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Flourensia cernua


American tarwort at Big Bend National Park, Texas.
Photo courtesy of Melody Lytle, Lady Bird Johnson Wildflower Center Native Plant Information Network.

Innes, Robin J. 2010. Flourensia cernua. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: [].



American tarwort
American tarbush

The scientific name of American tarwort is Flourensia cernua DC (Asteraceae) [39,68].




SPECIES: Flourensia cernua

American tarwort occurs primarily on plains and mesas throughout the Chihuahuan Desert, the second largest desert in North America. The Chihuahuan Desert occurs in southeastern Arizona, southern New Mexico, southwestern Texas, and Mexico [10,65,118]. In Mexico, American tarwort occurs in the states of Sonora, Chihuahua, Coahuila, Durango, San Luis Potosi, and Zacatecas [153]. One publication stated that American tarwort occurs in California [135], but this appears to be erroneous, as American tarwort is not included in California floras. NatureServe provides a distributional map of American tarwort .

American tarwort occurs in desert scrub and desert grassland habitats and their intergradations (e.g., [10,14,29,42,46,65,78,106,118,123]). In part a result of historical overgrazing, American tarwort has replaced grama (Bouteloua spp.) and tobosa (Pleuraphis mutica) grasses as dominants in many desert grasslands throughout its distribution. Thus, American tarwort is often considered "invasive" in desert grasslands [11,28,49,56,111]. See Other Management Considerations for more information on this topic.

American tarwort primarily occurs in the Chihuahuan Desert. Chihuahuan Desert scrub is the predominant plant community throughout the Chihuahuan Desert, covering about 70% of its area [55,86]. American tarwort is considered a characteristic or indicator plant species of Chihuahuan Desert scrub [10,29,65,118]. Chihuahuan Desert scrub is dominated primarily by American tarwort , creosotebush (Larrea tridentata), and viscid acacia (Acacia neovernicosa) [104,130]. Because secondary shrubs and cacti formed <10% of stands, Shreve [130] described areas of Chihuahuan Desert scrub as "uniform and monotonous".

Within Chihuahuan Desert scrub, American tarwort is most commonly associated with creosotebush. American tarwort may be dominant, codominant, or subdominant to creosotebush (e.g., [19,46,55,58,84,104,121,132,137,155]). In the Rio Grande Valley, New Mexico, 96% of habitats with American tarwort also had creosotebush with varying degrees of dominance. The authors found that the 2 species were associated more often than would be expected from random mixing (P=0.01) [41]. The abundance and distribution of American tarwort in relation to creosotebush depend partly upon site conditions. Creosotebush-American tarwort habitat is common on the plains below the desert mountains in the Chihuahuan Desert with decreasing American tarwort cover and increasing creosotebush cover in the warmest, most arid areas and increasing American tarwort and decreasing creosotebush cover in deep soils and along minor drainages [55,58]. For more information on this topic, see Site Characteristics.

American tarwort also occurs in desert scrub communities dominated by acacia (e.g., whitethorn acacia (Acacia constricta) [60,121,155], viscid acacia [104,105,137], catclaw acacia (A. greggii) [155]), honey mesquite (Prosopis glandulosa) [11,22,49], Berlandier wolfberry (Lycium berlandieri) [52,108,121], mariola (Parthenium incanum) [105,137,142], Wright's beebrush (Aloysia wrightii), littleleaf sumac (Rhus microphylla) [104], broom snakeweed (Gutierrezia sarothrae) [4,149], winterfat (Krascheninnikovia lanata) [52], or smooth-leaf sotol (Dasylirion leiophyllum) [158]. Throughout the Trans-Pecos region, American tarwort occurred in the viscid acacia shrubland alliance on lower foothills of mountains, mesas, and piedmont hills and ridges at 4,200 to 6,300 feet (1,280-1,930 m) [121]. American tarwort occurred in a cattle-grazed broom snakeweed-American tarwort -burroweed (Isocoma tenuisecta) community in the San Simon Valley, southeastern Arizona [150]. American tarwort -tobosa and American tarwort -burrograss associations occurred on gravelly clay loam [13], and the American tarwort -winterfat-Berlandier's wolfberry community occurred on fine loam [52] at the Jornada Experimental Range, south-central New Mexico. On the east-facing slope of Bishop's Cap Peak at 4,800 feet (1,465 m) in Dona Ana County, New Mexico, American tarwort occurred in Berlandier wolfberry-creosotebush communities on ledges below limestone cliffs [151]. American tarwort occurred "in abundance" in smooth-leaf sotol grassland on gravelly slopes of the Grapevine Hills in Big Bend National Park, Texas [158].

American tarwort commonly occurs scattered throughout desert grassland and in the desert grassland-desert scrub ecotone [29]. American tarwort occurs most commonly in and adjacent to desert grasslands dominated by grama, tobosa, and burrograss (Scleropogon brevifolius) (e.g., [10,13,21,32,41,43,49,67,110,155]). American tarwort occurred in a clay bottomland site in tobosa grassland at the Jornada Experimental Range, New Mexico [1]. American tarwort -tobosa-burrograss communities occurred as bands of vegetation on low slopes (<2% slope) of the watersheds of the Doña Ana Mountains located between unvegetated bands of soil [32]. In gypsiferous soil in New Mexico, scattered American tarwort occurred in the gyp dropseed (Sporobolus nealleyi) herbaceous alliance [121]. On the Stockton Plateau in western Texas, American tarwort occurred in banded vegetation as scattered individuals in burrograss and purple threeawn (Aristida purpurea) communities [92]. Also in western Texas, American tarwort occurred in the grama-muhly (Muhlenbergia spp.)-purple threeawn association [156]. In Big Bend National Park, American tarwort occurred in black grama-red grama-China grama (Bouteloua eriopoda-B. trifida-B. ramosa) grasslands [160]. In intermittently flooded depressions and floodplains throughout the Trans-Pecos region, American tarwort occurred as scattered individuals in the tobosa intermittently flooded herbaceous alliance [121].

The only mention of American tarwort occurring in pinyon-juniper (Pinus spp.-Juniperus spp.) woodlands was on the Gila National Forest in southwestern New Mexico [62].

Vegetation classifications describing plant communities where American tarwort is a dominant species follow:

Arizona: New Mexico: Texas:

Trans-Pecos region:



SPECIES: Flourensia cernua
The ecology of American tarwort has not been well studied except at the Jornada Plain in south-central New Mexico (e.g., [11,13,35,48,51,110,111,114,119,136]), the Arizona-New Mexico border near Portal, Arizona [16,17,18,19,20,149,150], and the Mapimi Biosphere Reserve in Durango, Mexico [36,89,99,101,146,147].


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., [15,69,85,118,153]).

Form and architecture: American tarwort is a native perennial C3 shrub [136]. It is many branched and densely leafy [69,118,135,153]. Its growth form may be erect or procumbent [135,153]. American tarwort plants do not have trunks; instead, the branches run obliquely from the base [88]. It grows from <1 foot (0.3 m) to >7 feet (2 m) tall [75,118,131,153].

American tarwort in the Burro Mountains, Engineer Canyon, Gila National Forest.
Photo courtesy of Russ Kleinman and Richard Felger.

Leaves: The thick leaves of American tarwort are alternate, simple, and elliptical [118,153]. American tarwort leaves may be up to 1 inch (2.5 cm) long [75,118]. Most researchers describe American tarwort leaves as winter deciduous (e.g., [26,45,66,113,152]). Near Las Cruces, New Mexico, American tarwort was described as "partially" winter deciduous because some American tarwort plants along arroyos (ephemeral stream beds) held some leaves throughout the winter [73].

Reproductive structures: American tarwort 's composite flower heads are small, solitary, and inconspicuous [135,153]. The yellow-disked flower heads lack ray flowers and are pendant [153]. Each flower head has up to 20 flowers [135,153]. The flowers are hermaphroditic [36]. The fruits are one-seeded achenes that are flattened, hairy, and about 0.3 inch (6 mm) long [75,89,118,131,153].

American tarwort flowers, achenes, leaves, and stems are highly resinous [135,146]. American tarwort produces a characteristic tar-like odor [118,135] due to the presence of secondary compounds [34].

