BOTANICAL AND ECOLOGICAL CHARACTERISTICS
SPECIES: Pleuraphis  mutica | Tobosa 



GENERAL BOTANICAL CHARACTERISTICS : 
Tobosa is a native, warm season, perennial, rhizomatous, sod-
forming grass [33,40,46,49].  Plants grow up to 3 feet (0.9 m), but 1 to
2 feet (0.3-0.6 m) is more the norm.  The smooth erect culms have
decumbent bases and are normally 1.2 to 3 feet (30-75 cm) tall [40,46].
Leaves are up to 6 inches (15 cm) long, stiff, harsh and hairless
[21,49], occuring mainly as a mass of basal leaves with only a few
located along the stem [11].  Rootstocks are strong, creeping, woody and
scaly.  Dense coarse and fibrous roots grow to depths of nearly 6 feet
(1.8 m) [37], but are poorly adapted to efficient absorption of moisture
from the soil [9].  The inflorescence consists of an erect spike 1.5 to
3 inches (4-8 cm) long, broad, and white, straw colored or occassionally
purplish [29,40,46].  Spikelets are upright in clusters of three [49],
with about 30 fascicles on each spike [8].  Each fascicle is shed as a
unit [8].


RAUNKIAER LIFE FORM : 
   Geophyte


REGENERATION PROCESSES : 
Throughout its range, tobosa is capable of both vegetative and
sexual means of reproduction.  Vegetative reproduction through the
initiation of coarse rhizomes is most prevalent.  Tobosa has very
low seed production [46], with very few seedlings found in wild
populations [9].  Very few fertile seeds are produced.  This high
percentage of sterility may be attributed to a high ovule abortion rate
[8], and from heavy parasitism of the inflorescence by fungi [18].  For
seeds that are fertile, many fail to germinate; percent germination is
reported at 55 % [48].  Seed germination and seedling survival are
dependent upon adequate moisture and soil temperature.  Fluctuating
climatic conditions of late spring and early summer may be responsible
for poor germination and lack of subsequent seedling success.  When
viable seeds of tobosa were planted in high vs low temperature
regimes, the percentage of emergence was much higher at a low soil
temperature.  Viable seeds planted in a low soil temperature of 102
degrees F (39 C) had a 73.3% emergence, while those planted in a high
soil temperature of 127 degrees F (53 C) had only 24 % emergence [41].
In this study, growth and survival of emerging seedlings was also
lowered by high soil temperatures.  Due to all these factors seedling
survival in the field is generally low, making vegetative reproduction
the primary means of propogation.

Vegetative reproduction is through a system of well developed, large
rhizomes connected to a coarse root system [9].  This allows tobosa
to spread vegetatively into adjacent communities, although this spread
is relatively slow.  On favorable sites, it may form a dense, coarse sod
[9,37].  Frequent clipping stimulates vegatative reproduction.  Canfield
[10] postulated that clipping aided in activating buds which otherwise
may have remained dormant, but this stimulus is dissipated if plants are
clipped too closely.  He found that clipping plants weekly to a 4 inch
(10 cm) stubble produced 110% more total herbage than plants clipped at
2 inches (5 cm).


SITE CHARACTERISTICS : 
Throughout its range, tobosa is found on both lowland and upland
sites.  Most characteristically though, it is found in soils that have
developed from basin fill material.  These are clay soils which are
tight and relatively impervious [37].  In these depressions where
surface runnoff accumulates and stands for a few days, it attains its
best development.  Here it is found in almost pure stands forming a
dense, coarse sod [9,21,37,49].  Although tobosa prefers areas
subject to flooding, it is one of the first grasses to die when
submersed for periods of several months [49].  Typically the majority of
pricipitation occurs in the summer months, with 50% or more occuring
from July through September [36,37].  It is found in mean annual
precipitation zones of 9 inches (23 cm) in New Mexico to 20 inches (50
cm) in Texas.

Tobosa also occurs on dry mesas, sandy-loam hills and rocky slopes.
On these dryer sites it grows in scattered stands of large, individual
tufts and is fairly resistant to drought [37,49].  During extended
drought periods though it suffers more than its associates.  It is
intolerant of shifting sand [9].  

Typically growing on clay soils it is commonly associated with honey
mesquite (Prosopis glandulosa).  Other common associates on adobe clay
soils include burrograss (Scleropogon brevifolius), alkali sacaton
(Sporobolus airoides) and sacaton (S. wrightii).  On sandy clay or
gravelly clay loams, tobosa is often found with tarbush (Flourensia
cernua), black grama (Bouteloua eriopoda), side-oats grama (B.
curtipendula), dropseeds (Sporobolus spp.) and muhly grasses
(Muhlenbergia spp.).

Elevational ranges for some western states are presented below
[21,23,25,36,52]:

from  2,460 to 5,906 feet (750 to 1800 m) in AZ
from  3,000 to 6,500 feet (914 to 1981 m) in NM
from  1,600 to 3,300 feet (488 to 853 m)  in TX


SUCCESSIONAL STATUS : 
Facultative Seral Species

Tobosa along with black grama (Bouteloua eripoda) are two of the
most common grasses in the semi-desert grassland [6].  Black grama is
most characteristically found on gravelly upland sites, while tobosa is
the climax diminant of lowland sites with heavy soils which receive some
surface runoff from upland sites.  Where it reaches climax on these clay
soils it often forms dense almost pure stands.  In some areas burrograss
(Scleropogon brevifolius) and dropseed (Sporobolus auriculatus), which
are indicitive of late seral stages on these clay soils, increase during
drought years as the tobosa stands do not receive surface runoff [9].
When sufficient moisture returns tobosa is able to recover and will
eventually dominate the stand.
   
Tobosa is able to outcompete other grass associates unless overgrazing
or drought occur [9].  In some areas the native grasses have been
replaced by introduced annuals due to grazing.  Brown [6], notes that in
these areas tobosa may be the only native grass still remaining,
but the reasons for this are not stated.  Vast areas of tobosa
range are now dominated by creosotebush (Larrea tridentata), tarbush
(Flourensia cernua), and honey mesquite (Prosopis glandulosa), having
the appearance of a short statured "soft chaparral".  Suppression of
fire and dispersal of seeds by cattle and rodents have caused this
disclimax vegetation which simulates the climax in terms of stability,
but is replaced by the true climax in the absence of disturbance
[6,16,47].  Generally plants survive burning to become an early seral
component of the postburn community.


SEASONAL DEVELOPMENT : 
Tobosa begins its growth in the late spring or early summer after
sufficient precipitation ensures high soil moisture.  In many areas the
first rains of summer are torrential in character and flood the low
swales occupied by tobosa; until the flooding occurs little or no growth
takes place [11]. Most of the growth begins from terminal nodes on solid
perennial stems, with only a small amount of growth occuring from
tillers [33].  Most growth occurs in the spring and about 20 % occurs in
the early fall [23,37].  Growth may occur any time during the frost free
season as long as sufficient moisture is available [10].  Tobosa is
drought tolerant and has the ability to become completely dormant as
soil moisture approaches the wilting point [26].

Related categories for Species: Pleuraphis mutica | Tobosa