BOTANICAL AND ECOLOGICAL CHARACTERISTICS
SPECIES: Ambrosia deltoidea | Triangle Bursage 



GENERAL BOTANICAL CHARACTERISTICS : 
Triangle bursage is a native, drought-deciduous, microphyllous shrub
growing up to 1.65 feet (50 cm) tall [24, 29].  Innumerable slender and
brittle branches spring up from the base forming a compact, roughly
hemispherical crown [27, 35].  Mature triangle bursage crowns contain
many dead branches which are shed only as a result of weathering [27].

The branches and young leaves of triangle bursage are tomentulose and
resinous.  Leaves become glabrous above with age.  Leaf length is 0.6 to
0.8 inches (1.5-2 cm) or smaller [24,27].

Staminate heads are borne terminally.  Two-flowered, pistillate
involucres are clustered below the staminate spikes or on lateral
branches.  They produce globose, spiny achenes [24,27].

Triangle bursage possesses one long taproot with well-developed laterals
[35].  Triangle bursage roots are slender, brittle, and longitudinally
ridged with cork [6].  They are confined to the soil above the caliche
hardpan--about 8 to 16 inches (20-40 cm) deep [6,35].  Besides roots
arising from the main root, Cannon [6] found 50 adventitious roots about
0.12 inches (3 mm) in diameter which arose from the root crown, growing
horizontally.  Filamentous rootlets formed on many roots of triangle
bursage after the soil had been moistened by rains, but such rootlets
were short lived [6].

Triangle bursage is relatively short lived for a desert shrub.  It has
an observed longevity of about 50 years [10,27].


RAUNKIAER LIFE FORM : 
   Phanerophyte


REGENERATION PROCESSES : 
Triangle bursage reproduces sexually.  Seed is produced abundantly, and
seedlings establish in open space.  In the Sonoran Desert, the relative
abundance of triangle bursage seedlings was some 45 times the density of
mature triangle bursage plants.  The density of mature triangle bursage
was similar to that of mature creosotebush, but 1,001 triangle bursage
seedlings were counted while only 124 creosotebush seedlings were found
[17].

Triangle bursage seeds are spiny and are probably dispersed by mammals.

One study reports that triangle bursage has very limited ability to
sprout after top-kill [18].  No other research describes the ability of
triangle bursage to sprout.

Triangle bursage reproduces from cuttings with the application of auxin
[15].


SITE CHARACTERISTICS : 
Triangle bursage commonly grows in open spaces [17] on pediments, upper
bajada surfaces, and basin floors on a variety of substrates including
volcanic, granitic, metamorphic, alluvial, and caliche soils.  There was
no significant difference in cover of triangle bursage among volcanic,
granitic, metamorphic, and alluvial substrates [21].

Triangle bursage is a frequent dominant on gravel outwashes, bajadas,
and the ecotone between upper and lower bajadas [3,17,35].  Triangle
bursage occurs almost entirely on very coarse soils that are relatively
uniform in composition with little differentiation into horizons
[3,14,15], although it occurs rarely on sand [27].  Precipitation
percolates rapidly through the soil and drains quickly away along the
underlying caliche layer [1,35].

Rainfall averages 4 to 12 inches (100-300 mm) annually with a bimodal
distribution [16,25].  Temperatures at Organ Pipe National Monument and
elsewhere in the Sonoran Desert fluctuate daily and seasonally.  The
mean frost frequency at the monument from 1956 to 1985 was 17 frosts per
year (standard deviation=7, range=5-34) [21].

Parker [22] studied the soil characteristics of sites dominated by
triangle bursage at Organ Pipe National Monument.  Overall, triangle
bursage was most abundant on flat terrain, but its cover was reduced on
the lower bajadas and in the western portion of the Monument where pH
was relatively high [22].  In the Ajo Mountains on south-facing slopes,
sites had low pH and high available magnesium.  In the Blanco Mountains,
Senita Basin, and Quitobaquito Hills, available magnesium was low.  In
the Bates Mountains, Puerto Blanco Mountains, lower elevations of the
Ajo Mountains, and flats in Senita Basin, triangle bursage grew on
north-facing slopes in volcanic soil.  In the Sonoyta Mountains,
triangle bursage grew on northwest- to northeast-facing slopes.  Leitner
[14] found that triangle bursage preferred the cooler, more humid
conditions of north-facing slopes at Punta Cirio, Sonora, Mexico,
although it occurred on both north- and south-facing slopes.  Triangle
bursage grows from 1,000 to 3,000 feet (300-900 m) [13].


SUCCESSIONAL STATUS : 
Facultative Seral Species

Triangle bursage is often the first plant to occupy an open area.
Recruitment of other species rarely occurs in the open.  Once
established, triangle bursage acts as a nurse plant for other species,
providing improved microhabitat and protection from herbivory [17,9].

Most mature triangle bursage plants have an associated perennial.
Eighty-two out of 103 triangle bursage plants lacking associated
perennials were small (and presumably young)--2 inches (5 cm) or less in
root crown diameter.  Only 2 of 101 triangle bursage plants that had
other perennials associated with them had basal diameters of 2 inches (5
cm) or less [17].

Succession in the desert is difficult to characterize.  Although changes
in relative abundances of species may occur, plants rarely disappear
from the ecosystem altogether because severe disturbance is usually
lacking [17].  McAuliffe [17] described changes in desert vegetation in
terms of community dynamics instead of succession.  For instance, in
communities codominated by triangle bursage, triangle bursage colonizes
open space and acts as a nurse plant for other perennial shrubs.  Some
larger, presumably older individuals within the population bear no
evidence of former associations with triangle bursage, probably because
they have outlived it.  Triangle bursage continues to colonize other
open space.  Relative abundance of species may be altered, but triangle
bursage presence is maintained [17].

Triangle bursage is known to establish on overgrazed desert grasslands.
Tueller [31] suggested that partial or complete protection from grazing
on triangle bursage-invaded rangeland will promote secondary succession.
Secondary succession results in the establishment of climax grasses and
palatable shrubs and a decrease in unpalatable shrubs such as triangle
bursage.

When growing among creosotebush and jumping cholla (Opuntia fulgida),
triangle bursage occupies a distinct root zone so interspecific
competition is minimized [35].  In one study, no roots of neighboring
plants grew near triangle bursage roots [6].


SEASONAL DEVELOPMENT : 
Most new leaves on triangle bursage develop and mature during the winter
and early spring [29].  Triangle bursage flowers from February to July
[33].  Seeds are produced after both summer and winter rains [27].

Related categories for Species: Ambrosia deltoidea | Triangle Bursage