Roots: American tarwort has tap roots [52,89]. It is able to exploit both shallow and deep soil moisture. American tarwort is often described as shallow rooted ([26], Muldavin 1998 cited in [121]) because >75% of adult American tarwort roots are located within the upper 16 inches (40 cm) of soil [101]. American tarwort roots spread horizontally up to 13 feet (4 m) [21,52]. As a result, American tarwort plants depend mostly on water from the upper soil layers [21,101], which is recharged by common, small rainfall events [52]. Thus, small rainfall events are likely critical to the establishment and survival of American tarwort [12]. In a field experiment in Durango, Mexico, American tarwort responded to increased water availability in the upper soil layers by significantly increasing stem growth (P<0.05) [101]. A small number of American tarwort roots are deep (up to >16 feet (5 m)) [52]. These roots can access water from horizons that recharge infrequently and may sustain individual plants during lengthy droughts [12,52].

American tarwort roots may be deepest in soils with abundant run-off water [52] or in sparsely vegetated areas [89]. In Durango, Mexico, American tarwort seedlings grown in unvegetated areas had significantly deeper root systems than seedlings grown under herbaceous cover (P<0.05) [89]. American tarwort seedlings can develop root biomass rapidly [101]. The deepest roots reported for a 1-year-old American tarwort seedling were 16.1 inches (41 cm) for a 2-inch (5 cm) tall shoot in Durango, Mexico [89].

Stand and age-class structure: Communities with American tarwort are usually open, with scattered shrubs and herbs and extensive bare interspaces, but vegetation may vary from very open to moderately dense (e.g., [10,14,55,65,89,121]). American tarwort may be locally or generally abundant [63,69]. It may occur in very open stands of widely-spaced individuals [11,78,106] or in dense clumps [30,75,78,89]. In Brewster County, Texas, the tobosa-American tarwort association was described as arid and open with American tarwort and other shrubs usually spaced 2 to 3 feet (0.6-0.9 m) apart [28]. In Hidalgo County, New Mexico, the nearest-neighbor distance between American tarwort plants averaged 0.79 feet (0.24 m) in dense American tarwort -dominated stands (3,800 American tarwort plants/ha) [129]. Although American tarwort commonly occurs with other shrubs (e.g., [41,82]), it may occur in pure or nearly pure stands [28,49,107,125,130]. American tarwort tends to occur in pure stands in areas with clay and silt soils [49] and along bottomlands [156]. Studies reported American tarwort cover ranging from 5% to 45% in American tarwort -dominated sites [36,51,67,89,99,101,108,111,129]. See Fuels for more information on American tarwort stand structure and composition.

Limited information is available on American tarwort 's age class structure as of this writing (2010). American tarwort was described as long lived [142,147], although a life span was not provided. In a photo taken of a American tarwort -dominated stand in 1937 and again in 2002 at the Jornada Experimental Range, some individual American tarwort plants were apparently still present after 63 years, indicating that American tarwort may live at least that long [119]. American tarwort plant age (number of growth rings) and stem diameter are positively correlated (R²=0.89, P<0.001). Based on this relationship, a 10-year-old juvenile American tarwort plant would have a stem diameter of approximately 0.8 inch (2 cm). High density of American tarwort seedlings (<1-year-old plants) indicates American tarwort may have high recruitment during some years. In Durango, Mexico, mean American tarwort seedling density was 1.9 seedlings/m²; the maximum reported seedling density was 85 seedlings/m² [89].

Physiology: American tarwort 's presence in arid environments suggests that it is highly tolerant of drought [45]. American tarwort tolerates flooding for short periods but cannot withstand long-term standing water [29]. American tarwort responds quickly to increased water availability by increasing photosynthesis and transpiration rates. Although American tarwort inhabiting arroyos in New Mexico responded quickly to cycles of water availability, rainfall that did not increase channel flow in the arroyos did not appear sufficient to provide sustained relief from drought [26].

Raunkiaer [120] life form:

American tarwort leaf emergence occurs in spring. The timing of spring leaf emergence depends on the distribution and amount of winter and spring precipitation, but green leaves are typically present in late May or early June. In south-central New Mexico, in years with a dry winter and spring, American tarwort plants reduced photosynthetic activity and produced small, scale-like leaves in May and early June. These plants shed the small leaves and produced a new set of larger leaves as moisture availability increased in summer [113]. During a year of abundant winter and spring precipitation at the Jornada Experimental Range, the growing season began earlier than usual, in early May [35]. American tarwort 's peak growing period appears coincident with the period of maximum rainfall (see Climate). Most American tarwort stem growth occurs in summer after the summer rains begin [35,134].

American tarwort generally flowers in fall at the end of the rainy season [35,89,134,147]:

General flowering period for American tarwort throughout its range
Location Flowering period
Arizona southeastern Arizona July-December [69]
New Mexico Jornada Plain, south-central New Mexico October [35]
Texas western Texas September-December, but usually October [118]
Mexico Mapimi Biosphere Reserve, Durango November [89,147]

After flowering, American tarwort growth wanes and leaves senesce [35,73]. American tarwort loses most leaves after freezing temperatures occur, although some desiccated leaves may remain on the plant until the following spring ([73], P.W. Hyder personal observation cited in [66]). Near Las Cruces, New Mexico, American tarwort leaves were completely senesced in December [73].

American tarwort fruits mature and seeds disperse in winter during the dry season [75,89]. In Durango, Mexico, American tarwort produced fruit from December to February and dispersed seeds during February and March. American tarwort seeds germinate in summer during the rainy season. In Durango, Mexico, American tarwort seeds germinated from July through August [89,147].


Most of the information regarding American tarwort regeneration processes originated from laboratory and field studies at the Mapimi Biosphere Reserve in the southern Chihuahuan Desert in Durango, Mexico [36,37,89,99,101,146,147]. Although American tarwort may reproduce vegetatively, information on vegetative regeneration is lacking as of this writing (2010), and it appears to reproduce primarily via seed [11,13,128,135].

Pollination and breeding system: American tarwort flowers are wind pollinated ([90], Mauchamp 1992 cited in [146]). American tarwort flowers are hermaphroditic and self-incompatable [36]. In field experiments, the production of viable seeds after selfing was low (2-4% of flowers produced a viable seed), whereas the production of viable seeds after cross-pollination was up to 20% (M. Ferrer unpublished data cited in [36]).

Seed production: American tarwort produces abundant seed annually [13,89]. However, the majority of American tarwort seeds are inviable [37,147]. In Durango, Mexico, individual American tarwort plants produced 0 to 22 viable seeds/plant from approximately 5,000 ovules per plant (Ferrer 2004 cited in [37]). The viability of seeds derived from outcrossing was higher than that of seeds derived from selfing (see Pollination and breeding system) [36]. Of those seeds that were viable, the proportion of seeds that germinated was low (9%) (Ferrer 2004 cited in [37]). For more information on this topic, see Germination.

Precipitation, temperature, and insect herbivory may affect American tarwort flowering and seed production. At the Jornada Experimental Range, American tarwort did not flower during 2 years of a 3-year study due to dry conditions and extensive defoliation by leaf beetles (Zygogramma tortuosa). During a year of abundant winter and spring precipitation, however, American tarwort herbage was lush and green, leaf beetle herbivory was less, and American tarwort plants flowered sporadically in mid-October [35]. In Durango, Mexico, American tarwort flowered in fall regardless of water availability [101]. Results from these studies suggest that insect herbivory in combination with dry conditions may have a greater impact on American tarwort flowering than water availability alone. In Durango, Mexico, seed quality (seed size and viability) increased with higher minimum temperatures during seed formation in winter. Increased precipitation during the same period also increased seed size but to a lesser extent [146]. Because seed quality affects germination and seedling development, between-year variation in minimum winter temperatures and, to a lesser extent, precipitation, affect American tarwort population dynamics.

Seed dispersal: Past researchers reported that American tarwort seeds were "relatively motile" [11] and dispersed by wind [19]. However, more recent studies reported that American tarwort seeds are not dispersed by wind, but are instead dispersed initially by gravity and secondarily by water [36,89].

