BOTANICAL AND ECOLOGICAL CHARACTERISTICS
SPECIES: Digitaria californica | Arizona Cottontop 



GENERAL BOTANICAL CHARACTERISTICS : 
Arizona cottontop is a native, perennial, long-lived, warm-season
bunchgrass [8].  It has slender, erect stems which are from 12 to 40
inches (30-100 cm) tall.  The leaves are normally 3 to 5 inches (8-13
cm) long, but may be up to 10 inches (25 cm) in length.  They are 0.125
to 0.25 inches (2-6 mm) broad [14,20].  The inflorescence is a narrow,
densely flowered panicle, usually 3 to 4.75 inches (8-12 cm) long [14].
Spikelets are normally borne in pairs on the rachis.  The cottony
seedhead is covered with long silky hairs [24], and the seeds are about
0.07 inches (1.7 mm) long.  Arizona cottontop's root system is finely
divided and branched.  It is concentrated mostly in the upper 8 inches
(20 cm) of soil, but in coarse-textured soils it extend down to about 40
inches (100 cm) [8].


RAUNKIAER LIFE FORM : 
   Hemicryptophyte


REGENERATION PROCESSES : 
Arizona cottontop reproduces from seed, and can be either
self-pollinated or outcrossed.  Over half of the florets of an Arizona
cottontop plant are self-pollinated.  The self-pollinated ovaries can
mature to viable seed even if lack of soil moisture prevents the panicle
from emerging from the sheath [8].  Seeds drop from the branches at
maturity [20].

In a seed longevity test, Arizona cottontop seeds kept under
uncontrolled conditions maintained over 80 percent germination for 3
years, with declining germination of about 6 percent per year for the
next 14 years, to less than 10 percent [8].  In another study of seed in
uncontrolled storage, Arizona cottontop showed 25 percent germination
after 25 years [36].

Arizona cottontop reproduces vegetatively from basal buds, which sprout
primarily in the spring, producing the year's crop of basal culms, most
of which mature the following summer [8].


SITE CHARACTERISTICS : 
Arizona cottontop is found on plains and hillsides on open,
well-drained sites [14,17].  Some of the diverse areas in which it
occurs include mesas and rocky hills in Arizona [24], deep hardland
range sites in Texas [5], and broad alluvial plains, fans, and river
bottoms in the Chihuahuan and Sonoran deserts [10].

Arizona cottontop grows on a wide variety of soils, including clayey loam,
sandy loam, and loose gravelly soils, as well as limestone ledges and
porphyritic hills.  However, it is more abundant and productive on
clay, sand, or sandy-loam subsoils than on shallow, stony, or cobbly
soils [8].

In the low-fertility soils of some desert sites, Arizona cottontop
thrives under mesquite shrubs, where the nitrogen, sulfur, and
phosphorus availability is much higher.  Cottontop shows evidence of
chlorosis and low fertility on some open desert sites [35].

Arizona cottontop has been recorded at the following elevations
[4,5,10,14,17,27,29,40]:

               Area       Elevation (feet)    Elevation (m)

          Arizona           1,000-6,000         305-1,830                     
          Chihuahuan           
            and Sonoran
            deserts             0-6,050           0-1,859
          Colorado          5,500-5,800       1,675-1,770                  
          Mexico            3,940-5,250       1,200-1,600
          New Mexico           4,300             1,310
          Texas             1,260-3,200         384-975

Within its geographical range, Arizona cottontop grows in a wide variety
of precipitation regimes, from areas of spring and summer maxima
separated by dry periods in Arizona, to high-summer, low-winter types in
Texas [8].  In much of its range, precipitation arrives during two
periods:  with winter frontal systems during November to March and with
summer convection storms from June to September.  However, across the
region annual precipitation and time of occurrence is extremely
variable, and extended drought is common.


SUCCESSIONAL STATUS : 
Facultative Seral Species

Arizona cottontop is listed as an important member of the semidesert
grassland climax vegetation [8].  It is also a climax species in the
shortgrass community in Texas [2], and is listed as a dominant in the
mixed-prairie association in north-central Texas [5].  It is a climax
decreaser, becoming less abundant where rangelands are overgrazed
[11,30].  On some sites it becomes the dominant species when protected
from grazing, greatly surpassing other grasses in abundance [8].
Arizona cottontop is usually found interspersed with other grasses [20],
but in some wetter sites can form almost pure stands [8].


SEASONAL DEVELOPMENT : 
Arizona cottontop is dormant during dry periods, but utilizes both
winter and summer precipitation.  However, most herbage is produced
during the summer growing season [8].  It responds quickly to spring and
summer rains, making rapid growth [20], and also responds with rapid
growth following winter rains [15].  Essentially all basal culms
produced in any given year sprout during the spring growing period;
summer growth on most basal culms is a continuation of growth on shoots
that sprouted in spring [8].  Shoots are produced throughout the growing
season.  Inflorescences begin emerging 2 to 3 weeks after growth starts
in the summer, and the topmost spikelets begin to fall within 5 to 8
days of full emergence.  All seeds on a panicle have usually fallen
within 7 to 8 days of first shatter.  New panicles are produced as late
in the fall as soil moisture is available [8].  Plants can set seed from
July to November when conditions are favorable [14].  Germinable seed is
therefore present whenever conditions are favorable for germination and
establishment.

Because Arizona cottontop has low-level apical dominance and a large
reservoir of buds at culm nodes, numerous axillary shoots develop over
the growing season.  Removing the growing point at the beginning of the
summer growing season stimulates the sprouting and growth of axillary
shoots [8].

Related categories for Species: Digitaria californica | Arizona Cottontop