BOTANICAL AND ECOLOGICAL CHARACTERISTICS
SPECIES: Ambrosia dumosa | White Bursage 



GENERAL BOTANICAL CHARACTERISTICS : 
White bursage is a native, drought-deciduous rhizomatous shrub growing
from 8 to 24 inches (20-60 cm) tall.  Its many slender, stiff branches
form a compact, hemispherical crown [14,41].  The leaves of white
bursage are small and deeply divided.  They may become so dry that it is
difficult to tell whether they are alive or dead [41].

The staminate and pistillate heads of white bursage intermingle
throughout the length of its racemes [41].  The pistillate heads are
two-flowered, producing obovoid fruits 0.2 to 0.8 inches (5-20 mm) long.
The burs generally contain 20-35 flattened, scattered, unhooked spines
that are about 0.08 inches (2 mm) long [33].

The root system of white bursage is derived from a segmented root crown,
and is mostly comprised of lateral roots [14].  Roots may grow 5 to 15
times the length of the stem [41] and extend to a depth of 28 inches (70
cm) [14].  Anderson [4] found the shoot to root dry weight ratio of
white bursage to be 1.38.  The leaf to root ratio was 0.18.

Definitive information on the longevity of white bursage is not
available in the literature.  Due to cloning, white bursage may be an
extremely long-lived shrub [32].  Some researchers, however, have
suggested that the longevity of white bursage is similar to that of its
noncloning congener, triangle bursage (A. deltoidea):  somewhat less
than 50 years [31,63].


RAUNKIAER LIFE FORM : 
   Phanerophyte
   Chamaephyte


REGENERATION PROCESSES : 
White bursage reproduces both vegetatively and sexually.

Vegetative reproduction:  Muller [32] and Wright and Howe [52] have
described vegetative reproduction in white bursage.  As white bursage
plants age, their crowns open irregularly with the successive deaths of
individual aerial shoots.  The clone slowly spreads to as much as 3.3
feet (1 m) in diameter.  The original seedling shoot branches
intricately at its base and below the soil.  The short rhizomes root
independently so that the death of the seedling stem and subsequent
rotting away of the original root crown disconnects the rhizomatous
shoots.  Windblown soil and organic debris accumulate about the base of
white bursage clones, producing a sizable mound over time [32].

Sexual reproduction:  White bursage flowers anytime during the spring,
summer, and fall if enough rain falls [1].  It produces seeds
abundantly, and seedlings establish in open space [31].  Large numbers
of white bursage seedlings emerge following heavy fall precipitation
[6].  In September of 1976, after a record rainfall near Ocotillo,
California, the density of white bursage seedlings was 466 plants per
acre (1,151 plants/ha) [55].

White bursage seeds have prickles that easily enter and remain in skin
and hair, so white bursage is probably dispersed by mammals.  Although
white bursage has moderately heavy fruits with low lofting ability,
requiring an air current of 87.9 centimeters per second, Maddox and
Carlquist [30] suggested that the tumbling ability of the fruits aids in
dispersal.

Germination experiments have been performed on white bursage.  Young and
Young [54] found that 30 days of moist stratification treatment at 35
degrees Fahrenheit (1.7 deg C) markedly improved white bursage
germination.  Graves and others [19] found that both moist sand
stratification and carbon treatments improved 7- and 14-day germination
of white bursage.


SITE CHARACTERISTICS : 
White bursage commonly grows on arroyos, bajadas, gentle slopes, valley
floors, and sand dunes at elevations up to 3,000 feet (900 m) throughout
the Sonoran and Mojave deserts [27,32,41,44,53].  It occurs on
calcareous, sandy, alluvial soil that is often underlain by a caliche
hardpan [3,13,29,41,54].  White bursage grows in pure stands or with
associates, especially creosotebush, in barren or open areas [24,31,41].

Temperatures in the Sonoran and Mojave deserts are variable and extreme.
At Puerto Libertad, Sonora, near the southern boundary of white bursage
distribution, the mean annual temperature is 68.37 degrees Fahrenheit
(20.2 deg C).  Daytime temperatures in the summer often reach 117
degrees Fahrenheit (47 deg C) [11].  In Rock Valley, Nevada, near the
northern boundary of white bursage distribution, temperatures range from
5 degrees Fahrenheit (-15 deg C) in winter to 117 degrees Fahrenheit (47
deg C) in summer [3].

Phenological events in the Sonoran and Mojave deserts are triggered by
rain.  In the Sonoran, rainfall averages 4 to 12 inches (100-300 mm)
annually with a bimodal distribution [29].  The Mojave gets more winter
than summer rain [29].  In Rock Valley, Nevada, rainfall averages 5.524
inches (138.1 mm), with 60 percent falling between September and February
[7].

Low soil oxygen may be a controlling factor in the distribution of
desert species.  White bursage was more tolerant of low soil oxygenation
than creosotebush [20].



SUCCESSIONAL STATUS : 
Succession in the desert is difficult to characterize because there is
no clear change in species composition over time.  For instance, in
creosotebush-white bursage communities, both white bursage and
creosotebush persist in the community even though changes in their
relative abundances may occur.

Most white bursage are located on bare soil away from other plants.
McAuliffe [31] found that 83 to 92 percent of all young white bursage in
creosotebush-white bursage communities were located in bare spaces.
White bursage was the principal colonizer of open spaces in those
communities.

Once established, white bursage acts as a nurse plant to creosotebush
and other desert species, providing improved microhabitat and protection
from herbivory [16,31].  McAuliffe [31] found that 85.5 percent of all
young creosotebush were rooted beneath the canopies of live white
bursage or positioned next to dead ones.  Most creosotebush
establishment apparently occurs while the white bursage are alive.  The
smallest creosotebush in McAuliffe's study were associated exclusively
with live white bursage.

Because of its colonizing ability, white bursage is a common pioneer on
disturbed areas in the Mojave Desert [36].  In a comparison between
vegetation in disturbed and undisturbed sites, white bursage was
subdominant to creosotebush on control sites and dominant on disturbed
sites 40 years after disturbance [36].  Vasek [50] noted that while
white bursage colonizes open space by large-scale seedling
establishment, creosotebush depends upon cloning or requires white
bursage for establishment.

Creosotebush-white bursage communities are probably adapted to continual
or relatively slight disturbance such as lightly shifting sand surfaces
[49,50].  If slight disturbance does occur, creosotebush-white bursage
communities recover quickly in terms of species composition [36].


SEASONAL DEVELOPMENT : 
White bursage leafs out in February or March; the mean leafing-out date
in Rock Valley, Nevada, was February 23.  The mean flowering date was
May 2 [46].  White bursage seeds usually germinate following heavy
September precipitation [6].  A minimum amount of rainfall is required
to induce germination.  For instance, a 1971 rain of 1 to 1.96 inches
(25-49 mm) was sufficient but neither an August 1972 rain of 0.68 inch
(17 mm) nor a July rain of 0.84 inch (21 mm) promoted germination [2].

White bursage has a drought dormancy period in the summer and may have
an induced dormancy period during the winter if freezing night
temperatures kill its leaves [1].

Related categories for Species: Ambrosia dumosa | White Bursage