American tarwort seeds are dispersed short distances. In Durango, Mexico, 90% of American tarwort seeds fell directly under the parent plant [89]. The most distant American tarwort seedling recorded from an adult was <16 feet (5 m) away; most were <10 feet (3 m) away. Controlled experiments with American tarwort seeds suggested that they may be dispersed short distances by ants [99]. Mauchamp and others [89] concluded that American tarwort seeds have no apparent adaptation for long distance dispersal by wind or animals.

Spread of woody plant seed by livestock has been implicated as a cause of increased shrub encroachment into desert grasslands [27], particularly for honey mesquite—whose seeds remain viable after passing through cattle digestive tracts [31]. Because livestock avoid eating American tarwort , including its seeds, it is unlikely that livestock disperse American tarwort seeds. For more information on this topic, see Palatability and nutritional value.

Seed banking: About 2% of American tarwort seeds germinated in the laboratory after 26 months of storage, suggesting that American tarwort may be capable of producing a short-term seed bank [147]. However, because American tarwort apparently produces seeds most years [101], it likely contributes seeds to the seed bank annually and may not rely on long-term seed banks. In a review of desert seed banks, Kemp [71] reported that most desert shrubs and other long-lived perennial plants such as American tarwort do not require seed banks because they produce seeds almost every year that can ensure the recruitment of new individuals into the population. Thus, when conditions are favorable for shrub establishment, the seed source may be primarily seeds produced during the previous season. American tarwort seems to be long lived, and desert shrubs such as American tarwort may be protected against climatic uncertainly by long life rather than by a seed bank [71].

In a review of desert seed banks, Kemp [71] reported that most (80-90%) seeds in desert seed banks occur in the upper 0.8 inch (2 cm) of soil and of those, most are located in the litter or upper 0.4 inch (1 cm) of soil. Highly clumped distributions of seeds in soils are common for desert seed banks in general, in part because of a lack of extensive dispersal. When seeds are dispersed, accumulations of seeds occur in depressions where water collects, in wind shadows of obstructions, or in animal caches [71]. Because American tarwort seeds typically fall beneath parent plants and are only minimally dispersed except secondarily by water, American tarwort seeds are likely to be highly clumped spatially in soil seed banks. In Durango, Mexico, in the American tarwort -western honey mesquite (P. glandulosa var. torreyana)/burrograss mixed grassland vegetation type, American tarwort seedlings were clearly aggregated with adults and American tarwort seed density in the upper 2 inches (5 cm) of soil decreased with increased distance from the base of an adult plant [89]:

Density of American tarwort seeds in the soil seed bank at varying distances from adult plants in Durango, Mexico [89]
Distance to plant base (m) Mean number of American tarwort seeds/m²
0.2 1,600
0.5 1,720
0.9 940
1.4 280
2.0 120
3.0 30

American tarwort seed densities in the soil seed bank were directly related to the density of adults. In habitat where American tarwort was dense, mean American tarwort seed densities in the seed bank in June—after seed dispersal and prior to the summer rains—were up to 153 American tarwort seeds/m². Where American tarwort was sparse, mean American tarwort seed densities in the seed bank were up to 5.8 seeds/m². No American tarwort seeds were found in habitats without American tarwort . After the summer rains in August, however, no American tarwort seeds were found in the soil seed bank in any habitat. The authors suggested that this indicated substantial local displacement of seeds by water flow [89]. The authors did not report whether American tarwort seeds germinated from the seed bank.

In general, granivores such as ants and small mammals substantially reduce desert seed banks by consuming and dispersing seeds [71]. As of this writing (2010), it is unclear whether small mammals and other granivores consume or disperse American tarwort seeds; however, granivores likely avoid unpalatable or toxic seeds. Thus, species such as American tarwort —whose seeds are likely unpalatable—are expected to have more persistent seed banks because they are less susceptible to granivory [71]. For more information on this topic, see Palatability and nutritional value.

Germination: American tarwort has extremely low germination rates (<10%) compared with the average recorded for arid-land shrubs of the family Asteraceae ([146], Mauchamp 1992 cited in [147], Ferrer 2004 cited in [37]). Maximum germination rate of American tarwort seeds reported under the best laboratory conditions was 39% [147].

American tarwort seed germination peaks during the summer rainy season. In the laboratory, American tarwort seed germination increased when seeds were more than 3 months old, peaked for seeds 5 and 8 months old, and decreased for older seeds (14-26 months old). The peak in American tarwort seed germination corresponded with the peak of the summer rainy season, suggesting that percent germination of American tarwort seeds is low immediately after dispersal in winter and increases after cold winter temperatures and summer rains. Constant moisture increased American tarwort seed germination in the laboratory, suggesting that American tarwort seeds might have seed quiescence, where seeds only germinate when soil moisture has increased after the onset of the rainy season. In the laboratory, American tarwort seed germination was inhibited by a low ratio of red:far-red light, similar to that which occurs under dense canopies, suggesting that American tarwort seed germination is inhibited in areas with a dense canopy or litter cover [147].

Seedling establishment and plant growth: American tarwort reportedly grows at a slow rate [11,13]. Mauchamp and others [89] defined American tarwort seedlings as <1-year-old plants with stems <0.1 inch (2 mm) in diameter, and juveniles as 1- to 10-year-old, nonflowering plants, <20 inches (50 cm) tall with stem diameters between 0.1 to 0.8 inch (2-20 mm) [89].

American tarwort apparently reaches its best growth in full sun. In Durango, Mexico, American tarwort seedlings growing in full sun were significantly larger than shaded seedlings (P<0.05 for all variables). American tarwort seedlings emerged in shaded and unshaded sites, but as the dry season progressed, shaded American tarwort seedlings appeared more susceptible to drought-caused mortality than seedlings in full sun. Survival rates of seedlings beneath the canopy of adults were significantly lower than those located outside the canopy (P<0.05 for all variables) [89]:

Survival rates (%) of American tarwort seedlings under and outside of adult American tarwort canopy cover from 60 to 300 days after the end of the rainy season (15 September) [89]
Days Under adult canopy Outside adult canopy
60 28 64
140 16 53
220 3 27
250 0 18
300 0 3

These data suggest that the probability of a American tarwort seedling reaching the juvenile stage was very low under any conditions [89].

American tarwort grows taller on moist than dry soil [41,75,131]. In the Rio Grande Valley, American tarwort occasionally occurred on sandy soil and infrequently on shallow soil underlain by caliche, but reached its best development as a community on soils that received some run-off water. The largest individuals were observed in or along washes, although average height of plants along washes did not differ significantly from that of upland plants [41]. Because desert grasses and forbs generally depend on water from the same soil layers as American tarwort , American tarwort likely competes directly with these species for water. Thus, the ability of American tarwort seedlings to rapidly develop root biomass was hypothesized to be critical for establishment [101]. Roots of American tarwort seedlings can grow >16 inches (41 cm) in <1 year (see Roots) [89].

Herbivory may be an important cause of American tarwort mortality. In Durango, Mexico, browsing did not cause seedling mortality, but many dead juvenile American tarwort plants were observed with all of their branches browsed, most likely by jackrabbits [89]. On the piedmont slopes of the Doña Ana Mountains, jackrabbit herbivory reduced the size and vigor of American tarwort plants, and 47% of browsed individuals died [125].

