BOTANICAL AND ECOLOGICAL CHARACTERISTICS
SPECIES: Larrea tridentata | Creosotebush 



GENERAL BOTANICAL CHARACTERISTICS : 
Creosotebush is a native, drought-tolerant, evergreen shrub growing up
to 13.2 feet (4 m) tall [79].  Its numerous branches are brittle and
densely leafy at the tips [41,79].  Because of leaf and stem alignment,
creosotebush provides little shade during the full desert sunshine [70].

The leaves of creosotebush are thick, resinous, and strongly scented
[8,61] .  Flowers are solitary and axillary [61].  Fruits are globose,
consisting of five united, indehiscent, one-seeded carpels which may or
may not break apart after maturing [13,68,79].  Each carpel is densely
covered by long trichomes [68].

The root system of creosotebush consists of a shallow taproot and
several lateral secondary roots, each about 10 feet (3 m) in length and
8 to 14 inches (20-35 cm) deep.  The taproot extends to a depth of about
32 inches (80 cm); further penetration is usually inhibited by caliche
[41,114].  Barbour [10] found that root growth decreased as pH increased
above 8.0.  Optimum root growth occurred at acid pH; however, only one
of the topsoils from which seeds were gathered exhibited acid pH.  Root
growth was inhibited by high concentrations of salt (>10,000 ppm).
Creosotebush roots require relatively large amounts of oxygen for growth
[66].

Creosotebush is known to attain ages of several thousand years; some
creosotebush clones may be the earth's oldest living organisms.  The age
of the largest clone in Johnson Valley, California, is estimated at
9,400 years [101].  McAuliffe [71] estimated the average longevity of
creosotebush to be 1,250 years at a study site in Dateland, California,
and 625 years at a San Luis site.


RAUNKIAER LIFE FORM : 
   Phanerophyte


REGENERATION PROCESSES : 
Creosotebush reproduces both vegetatively and sexually.

Vegetative reproduction:  Creosotebush achieves its status as one of the
most stable members of desert communities by cloning.  When drought is
extreme, old branches and roots of creosotebush die back.  When rains
return, branches are replaced by sprouts originating near the outside of
the root crown.  Creosotebush clones gradually expand to form rings many
meters in diameter [32,63].  Creosotebush may occasionally sprout from
its root crown after disturbance.  New sprouts were produced by
creosotebush on a Mojave Desert site that had been denuded by grading
[89].

Sexual reproduction:  Age distribution in many stands of creosotebush
indicates that germination and survival under natural conditions are
rare [11,66].  Sexual reproduction may be especially rare at the upper
elevational limits of creosotebush [104].

Creosotebush requires summer rains for successful sexual reproduction.
The flowering success of creosotebush is greatest with moderate
rainfall.  In years of high rainfall, a high proportion of flowers is
diseased [13].

Creosotebush seeds are primarily adapted for tumbling rather than for
animal dispersal or lofting [68].  The stiff trichomes radiate equally
in all directions so that little wind is required to send the seeds
tumbling.  The trichomes are not stiff enough to penetrate animal skin,
and the seeds are too heavy for lofting.  However, Chew and Chew [29]
suggested that the shucking and burial of creosotebush seeds by rodents
may facilitate the germination and survival of creosotebush.  Shreve
[91] noted poor creosotebush reproduction on level plains.  More
seedlings established if the soil surface was broken or scarred.
Leitner [116] found creosotebush more abundant on southern or northern
slopes of a pediment in Sonora, Mexico, than in washes.  Rock crevices
and irregularities of the pediment may provide protection and footholds
for wind-tumbled seeds.

Germination of creosotebush is related to rainfall.  A minimum of about
1 inch of rainfall seems necessary to induce germination.  A 1971 rain
of 1 to 1.96 inches (25-49 mm) in the Mojave Desert 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 of creosotebush seeds [2].  If less
than 2 to 3 inches (50-80 mm) or more than 6 inches (150 mm) of rain
fall during the summer, germinability of seeds is usually less than 20
percent.  If 3 to 6 inches (80-150 mm) fall, germination is 20 to 60
percent.

