139,155,157]. In addition, large expanses of grasslands in the Southwest are now dominated by nonnative invasive plants, such as Lehmann lovegrass (Eragrostis lehmanniana) and red brome (Bromus rubens) [11,128]. A review stated that tobosa is one of the few native grasses that have competed well with nonnative grasses .
Because tobosa is relatively tolerant of livestock grazing, it apparently did not suffer as large a reduction as many other native grasses in the Southwest, such as gramas [128,139]. Some authors suggest that tobosa has either remained unchanged  or increased since the introduction of livestock grazing . According to Dick-Peddie , most tobosa grasslands have withstood historical livestock grazing so the composition and structure of their vegetation in the 1990s was "virtually as they were in presettlement times". On grazed sites across 6 counties in southwestern New Mexico, tobosa and other perennial grass cover was similar during 48 years . In contrast, tobosa cover decreased at the Jornada Experimental Range during 80 years. In the 1910s, tobosa dominated an estimated 5,965 acres (2,414 ha) at the Jornada Experimental Range; 83 years later, tobosa covered only 215 acres (87 ha), a 72% decrease. The decrease was attributed to encroaching shrubs, particularly tarbush . Humphrey  proposed that fires must at one time have been the primary factor that maintained tobosa communities as pure grassland rather than as a grassland with shrubs, noting that since the late 1800s and early 1900s the vegetation composition of many tobosa swales and tobosa flats had changed from one of pure grasses to grass-shrub mixtures. For more information on this topic, see Successional Status.
Livestock grazing: Communities with tobosa are commonly grazed by livestock. Canfield  described tobosa and black grama as the most important grasses in determining livestock grazing capacity in desert rangelands. A rotation developed in southern New Mexico utilizes tobosa during the summer and black grama during the winter .
Generally tobosa withstands grazing well and is considered an "increaser" [19,154,175]. Brown and Schuster  considered tobosa an increaser when grazed on the Texas High Plains because its density was higher in a grazed honey mesquite community (136 lbs/acre) than an ungrazed Pinchot juniper community (112 lbs/acre). On the Rolling Plains, tobosa production in livestock grazed tobosa-buffalo grass grasslands was 717 pounds/acre higher than in grasslands where grazing was excluded for 27 years . However, tobosa's response to grazing may depend on site characteristics or type of livestock. On the Edwards Plateau, tobosa density increased on bottomland sites that were heavily grazed by sheep and cattle, but its density decreased under sheep grazing on upland sites . In southern New Mexico, cattle grazed a tobosa swale during 2 consecutive years, which reduced grass cover to small areas; however, grass cover increased substantially during postgrazing years 1 and 2 . Other researchers reported that grazing had no affect on tobosa abundance. Tobosa frequency on rangeland in south-central New Mexico was similar on grazed sites and on sites where livestock and pronghorn were excluded for 38 years .
Experiments that used clipping to simulate grazing did not show consistent results. In a field experiment, a pure stand of tobosa clipped to 4 inches (10 cm) had a higher cumulative yield at weekly clippings than at less frequent clippings, even after 11 years of treatment. However, only 1 plot was clipped at each frequency, so there was no treatment replication . In a common garden experiment, mean tobosa production in clipped plots (191 g/m²) was similar to that in control plots (243 g/m²) . A review stated that studies of different grazing intensities on tobosa in southern New Mexico found that protected areas had the lowest basal area of 4 grazing intensities, while stands that were intermediately grazed had the highest basal area . Campbell  concluded that 60% of tobosa herbage production may be utilized by cattle each summer without injury, whereas other researchers concluded that maximum utilization of tobosa during any year should probably not exceed 50% . At the Jornada Experimental Range, tobosa maintained its greatest cover when 40% to 55% of its annual vegetation growth was grazed .
Mowing: Mowing may reduce tobosa litter and increase its production. On the Edwards Plateau, tobosa increased tillering from basal nodes after mowing in early spring. In July and September, the number of tillers produced by mowed tobosa plants was greater than that produced by unmowed plants. The average number of leaves per tiller was higher on mowed than unmowed plants in July, but the reverse was true in September. Apparently, the unmowed plants were not influenced as much by low moisture conditions during September as mowed plants. Mowing increased rhizome production in July and September; 22 new rhizomes were produced by mowed plants, whereas unmowed plants only produced 1 rhizome. This rhizome was longer than the rhizomes of mowed plants .
Water management: Practices that maintain or increase water spreading may increase tobosa distribution [64,74]. Gullies formed in tobosa flats drain off water and may reduce tobosa. Dams with spreader wings may increase the area flooded and thus promote tobosa growth . The high grass and litter cover in tobosa grasslands may help promote water infiltration and reduce evaporation .Herbicides: Herbicides are often used in tobosa habitats to kill trees, shrubs, and succulents (e.g., [29,32,62,121,122,131,136,161,171,182]). Effects of these treatments vary in the 1st posttreatment growing season (e.g., [32,121,165,171]) but in later growing seasons, tobosa abundance is generally similar among treatments and controls . For information on the combined effects of herbicide application and prescribed fire in communities with tobosa, see Plant response to fire.
|Fire regime information on vegetation communities in which tobosa may occur. This information is taken from the LANDFIRE Rapid Assessment Vegetation Models , which were developed by local experts using available literature, local data, and/or expert opinion. This table summarizes fire regime characteristics for each plant community listed. The PDF file linked from each plant community name describes the model and synthesizes the knowledge available on vegetation composition, structure, and dynamics in that community. Cells are blank where information is not available in the Rapid Assessment Vegetation Model.|
|Vegetation Community (Potential Natural Vegetation Group)||Fire severity*||Fire regime characteristics|
|Percent of fires||Mean interval
|Surface or low||15%||67|
|Desert grassland with shrubs and trees||Replacement||85%||12|
|Plains mesa grassland||Replacement||81%||20||3||30|
|Plains mesa grassland with shrubs or trees||Replacement||76%||20|
|Shortgrass prairie with shrubs||Replacement||80%||15||2||35|
|Shortgrass prairie with trees||Replacement||80%||15||2||35|
|Interior Arizona chaparral||Replacement||100%||125||60||150|
|Salt desert scrubland||Replacement||13%||200||100||300|
|Southwestern shrub steppe||Replacement||72%||14||8||15|
|Surface or low||15%||69||60||100|
|Southwestern shrub steppe with trees||Replacement||52%||17||10||25|
|Surface or low||25%||35||25||100|
|Pinyon-juniper (mixed fire regime)||Replacement||29%||430|
|Surface or low||6%||>1,000|
|Pinyon-juniper (rare replacement fire regime)||Replacement||76%||526|
|Surface or low||4%||>1,000|
|Vegetation Community (Potential Natural Vegetation Group)||Fire severity*||Fire regime characteristics|
|Percent of fires||Mean interval
|South-central US Grassland|
|Southern shortgrass or mixed-grass prairie||Replacement||100%||8||1||10|
|South-central US Shrubland|
|Shinnery oak-mixed grass||Replacement||96%||7|
|Southwestern shrub steppe||Replacement||76%||12|
|South-central US Woodland|
|Surface or low||91%||6|
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.
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.
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 [59,90].
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