Vegetative regeneration: As of this writing (2010), information regarding American tarwort vegetative regeneration is largely anecdotal. Barrow [5] stated that American tarwort "commonly" propagates asexually. On the plains below the Guadalupe Escarpment, prolonged freezing weather killed American tarwort plants "to ground level" [45], indicating some sprouting may have occurred. Campbell's [13] observation that American tarwort can survive wind action exposing its roots "because it spreads vegetatively by sprouts" suggested that American tarwort may spread by sprouting from roots. He further stated that American tarwort "spreads by vegetative means so that its very steady, once it becomes established" [13]. However, Scifres [128] reported that American tarwort sprout development was from stem sections, and that American tarwort spreads primarily via seeds. Noting that American tarwort is easily killed by herbicide, Tschirley [140] concluded that American tarwort was a "weak sprouter". As of this writing, no herbicide study reported American tarwort sprouting after herbicide treatment (e.g., [33,50,60,102,103,127,139]). In the San Simon Valley of southeastern Arizona, mammal (black-tailed jackrabbit, desert cottontail, and white-throated woodrat) browsing appeared to stimulate new growth on American tarwort , with new stems growing below the browsed points [20]. In contrast, on the piedmont slopes of the Doña Ana Mountains, New Mexico, American tarwort plants that were moderately to heavily browsed by jackrabbits showed no evidence of new stem growth below browsed points, although the leaves on browsed stems were visibly larger than the leaves on unbrowsed plants [125]. Other researchers reported that American tarwort can replace its leaves after defoliation. American tarwort in western Texas and southeastern Arizona apparently compensated for insect-caused damage to leaves in early spring by producing new foliage by midsummer [122]. After herbicide application in late winter, American tarwort refoliated several times during the 1st posttreatment growing season on the western Edwards Plateau, Texas [145]. In south-central New Mexico, after a dry winter and spring, American tarwort plants produced small, scale-like leaves in May and early June. These plants shed the small leaves and produced a new set of larger leaves as moisture availability increased in summer [113].

Apparently, American tarwort prefers full sun, fine to medium soil textures, soil pH from moderately alkaline to strongly alkaline, and shallow to moderate soil depths.

Topography: American tarwort -dominated communities commonly occur as bands on alluvial flats and plains below the slopes of hills and mesas, in patches on the edges of tobosa or burrograss swales, along drainages that dissect the plains, and on piedmonts and mesas where it often occurs within creosotebush habitat. Rarely, American tarwort habitats occur on steep, rocky slopes in a transition zone between creosotebush communities and upland desert grasslands. American tarwort also occurs near terrace edges and erosional escarpments where shifting sands, often dominated by mesquite (Prosopis spp.), encroach upon tobosa and burrograss habitats [11,13,69,75,84,121,153].

Habitats dominated by American tarwort typically occur on nearly level to gently sloping sites on all aspects. In the basin and range physiographic province in southeastern Arizona, American tarwort occurred most frequently on flat (<10% slope) and dissected topography on alluvial landforms, but occurred on a range of slope angles averaging <5% to >45% and on all aspects [104].

Elevation: Across its distribution in the United States, American tarwort occurs from 2,300 to 6,900 feet (700-2,100 m) [15,43,69,104,118,121]:

Elevational ranges reported for American tarwort across its geographic distribution in the United States
Location Elevation range


Cochise County 3,500-5,000 feet (1,100-1,500 m) [69]
southeastern Arizona 4,300-5,400 feet (1,300-1,650 m) [121]
southeastern Arizona primarily from 3,800-5,000 feet (1,200-1,500 m) [104]
New Mexico throughout 3,500-6,500 feet (1,100-2,000 m) [43]
throughout 4,600-6,900 feet (1,400-2,100 m) [15]
Texas Trans-Pecos 2,300-6,500 feet (700-2,000 m) [118]
Del Norte Mountains 4,500-4,700 feet (1,370-1,430 m) [14]

Soils: American tarwort dominates soils that are shallow to moderately deep, fine-textured silt or clay loams, often calcareous, and sometimes with impermeable or semi-permeable caliche or argillic horizons [121]. Variation in parent materials and exclusion of American tarwort by creosotebush in the most xeric, shallow soils may in part determine American tarwort 's distribution and abundance [19].

Texture: American tarwort grows in soils with a wide range of textures [108]. Most soils supporting American tarwort are moderately fine-textured to moderately coarse-textured [11]. American tarwort often dominates "heavy" silt or clay soils with some gravel near the surface (e.g., [1,9,13,28,32,49,67,108,111,155]). American tarwort also grows on sands and sandy loams [6,11,16,67]. In the San Simon Valley, American tarwort dominated a site with deep, well-drained, gravelly sandy loam soils [16]. American tarwort occurred occasionally on mesquite sand dunes in southeastern New Mexico [11]. A creosotebush-American tarwort community occurred on sandy or gravelly soil overlying a bed of Tornilla Clay in the Tornilla Clay beds area of western Texas [106].

Depth: Soils supporting American tarwort are shallow to moderately deep, although American tarwort seems to prefer moderately deep soils [11,19,121]. American tarwort grows intermixed with creosotebush, but American tarwort dominates on soils that are deeper than those in areas where creosotebush dominates [19,55]. In the San Simon Valley, where the soil was shallow (9 inches (24 cm) deep) over a dense, continuous caliche layer, creosotebush dominated the shrub cover (84%) and American tarwort occurred with 5% cover. In areas where the soil was deeper (24 inches (60 cm) deep), finer textured, and had less carbonate and a discontinuous caliche layer, American tarwort dominated the shrub cover (44% cover) and creosotebush was sparse (<1%) [19]. In Cochise County, Arizona, a soft, unconsolidated caliche layer was considered typical of American tarwort communities [67].

pH and parent materials: American tarwort occurs on mildly to strongly alkaline soils (range: 7.7-8.7) [9,28,67,108,130,132]. On a south-central New Mexican bajada, the creosotebush-American tarwort association occurred on soils with a pH of 7.7 [132]. In Brewster County, Texas, the creosotebush-American tarwort association occurred on soils with a pH ranging from 8.1 to 8.6 and the tobosa-American tarwort association occurred on soils with a pH of 8.7 [28]. American tarwort communities occurred on soils with pH of 8.4 at the Jornada Experimental Range [108]. In Cochise County, Arizona, average soil pH was 8.5 in American tarwort communities [67].

American tarwort occurs in soils derived from many parent materials. American tarwort most often occurs in limestone or calcareous soils [11,19,43,69,104]. Limestone is the predominant parent material in the Chihuahuan Desert, although gypsum and igneous rocks are parent materials in some areas [84,86,132]. American tarwort may occur on soil derived from igneous rocks but is absent or rare on gypsic soils [11,157]. O'Laughlin [108] concluded that gypsiferous soils were the only soils from which American tarwort has been found to be consistently absent. Unusually, Reid and others [121] reported scattered American tarwort on gypsiferous soil in the gyp dropseed alliance in New Mexico. American tarwort frequently occurs on alluvial soils. In the Del Norte Mountains, Texas, American tarwort was most common on deep, fine alluvial soils [14]. In southeastern Arizona, American tarwort occurred most frequently on alluvium parent materials (73% of plots) [104]. Calculations by Chew and Chew [19] using the data of Gardner (1959 cited in [19]) gave the following creosotebush:American tarwort cover ratios in soils of different parent materials: 1.4 on alluvium, 1.2 on limestone conglomerate, 0.9 on limestone, and 0.2 on shale. American tarwort was sparse on transported andesite (Gardner 1959 cited in [19]).

Moisture: American tarwort prefers relatively moist sites that receive some run-off water. Bajadas, or piedmont plains, are the weakly sloping plains that connect the mountains and hills with bottomland playas (ephemeral lakes), arroyos, and floodplains. Bajadas receive some water by sheet flow and also lose some water by run-off [100]. American tarwort often occurs on bajadas and in and on the edge of adjacent bottomlands [100,136]. American tarwort appears to avoid poorly drained areas such as playas and excessively drained areas such as high-elevation mountain slopes, preferring the moderately well-drained bajadas [100]. American tarwort often occurs within and adjacent to tobosa and burrograss grasslands [10,13,28,43]. These grasslands are often called "tobosa swales" or "burrograss swales" because they occur in depressions where surface run-off accumulates and stands for several days [13]. On the Jornada Plain, sites with abundant run-off water from high-elevation areas in the Jornada Plain typically supported abundant stands of tobosa with lesser amounts of American tarwort , but sites that were occasionally flooded by run-off water were dominated by American tarwort [11]. In Cochise County, Arizona, American tarwort individuals were more abundant and visibly more vigorous in that portion of the American tarwort community nearest the ecotone with tobosa, where soil moisture availability appeared to be more favorable [67]. In New Mexico, American tarwort occasionally occurred on the edges of dried playa beds as long as the soil was not sandy, with burrograss extending toward the center of the bed. American tarwort did not establish in the center of the playas, apparently because it cannot tolerate standing water [29] and/or possibly high salt concentrations [72] that sometimes occur in playas. American tarwort also frequently occurred along arroyo margins in southern New Mexico [26,73,111]. In south-central and southwestern New Mexico, American tarwort occurred in clay soils on barren or sparsely vegetated flats and bottomlands where moisture was often abundant due to run-off water from surrounding areas; in these areas, soil erosion was often "severe" and deep gullies were common [9]. American tarwort occurred in washes in Brewster County, Texas [28] and the Rio Grande Valley [41]. Herbel [58] described American tarwort as a meshophyte and creosotebush as a xerophyte in the creosotebush-American tarwort association. In contrast, Gehlbach [45] described American tarwort as one of the most xerophytic dominant plants of the Guadalupe Escarpment of New Mexico and Texas.