Germination experiments have been conducted on creosotebush seeds from
all three southwestern deserts.  Barbour [10] found that the average
creosotebush mericarp contained one seed, and viability ranged from 15
to 76 percent.  The presence or absence of mericarp about the seed had
no effect on germination.  Germination was two times higher in darkness
than under light, and optimal germination temperature was 73.4 degrees
Fahrenheit (23 deg C).  Optimum salinity was 500 parts per million of
sodium chloride.  Germination was not affected by pH.  Creosotebush
seeds may lose viability if they remain in topsoils during the summer;
seeds from the Sonoran and Chihuahuan deserts showed decreased
germination as storage temperature increased.


SITE CHARACTERISTICS : 
Creosotebush commonly grows on bajadas, gentle slopes, valley floors,
sand dunes, and in arroyos [23,34,107] at elevations up to 5,000 feet
(1,515 m) [61,79] throughout the Sonoran, Mojave, and Chihuahuan
deserts.  It occurs on calcareous, sandy, and alluvial soils that are
often underlain by a caliche hardpan [21,43,45,48,67].

Temperatures in the southwestern deserts are variable and extreme.  Near
the southern boundary of creosotebush distribution, at Puerto Libertad,
Sonora, the mean annual temperature is 68.37 degrees Fahrenheit (20.2
degrees C).  Daytime temperatures in the summer often reach 117 degrees
Fahrenheit (47 deg C) [26].  In Rock Valley, Nevada, near the northern
boundary of creosotebush 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 southwestern deserts are triggered by rain.
In the Sonoran Desert, annual rainfall averages 4 to 12 inches (100-300
mm) and is distributed bimodally [67].  The Mojave Desert gets more
winter than summer rain [67]; in Rock Valley, Nevada, rainfall averages
5.524 inches (138.1 mm), with 60 percent falling between September and
February [18].  The Chihuahuan Desert is slightly less dry; in the Rio
Grande Valley, New Mexico, rainfall averages from 8.5 inches at San
Marcial to slightly more than 10 inches at Socorro.  Two-thirds to
three-fourths of the precipitation falls between April 1 and September
30 [43].

Low soil oxygen may be a controlling factor in the distribution of
desert species.  Creosotebush is less tolerant of low soil oxygen than
white bursage [46].  Lunt [66] attributes the exclusion of creosotebush
from fine-textured and poorly drained soils to its high oxygen
requirement.


SUCCESSIONAL STATUS : 
Creosotebush density and cover are generally decreased by disturbance.
In a comparison between vegetation on disturbed and undisturbed Mojave
Desert sites, creosotebush was dominant on all control sites and
subdominant to white bursage on disturbed sites [84].  Webb [104] noted
that desert succession can be described using life-history strategies:
Species with high recruitment and mortality rates, such as white
bursage, are dominant in the colonizing stage and species with low
recruitment and mortality, such as creosotebush, eventually dominate the
landscape, although colonizing species usually remain present.

Creosotebush uses white bursage as a nurse plant.  McAuliffe [71] found
that 85.5 percent of all young creosotebush were rooted beneath the
canopies of live white bursage or positioned next to dead ones.  The
smallest creosotebush plants in McAuliffe's [71] study were all
associated with live white bursage.  Most creosotebush establishment
apparently occurs near live white bursage.

Recruitment of creosotebush is infrequent.  Despite the abundance of
potentially suitable areas beneath white bursage, McAuliffe [71] found
young creosotebush beneath only 1 percent of all white bursage.  Total
densities of young creosotebush were between 12 and 15 plants per
hectare.  The density of white bursage plants was ten times that of
creosotebush.  Although large-scale creosotebush seedling establishment
does not occur after disturbance, relict creosotebush usually increases
in size by cloning [100,101,104].  Creosotebush canopies may grow to
exceed the coverage of white bursage by more than six times [71].

Creosotebush exhibits root-mediated allelopathy.  In a laboratory study,
creosotebush test roots grew freely through soil occupied by white
bursage roots, but white bursage test roots grew at reduced rates into
soil occupied by creosotebush [69].  Mature creosotebush may be
allelopathic to their own seedlings, encouraging an open community
structure [71].


SEASONAL DEVELOPMENT : 
Creosotebush leafs out in response to spring, summer, or fall rains [1].
Creosotebush usually flowers in May [1] in the Mojave Desert, but it can
flower anytime during the summer if it receives enough rain [1,3,9].  In
the Sonoran Desert, most creosotebush seeds are shed in the summer, but
creosotebush in the Chihuahuan Desert does not shed its seeds until fall
[10].  Creosotebush seeds germinate after rains from mid-June to
mid-September in the Mojave Desert [2].

Related categories for Species: Larrea tridentata | Creosotebush