BOTANICAL AND ECOLOGICAL CHARACTERISTICS
SPECIES: Aristida purpurea | Purple Threeawn 



GENERAL BOTANICAL CHARACTERISTICS : 
Purple threeawn is a warm-season, native perennial bunchgrass
[5,8,22,26].  It is a mid-grass, with 6- to 12-inch (15-30.5 cm) culms.
Leaves primarily grow in basal tufts, but there are a few culm leaves.
The inflorescence is a panicle.  Florets have sharp-pointed lemmas with
stiff, hairy calluses and three-parted awns.  Awns are 1 to 5 inches
(2.5-13 cm) long [15,25,56].  Roots are moderately deep.  On widely
scattered sites on short- and mixed-grass prairies from South Dakota to
Kansas, Weaver [58] found that maximum depth of purple threeawn roots
averaged 4 feet (1.2 m).

Purple threeawn is highly competitive during droughts lasting only a few
years [19,39].  It tends to decrease during periods of extended drought.
In eastern Colorado during the drought of 1931-1937, Fendler threeawn
nearly disappeared from the Fendler threeawn/blue grama-buffalograss
communities it once dominated.  It reestablished during the 1940's, a
decade when regional precipitation was mostly above normal [39,59].


RAUNKIAER LIFE FORM : 
   Hemicryptophyte


REGENERATION PROCESSES : 
Purple threeawn regerates by seed and by tillering [15,19].  Seed crops
are usually plentiful [53].  With adequate summer and fall rainfall,
plants in the Southwest may produce two seed crops:  one in spring and
one in fall [34,43].  Second seed crops are rare, however, because
late-season rains are seldom abundant enough to support a second seed
crop [34].  Upon seed shatter, the seed falls nears the parent plant or
is dispersed by animals when the long seed awns catch on their hides
[56].  The combination of divergent awns and a sharp-pointed callus
promotes rapid penetration of seed into soil [15,16].  Purple threeawn
apparently maintains a persistent seedbank [35].  

Seed usually germinates in spring, but may germinate in fall in warm
climates.  There is no light requirement [14,15,30], but high
temperatures are required for germination.  In northern locales,
temperatures high enough to stimulate purple threeawn germination
generally occur only in spring on flats and low-elevation, south- and
west-facing slopes [15].  In the laboratory, 3-month-old seed from
southeastern Montana showed optimum germination at 69 degrees Fahrenheit
(20 deg C).  Older seed germinated best at 77 degrees Fahrenheit (25 deg
C).  Temperatures below 50 degrees Fahrenheit (10 deg C) and above 86
degrees Fahrenheit (30 deg C) inhibited germination.  Neither
stratification nor light had significant effects on germination [14].
Seed from southern Idaho showed 92 percent germination within 10 days
with day/night temperatures of 109 and 43 degrees Fahrenheit (43 and 23
deg C).  Germination was less than 5 percent after 30 days at room
temperature.

Seedlings rapidly grow deep roots.  Greenhouse seedlings attained a
primary root length of 19 inches (37 cm) in 30 days, then began
developing secondary roots that grew downwards with little lateral
development.  Purple threeawn seedlings may not tolerate wet soils.
Purple threeawn seedlings subjected to 1 week in saturated soil followed
by 3 weeks in soil at field capacity showed no growth during the 4-week
period.  In contrast, bluebunch wheatgrass (Pseudoroegneria spicata)
seedlings did not grow during saturation but grew well when soil
moisture was at field capacity [15,16].


SITE CHARACTERISTICS : 
Purple threeawn is most common on coarse-grained, xeric soils [15,16].
In the Intermountain region, it often dominates grassland communities on
gravelly or sandy soils [16,28].  Purple threeawn is also common on
disturbed sites such as roadsides and railway rights-of-way [28].  In
Colorado and the Southwest, purple threeawn is a relatively minor
species generally confined to xeric sites.  It is also described as a
minor species in the Pacific Northwest, usually occurring on sandy and
gravelly soils [16].

Although purple threeawn generally grows on rocky or sandy soils, it may
occur on soils of other textures [28,31].  Tisdale [51] reported that in
canyon grasslands of southern Idaho, the Fendler threeawn/Sandberg
bluegrass community type occurred on sandy to silty loams that were
deeper and lower in organic matter and pH than soils of surrounding sand
dropseed (Sporobolus cryptandrus)/Sandberg bluegrass communites.

Elevational ranges of purple threeawn in several states are as follows:

Arizona - 1,000 to 5,000 feet (305-1525 m) [28]
California - below 6,600 feet (2,000 m) [26]
Colorado - a few specimens have been collected from 5,300 to 6,800 feet
           (1,615-2,070 m) [24]; actual elevational range may be greater
Idaho - below 2,800 feet (853 m) [15]
Utah - 2,700 to 7,655 feet (820-2,320 m) [61]


SUCCESSIONAL STATUS : 
Purple threeawn is seral on most sites but is a component of stable
plant communities on some sites.  It is one of the first grasses to
establish on abandoned fields and other disturbed sites [25].  In
creosotebush (Larrea tridentata) communities of southwestern Nevada, it
was more common on disturbed sites than on undisturbed sites [20].  In
eastern Washington, it invaded and dominated an adandoned roadway in a
bluebunch wheatgrass-sand dropseed habitat type [31].  In northeastern
Arizona, purple threeawn was one of the first grasses to colonize
volcanic cinders [15].

In Intermountain grasslands and shortgrass prairie, purple threeawn is
generally a minor component of undisturbed plant communities protected
from livestock grazing [38].  Purple threeawn tends to increase with
heavy grazing and may persist after livestock grazing has stopped
[15,31].  Bluebunch wheatgrass-sand dropseed-purple threeawn communities
of the Oregon-Idaho border are stable, covering expansive areas where
grazing was historically heavy [31].  Purple threeawn sometimes
dominates stable communities on undisturbed sites, however.  Daubenmire
[9] described a Fendler threeawn-Kentucky bluegrass community in eastern
Washington as a possible "edaphic climax."  In eastern Colorado, a
stable Fendler threeawn/blue grama-buffalograss community develops on
sandy loam soils within what is otherwise buffalograss-blue grama
prairie on clay [39].


SEASONAL DEVELOPMENT : 
Purple threeawn generally grows in spring and early summer [25].  In
southern Idaho, it began growth in late March and flowered in mid-June.
Seeds reached milk-dough stage in mid-August and dehisced in September.
Plants stayed green all summer and did not put on new growth with fall
rains [15,16].  Similar development is reported in southeastern Montana
[14] and on Colorado shortgrass steppe [10].

Fendler threeawn had two growth periods on the Chihuahuan Desert of
southwestern New Mexico.  It first began growth in mid-March, flowered
in mid-April, and set mature fruit in May.  A second period of growth
occurred from mid-July through mid-September [34].

Related categories for Species: Aristida purpurea | Purple Threeawn