Differences in soil moisture preference may in part explain American tarwort and creosotebush distribution, with creosotebush tending to dominate the most xeric sites and American tarwort tending to dominate more mesic sites [58]. In San Luis Potosi, Mexico, creosotebush occurred in nearly pure stands except in places with high soil moisture, where American tarwort occurred with creosotebush [130]. At the Jornada Experimental Range, American tarwort dominated sites primarily on clay loam soils with some gravel near the surface on sites that received some flood water. In contrast, American tarwort occurred within creosotebush-dominated habitat on a variety of soils including low rolling ridges of the foothills and alluvial fans; well-drained areas of deep sand; deep, "heavy" soil; shallow, stony soils underlain by caliche; and arroyo beds [111].

Climate: American tarwort primarily occurs in the Chihuahuan Desert. Average annual precipitation in the Chihuahuan Desert ranges from 5.9 to 15.7 inches (150-400 mm) [86]. Most (>50%) precipitation in the Chihuahuan Desert occurs during convective thunderstorms during July to September, when evapotranspiration rates are high [10,19,65,111]. Evaporation rates during summer may be up to 10 times the precipitation [42,111]. Thunderstorms are frequently of high intensity, so much of the rainfall becomes surface run-off and thus is unavailable to plants [111]. Flash flooding and water erosion often occur in summer [159]. Most winter precipitation occurs as low-intensity rainfall and occasionally as snow in winter and early spring [111]. Late spring [19,111] and late fall [111] are commonly dry.

Interannual variability in precipitation is high, and drought is common in the Chihuahuan Desert [10,31,111,121]. Temperatures are often extreme and vary from -9 to 108 °F (-23 to 42 °C) [31]. Mean maximum monthly temperature (97 °F (36 °C)) occurs in June. Temperatures are ameliorated in July by the beginning of summer rains. Mean minimum monthly temperature (56 °F (13 °C)) occurs in January [19,31,111]. The frost-free season averages 200 days/year, but the effective growing season (the duration of favorable soil moisture and temperatures) is typically <90 days and occurs from July to September [31,111].

Weather extremes may kill American tarwort . In the San Simon Valley, an unusually long period of freezing weather (4 days) killed American tarwort stems [18]. On the plains below the Guadalupe Escarpment, prolonged freezing weather killed American tarwort plants "to ground level" [45]. At the Jornada Experimental Range, some American tarwort plants died after successive years of drought, which set back American tarwort encroachment into tobosa and other swale habitats [110]. In Durango, Mexico, water-stressed American tarwort seedlings had no leaves in May, whereas unstressed American tarwort seedlings retained their leaves [89].

Primary disturbances in communities with American tarwort are grazing, fire, drought, and flooding [72,123,141]. American tarwort has increased in density and distribution due to overgrazing by livestock, reduced fire frequency and severity, and climate change since the late 1800s [11,27,29,121]. Thus, American tarwort is often considered "invasive" in desert grasslands [11,28,49,56,111], and desert grasslands that have become dominated by shrubs are often referred to as "degraded" because they have increased soil erosion and reduced forage value for livestock and wildlife [14,18,27,104,162]. American tarwort may spread into black grama grasslands [11] but primarily spreads into communities on clay soils dominated by tobosa and burrograss [11,51,111]. While American tarwort has spread into many grasslands, creosotebush has encroached into American tarwort habitats in some areas since the late 1920s [11,48].

American tarwort primarily increases in desert grassland communities that have been "degraded" due to fire exclusion, overgrazing, or soil erosion. Herbel [58] noted that American tarwort spread into sites dominated by black grama or tobosa, after which sites are deteriorated by soil erosion and the formation of drainages. He described the creosotebush-American tarwort community as a "persistent subclimax" of the black grama-tobosa shrub steppe [58]. In an early study at the Jornada Experimental Range, the American tarwort -tobosa community was described as a "climax" plant community, and the American tarwort /burrograss community was described as seral to this climax [13]. Subsequent studies on the Jornada Experimental Range reported that American tarwort quickly spread into areas where the conversion of desert scrub to desert grasslands was attempted by use of mechanical methods [111,141]. In a study of the Big Bend Region of Texas, Muller [106] concluded that the creosotebush-American tarwort association is capable of forming a stable climax community. In Brewster County, Texas, the creosotebush-American tarwort association was considered climax vegetation for the desert plains in the region [28]. In a 1975 review, Tueller [141] concluded that in southern desert grasslands, a climax community dominated by grasses retrogressed after disturbance to a "disclimax" of short-lived perennial grasses, half-shrubs, cacti, and shrubs including American tarwort , creosotebush, viscid acacia, and mesquite. Another study described desert scrub as part of the desert grassland disclimax in which shrubs become dominant in grassland communities following disturbance [111].

As American tarwort cover increases, grass cover typically declines [11,41]. In a tobosa-American tarwort association in Brewster County, Texas, American tarwort and other shrubs were usually spaced 2 to 3 feet (0.6-0.9 m) apart, and bare ground was particularly abundant around American tarwort plants [28]. This suggests that habitats with greater American tarwort dominance are likely to have increased bare ground and less grass cover. However, American tarwort spread into desert grasslands appears to reduce grass cover less than mesquite or creosotebush spread [11,41]. In the Rio Grande Valley, grass cover in American tarwort communities was usually greater than it was on sites dominated by creosotebush or other shrubs in habitats with similar shrub cover. Grass cover in American tarwort communities (primarily burrograss and tobosa) ranged from 0.06% to 1.58%, and that on creosotebush communities (primarily fluff grass and burrograss) ranged from 0% to 0.77% [41]. Shrub cover in mesquite communities was greater and grass cover less than on American tarwort or creosotebush communities. The author noted, however, that as American tarwort cover increased, grass cover declined. American tarwort 's association with high grass cover was attributed to its preference for more favorable sites [41], such as those with deep and moist soils. At the Jornada Experimental Range, Buffington and Herbel [11] observed that "some areas invaded by American tarwort still have a good understory of grass", but noted that livestock forage production was "negligible" in many creostebush-dominated areas. In the Rio Grande Valley, American tarwort -dominated habitats always had some grass among the American tarwort plants, and American tarwort plants branched from ground level; whereas in creosotebush-dominated habitats with American tarwort , grass cover was lower and American tarwort plants appeared to have trunks because soil had eroded from their roots [41].

Creosotebush spread may reduce habitat quality for American tarwort as well as for grasses. In creosotebush-American tarwort communities in southeastern Arizona, a negative correlation between creosotebush density and American tarwort density indicated that creosotebush excluded American tarwort in the habitat (r= -0.43; P<0.01) [19]. In the Rio Grande Valley, creosotebush populations were expanding and dominance was increasing in areas dominated by American tarwort . Young creosotebush plants grew on the edge of American tarwort communities, but no young American tarwort plants grew in creosotebush communities. This suggested that creosotebush spread into American tarwort communities was due to changes in the complex of soil factors affecting these species distributions, especially the loss of surface soil due to erosion. Mesquite did not appear to be encroaching into American tarwort communities [41]. In a study of vegetation changes from 1858 to 1963 in desert grasslands of the Jornada Plain in southeastern New Mexico, creosotebush spread and excluded American tarwort in communities historically dominated by American tarwort . Mesquite also spread into American tarwort communities. Grass cover decreased with the spread of both American tarwort and creosotebush, although "there has been a much greater reduction of grass cover on creosotebush and mesquite areas than on American tarwort areas". Furthermore, where American tarwort spread into grasslands, loss of grass cover was slow, in contrast to the rapid replacement of grasses by mesquite [11]. In a subsequent study of vegetation changes in the Jornada Basin to 1998, Gibbens and others [48] agreed that much of the increase in creosotebush over the 150-year study "occurred at the expense of American tarwort ", particularly on bajada slopes. Overall, American tarwort declined in areas where it was once dominant and spread in areas formerly dominated by burrograss and tobosa [48]. Due to its tendency to be replaced by creosotebush in some disturbed habitats over the past 100 to 120 years, Dick-Peddie [29] proposed that the absence of American tarwort in desert scrub may indicate succession from desert grassland to desert scrub.

American tarwort cover is often greater on undisturbed sites than on severely disturbed sites. In southeastern Arizona, where American tarwort was removed from a site in San Simon Valley, American tarwort density was higher on ungrazed sites (6.3 plants/60 m²) and grazed sites (7.9 plants/60 m²) than on root-plowed sites (0.4 plants/60 m²) 16 years after disturbance [126]. Another study in the San Simon Valley reported higher American tarwort cover on undisturbed sites (6.0 plants/500 plots) than disturbed sites (grazed, burned, and burned and grazed sites; 0-2.2 plants/500 plots) 2 years after disturbance [149]. See Plant response to fire for more information on this study.

Establishment of American tarwort after disturbance is apparently slow, but American tarwort can dominate a site within 60 years after disturbance. Rango and others [119] examined American tarwort habitat in the Jornada Basin where all shrubs were removed by grubbing in 1937 and 1939 and noted that 63 years after the 2nd grubbing, cover of American tarwort , Berlandier's wolfberry, and bush muhly (Muhlenbergia porteri) increased to approximately the same as that on control plots the year of the treatment. Control sites had many small shrubs, whereas treated sites had few but large shrubs [119]. In another study in the Jornada Basin at the Jornada Experimental Range, a desert grassland site likely historically dominated by black grama, bush muhly, and spike dropseed (Sporobolus contractus) became dominated by American tarwort within 57 years of increased cattle grazing and periods of extended severe droughts. Within 140 years, the site was dominated by creosotebush with honey mesquite and American tarwort as subdominants [114].


SPECIES: Flourensia cernua

Immediate fire effect on plant: As of this writing (2010), no direct observations of immediate fire effects on American tarwort were recorded in available literature. However, fire likely kills American tarwort

Postfire regeneration strategy [133]:

Fire adaptations and plant response to fire: Fire adaptations: Due to its high resin content (see Botanical description) and "weak" sprouting ability (see Vegetative regeneration) fire likely kills American tarwort plants. American tarwort likely establishes from seed after fire. It is unclear as of this writing (2010) whether American tarwort seeds are killed by fire. Information on seed sources (on- or off-site) was limited. Some evidence suggests that American tarwort lacks a persistent (>2 years) seed bank. This, coupled with short dispersal distances of seeds, suggests that establishment of American tarwort by seed after extensive, stand-replacing fire is likely very slow. Anecdotal evidence that American tarwort may spread by sprouting from roots [13] suggests that it could reproduce vegetatively after fire as long as the roots are not damaged; however, no studies as of this writing (2010) reported American tarwort sprouting after fire. Due to limited studies, it is uncertain how quickly American tarwort establishes after fire. Studies are needed on American tarwort fire ecology.

Plant response to fire: Because American tarwort increases slowly after disturbances that remove shrubs and may not regain dominance for up to 60 years (see Successional Status), American tarwort likely decreases after fire and then increases slowly over time. Droughts likely slow the rate of American tarwort recovery following fire, but as of this writing (2010), no information is available. At the McGregor Missile Range, Fort Bliss, Texas, prescribed fire was applied in June in a creosotebush-American tarwort community. Immediately after the prescribed fire, grass and shrub cover were reduced and bare ground increased. American tarwort mortality averaged 29.1%. No information was provided on prefire cover or the fire's severity or patchiness [98]. A late May prescribed fire in a desert grassland with low American tarwort cover (<1%) reduced shrub cover to zero, reduced grass cover from 46% to 2%, and increased bare ground cover from 14% to 51% during the 1st postfire year [38]. In the San Simon Valley, prescribed burning was applied in June to a grazed and an ungrazed desert scrub community on a site historically dominated by desert grassland. Because there were insufficient fuels to carry a fire, all individual shrubs, grasses, and standing dead annuals were burned individually with a propane torch, resulting in 100% open ground and all plants at least top-killed. On ungrazed sites, American tarwort cover was 0% immediately after burning; 2.2% 3 months after burning; and 0.8% 2 years after burning. This indicated substantial American tarwort mortality after initial postfire establishment. It was unclear whether postfire establishment was by sprouts or seedlings. Grazed sites (both burned and unburned) had no American tarwort and untreated sites had the highest American tarwort cover throughout the study [149]:

Mean American tarwort cover across 4 treatments 3 and 24 months after prescribed burning and livestock grazing [149]
Time since treatment
Number of American tarwort plants/500 plots
Burned Grazed Burned and grazed Untreated
3 months 2.2 0 0 4.8
24 months 0.8 0 0 6.0


Fuels: Stand structure in plant communities where American tarwort occurs is typically open with scattered shrubs, grasses, and forbs, and extensive rock and bare ground. Vegetation may vary from very open to moderately dense (e.g., [10,14,55,65,121]). Potential fire spread depends greatly on stand structure, which is influenced by site conditions. The frequency and severity of fires in desert ecosystems are highly contingent on the degree and rate of grass growth to serve as fuels, which may be determined by the timing and amount of precipitation prior to and following fires, plant physiology, soil characteristics, and the degree of livestock grazing [31,64,91]. In general, productivity—and consequently fuel load—is low in Chihuahuan Desert communities except when there is a year of exceptional winter rains resulting in a heavy stand of annuals to serve as fuels [65].

Total plant cover in communities where American tarwort occurs is typically sparse (<40%). Rock and/or bare ground cover commonly exceeds 60%, and litter cover is often <10% (e.g., [4,41,51,67,92,129]). At the Jornada Experimental Range, American tarwort -dominated communities had 17% total shrubs, 21% herbs, 57% bare ground, and 5% litter [51]. In Hidalgo County, New Mexico, American tarwort occurred in creosotebush-broom snakeweed habitat with approximately 20% total shrubs, 70% rocks and bare ground, and 10% litter [4]. On the Stockton Plateau in western Texas, in American tarwort -burrograss communities, cover ranged from 13% to 21% plants, 69% to 82% mineral soil and rock, and 2% to 4% litter. In American tarwort -purple threeawn communities, cover ranged from 32% to 43% plants, 27% to 34% rock and mineral soil, and 32% to 40% litter [92]. In Cochise County, Arizona, American tarwort -dominated communities had a mean cover of 32% perennials, 10% annuals, 61% bare soil, 20% rock, and 7% litter [67]. Grass cover typically decreases as American tarwort cover increases, suggesting that American tarwort -dominated habitats are likely to have sparse, widely spaced fine fuels, and fire is unlikely to carry well. However, American tarwort -dominated habitats may be more likely to carry fire than creosotebush-dominated habitats because of relatively greater fine fuel loads in American tarwort -dominated habitats [11]. See Successional Status for more information on this topic.

The amount of resin a plant contains affects how readily and intensively it burns. Although not reported as of this writing (2010), the "extremely" resinous foliage and stems of American tarwort [35] may make it highly flammable.

American tarwort growth habit.
Photo courtesy of Russ Kleinman and Deming Gustafson, Burro Mountains, Hoodoo Canyon, Gila National Forest

Fire regimes: Historically, fire was a natural event in Chihuahuan Desert ecosystems, although some parts of the Chihuahuan Desert may have never experienced fire historically [12,29,64,95]. Lightning-caused fires occurred just prior to and throughout the growing season, particularly during June when dry conditions persist and high winds are common [31].

American tarwort occurs as scattered individuals in Chihuahuan Desert grasslands where fire was historically frequent (<10-year intervals) [31,163]. Fire-return intervals coincided with the time required for herbaceous fuels to accumulate to the point where they could carry fire (Cable 1967 cited in [31]). Historically, frequent fires interacted with other factors including topography, soil, herbivory, and herbaceous plants to restrict woody plant establishment in grasslands [29,63,64,65,95]. Even occasional fires in desert grasslands may have prevented establishment of woody species, thus maintaining grasslands [65,112,144]. American tarwort occurs most frequently in desert scrub communities. Historically, Chihuahuan Desert scrub was restricted to ravines, knolls, and sites where conditions were unfavorable for grasses, and consequently where fuels were sparse and would not carry fire over large areas [19,44,86]. Fire-return intervals in desert scrub were greater than in desert grasslands on average [112]. When fires occurred they were typically stand replacing. Desert scrub was most likely to burn in the ecotone where it bordered desert grassland [65].

Burgess [12] commented that "it would be assume that fire is a universal feature of desert grasslands". Some parts of the Chihuahuan Desert may have never experienced fire historically due to a combination of patchy vegetation and low fuel loads [12,29,64]. On the Jornada Plain, no extensive fires were reported historically, and fire was not considered a factor in the maintenance of these grasslands [11]. No fires were reported historically at the Mapimi Biosphere Reserve, Durango, Mexico [101]. Grassland fires leave behind little to no direct evidence of fires so estimates of historic fire frequency and extent are based on historical accounts of early settlers and indirect evidence [64,95]. Nonetheless, most researchers agree that fire has historically been common in most desert grasslands [95]. For reviews of fire history in North American desert ecosystems in general and Chihuahuan Desert ecosystems in particular, see these sources: [7,31,95]. See the Fire Regime Table for further information on fire regimes in vegetation communities in which American tarwort may occur. Find further fire regime information for the plant communities in which this species may occur by entering the species name in the FEIS home page under "Find Fire Regimes".

Since the late 1800s, frequency and severity of fires in Chihuahuan Desert ecosystems was lessened directly by fire exclusion and indirectly by livestock grazing. Overgrazing by livestock decreased cover and biomass of herbaceous species necessary to carry fire. Grasses stabilize desert soils, and reduction of grasses by overgrazing led to soil erosion and reduced likelihood that grasses would return to dominance. Drought periods and possibly gradual warming of the climate of the Southwest enabled trees, shrubs, and cacti to spread into desert grasslands (e.g., [7,19,29,31,31,64,65,70,87,118]). American tarwort spread into desert grasslands of the Jornada Plain from 1858 to 1963 may have been due, in part, to reduced fire frequency resulting from overgrazing and a reduction of fine fuels [11].

Prescribed fire is often recommended for preventing the encroachment of shrubs such as American tarwort into desert grasslands [7,64]. Because American tarwort may not reproduce until it is >10 years old (see Seedling establishment and plant growth), prescribed fires occurring at <10-year intervals are likely to eliminate American tarwort . Humphrey [64] surmised that prescribed burning in desert grasslands at 5- to 10-year intervals would reduce abundance of shrubs such as American tarwort and maintain desert grasslands. He cautioned, however, that heavily grazed desert grasslands typically do not have enough fuel to effectively carry a prescribed fire that would be "hot" enough to kill large shrubs [64]. In a 1993 review of fire effects on mixed desert grass-scrub communities, prescribed fire was not recommended for desert habitats with moderate to high shrub cover because there is typically not enough fine fuel to carry a fire. The authors suggest a minimum fine fuel load of 600 lb/acre (674 kg/ha) to support prescribed fire for controlling shrubs. Burroweed, broom snakeweed, creosotebush, young mesquite, and cactus are mentioned. The researchers recommended applying prescribed fire during dry seasons that follow 1 to 2 years of above-average summer precipitation, when enough fuel has accumulated and grasses are most likely to increase after fire [144]. Summer prescribed fires were generally considered more likely to damage shrubs than winter fires [144]. A 1975 review recommends against prescribed fire in several areas in the Chihuahuan Desert because fires that kill shrubs such as American tarwort usually kill grasses too, and fires that spare grasses may not kill shrubs [87]. Although their study site did not include American tarwort , Rogers and Steele [124] recommended a conservative view toward use of fire in deserts because of slow recovery rates.

Presence of nonnative invasive species such as lovegrass (Eragrostis spp.) may alter fire regimes in desert grasslands and limit the effectiveness of prescribed fire for restoring native grasses [93]. Lehmann lovegrass (E. lehmanniana), common in parts of the Chihuahuan Desert including the Jornada Experimental Range [93], increases after fire, produces more fine fuel than native grass species, and probably contributes to increased fire spread [95]. In a review, McPherson [95] suggested that a positive feedback pattern may develop between lovegrass and fire, resulting in further reduced forage in desert ecosystems for livestock and wildlife. Such as feedback loop could cause fires to occur more frequently than in the 1900s; however, fires would likely still be less frequent than occurred historically. For more information on the interactions of fire and nonnative invasive species, see the FEIS review for Lehmann lovegrass and other species of interest.


SPECIES: Flourensia cernua


Information on state- and province-level protection status of plants in the United States and Canada is available at NatureServe.

American tarwort is unpalatable and not generally used by wildlife or livestock, unless little else is available [75,131].

Palatability and nutritional value: American tarwort leaves have high concentrations of plant secondary compounds [74]. The peppery, bitter quality of American tarwort herbage makes it unpalatable to wildlife and livestock [25,69,75].

Wildlife: American tarwort is browsed by black-tailed jackrabbits [20,89,125,135], desert cottontail, and white-throated woodrats [20]. Heavy jackrabbit browsing may decrease American tarwort abundance. At the Jornada Experimental Range, American tarwort cover decreased in lagomorph-grazed areas compared to exclosures, probably due to browsing of the American tarwort canopy and seedlings [47]. On the piedmont slopes of the Dona Ana Mountains, New Mexico, jackrabbits preferentially browsed American tarwort in creosotebush-American tarwort communities [125]. Chew and Chew [20] reported that in addition to eating American tarwort , black-tailed jackrabbits, desert cottontail, and white-throated woodrats in the San Simon Valley "pruned" American tarwort , removing an average of 46% to 48% of the volume of individual American tarwort plants along transects. Some American tarwort plants were browsed close to the ground. However, much of the American tarwort browse was uneaten and found lying on the ground next to the plants [20].

American tarwort fruit and browse may be toxic to some wildlife species, particularly if they feed on it exclusively. Pronghorn antelope in the Trans-Pecos region of Texas reportedly died from American tarwort consumption because of a lack of other available forage [53]. As of this writing (2010), it is unclear whether small mammals and other granivores eat American tarwort seeds. Captive Merriam's kangaroo rats obtained from the San Simon Valley ate American tarwort seeds when presented with pure diets of the seeds. However, they lost weight rapidly [20]. American tarwort seed predators include fly (Euarestoides acutangulus) and beetle (Smicronyx spretus and S. profusus) larvae ([122], Mauchamp 1992 cited in [147]) and possibly ants (order Hymenoptera) [99].

American tarwort is host or alternate host to >90 species of insects, spiders, and arthropods throughout its distribution in the United States and northern Mexico [17,35,122]. Many of these feed on American tarwort and some, such as the leaf beetle Zygogramma tortuosa, may cause defoliation leading to reduced seed production in American tarwort [35]. For more information on this study, see Seed production. Two invertebrates, a grasshooper (Ligurotettix planum) and a moth (Bucculatrix flourensiae), apparently feed exclusively on American tarwort , although they do not seem to affect American tarwort growth, seed production, or mortality [122].

Livestock: American tarwort is unpalatable and not generally used by livestock, unless little else is available [75,131]. The buds, flowers, fruit (immature or ripe), and possibly seeds are toxic to domestic sheep, domestic goats, and cattle [30,75,131,135]. American tarwort herbage appears less toxic to livestock than other parts of the plant. Cattle, domestic sheep, and domestic goats can apparently consume modest quantities (≤30%) of American tarwort forage in mixed diets without ill effects [2,61,74,83]. However, a diet exclusively of American tarwort leaves may lead to malnutrition, weight loss, and possibly death [30]. Some researchers reported high nutritive value of American tarwort leaves that met the requirements of livestock but also high values of "anti-nutritional factors" such as phenolics [30,35].

Cover value: American tarwort provides shade for animals in summer [75]. It is used as escape cover and foraging sites for lizards, such as the western whiptail and side-blotched lizard [4,115], small mammals, such as the white-throated woodrat, cactus mouse, western harvest mouse, and Marriam's kangaroo rat [4,8], and birds such as sparrows and northern mockingbird [138]. In Fort Bliss, Texas, 5 species of birds nested in American tarwort , and American tarwort was the 4th most frequently used nest plant species in both arroyos and upland habitats [76]. Despite the fact that American tarwort provides "few hiding places" [109], the Pecos clicker grasshopper (Ligurotettix planum) lives and feeds almost exclusively on American tarwort [109,122,129].

No information is available on this topic.

In northern Mexico, American tarwort leaves and flower heads were used historically to make a decoction for treating indigestion [69,77,118,135,153].

American tarwort is unpalatable to livestock, and it increases as overgrazing reduces palatable forage plants, often becoming dominant on overgrazed sites [18,29,43,121]. Overgrazing reduces grass cover in desert grasslands below the levels where they can carry fire, thus reducing fire frequency and severity [64,163]. Mechanical controls (e.g., hand grubbing, bulldozing, and root plowing) and subsequent seeding [59,96,126], herbicide application (e.g., [33,50,60,102,103,127,139,145,154]), and prescribed fire [38,98,149] have been used in American tarwort -dominated areas to reduce or eliminate American tarwort and other shrubs and increase grasses (reviews by [56,57,148]). Biological control methods have been considered for American tarwort [27,122], but as of this writing (2010) no such methods have been implemented either in the United States or Mexico. Results of studies attempting desert grassland restoration in habitats with American tarwort were varied. Treatments were generally successful at reducing or removing American tarwort in the short term, but in some cases desirable native grasses (e.g., black grama) did not increase, unpalatable native (e.g., fluff grass and broom snakeweed) or nonnative (e.g., lovegrass) grasses increased or remained the same, and American tarwort and other shrubs returned [13,27,96,126,148,150]. The rate, direction, and amount of vegetation change observed after removal of either grazing or shrubs varies depending on many complex and interrelated factors, including weather, initial and posttreatment species composition and abundance, and nearby seed sources [6,149].

Some researchers reported reduced American tarwort and other shrub cover and increased perennial grass cover in habitats where grazing was excluded. Near the Black Mountains in Catron County, New Mexico, 25 years of grazing exclusion in honey mesquite-American tarwort habitat eliminated American tarwort , reduced honey mesquite, decreased fluff grass, and increased black grama [117]. In the San Simon Valley, American tarwort -dominated areas protected from livestock grazing for 18 to 19 years had increased abundance of palatable perennial grasses, particularly black grama, compared to grazed sites (P<0.02) [18].

In contrast, others researchers reported little or very slow change in cover of shrubs and grasses with protection from grazing for up to 20 years in habitats with American tarwort (e.g., [6,18,59,149]). In southeastern Arizona, American tarwort and unpalatable fluff grass were more abundant on sites grazed by cattle during and prior to the study than on sites ungrazed for 16 years (P<0.05 for both variables). Palatable native perennial grasses such as bush muhly and Arizona cottontop (Digitaria californica) were similar between grazed and ungrazed sites; thus grazing exclusion did not improve cattle forage. In broom snakeweed-American tarwort -burroweed (Isocoma tenuisecta) communities in the San Simon Valley, there was little difference between a site ungrazed for 20 years and a nearby grazed site. However, a site ungrazed for 39 years had significantly higher native perennial grass cover (e.g., black grama, tobosa, cane bluestem (Andropogon barbinodis)) and significantly lower shrub cover than a nearby grazed site, and nearly all the increase in native perennial grass cover occurred during the latter 20 years (P<0.03 for all variables). American tarwort cover was similar on grazed (4.8%) and ungrazed (4.4%) sites after 39 years of grazing exclusion [150].

Precipitation patterns influence plant growth and persistence in desert ecosystems [110], and weather likely plays a substantial role in the effects of disturbance on American tarwort populations. In Big Bend National Park, historic records over a 26-year period after removal of livestock from desert grassland communities indicated that during the first 5 years of livestock exclusion, a period of drought, little change in cover was observed. American tarwort and creosotebush cover increased from 5 to 12 years after livestock exclusion, a period of summer drought and frequent wet winters. From 12 to 26 years after livestock exclusion, cover and density of forbs, perennial grasses, and most shrubs except creosotebush increased when summers were wetter than average. No control plot was available for comparison, so the authors could not isolate the effects of livestock grazing exclusion from the effects of weather [161].

Extended dry periods strongly influence dominance in desert plant communities. Buffington and Herbel [11] speculated that periodic drought most likely contributed to American tarwort spread into burrograss and tobosa grasslands [11]. Typically, the lower, most poorly drained parts of basins are dominated by tobosa. On bajadas, the slightly better drained areas fringing these flats, American tarwort often dominates, replacing tobosa. Thus during periods of sustained drought, as basins become drier, American tarwort is able to spread into tobosa swales. Further up slope, on excessively drained sites, creosotebush replaces American tarwort . As bajadas become progressively drier due to sustained drought, creosotebush spreads into American tarwort -dominated sites [24].

Long-term climate changes are likely to cause vegetation changes in desert ecosystems. Rising carbon dioxide levels predicted with global climate change are predicted to favor C3 woody plants such as American tarwort more than herbaceous plants, particularly C4 grasses [27,91]. This relationship may partially account for the increase in shrubs such as American tarwort in desert grasslands in the early 1900s [27]. However, it was difficult to determine whether vegetational changes were due to carbon dioxide, climate, or other causes such as overgrazing [27]. Increasing greenhouse gas concentrations in the atmosphere and increasing temperatures may result in either increased fire frequency in desert ecosystems due to hotter, drier weather, or to decreased fire frequency due to decreased fine fuel production. Climate may not only become drier but also more variable as a result of increasing greenhouse gases. The impacts of this change are likely to be complex and difficult to predict [95].


SPECIES: Flourensia cernua
The following table provides fire regime information that may be relevant to American tarwort habitats. Find further fire regime information for the plant communities in which this species may occur by entering the species name in the FEIS home page under "Find Fire Regimes".

Fire regime information on vegetation communities in which American tarwort may occur. This information is taken from the LANDFIRE Rapid Assessment Vegetation Models [81], 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.
Southwest South-central US
Vegetation Community (Potential Natural Vegetation Group) Fire severity* Fire regime characteristics
Percent of fires Mean interval
Minimum interval
Maximum interval
Southwest Grassland
Desert grassland Replacement 85% 12    
Surface or low 15% 67    
Desert grassland with shrubs and trees Replacement 85% 12    
Mixed 15% 70    
Plains mesa grassland Replacement 81% 20 3 30
Mixed 19% 85 3 150
Plains mesa grassland with shrubs or trees Replacement 76% 20    
Mixed 24% 65    
Southwest Shrubland
Desert shrubland without grass Replacement 52% 150    
Mixed 48% 165    
Southwestern shrub steppe Replacement 72% 14 8 15
Mixed 13% 75 70 80
Surface or low 15% 69 60 100
Southwestern shrub steppe with trees Replacement 52% 17 10 25
Mixed 22% 40 25 50
Surface or low 25% 35 25 100
South-central US
Vegetation Community (Potential Natural Vegetation Group) Fire severity* Fire regime characteristics
Percent of fires Mean interval
Minimum interval
Maximum interval
South-central US Grassland
Desert grassland Replacement 82% 8    
Mixed 18% 37    
South-central US Shrubland
Southwestern shrub steppe Replacement 76% 12    
Mixed 24% 37    
*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.
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 [54,80].

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