Download Larrea tridentata (Sesse` and Moc. ex DC.) Coville creosote bush

Larrea tridentata (Sesse' and Moc. ex DC.) Coville
ZYGOPHYLLACEAE
Synonyms:
creosote bush
Covillea tridentata (Sesse' & Moc. ex DC.) Vail
Covillea glutinosa (Engelm.) Rydb.
Larrea divaricata Cav.
Larrea glutinosa Engelm.
Larrea mexicana Moric.
General Description.—Creosote bush is also
know by the names gobernadora, hediondilla
(Spanish for little stinker), and guamis. It is
sometimes erroneously referred to as greasewood.
It is an evergreen resinous shrub of warm deserts.
It is a well branched shrub with no defined trunk,
typically reaching heights of 1 to 1.5 m, but can
grow as high as 3 m. It is strongly scented,
especially noticeable after a rainstorm. The stems
are gray with black, somewhat swollen bands at
the nodes; the twigs and young branches are
relatively flexible. The leaves are opposite,
bifoliately compound, each leaflet joined at the
base, divergent, but the tips often point toward
each other. Leaflets generally are olive green in
color, to 1 cm in length, 3 to 4 mm wide,
asymmetrical, oblong to obovate in shape, and
broader away from the base (Carter 1997, Correll
and Johnston 1970, Hickman 1993, Kearney and
others 1951, Martin and Hutchins 1980-81). The
leaves are often glossy with a thick resinous
coating. Stipules are persistent. The South
American L. divaricata is so similar to L.
tridentata that some researchers have considered
them the same species (Mabry and others 1977).
These two species have been demonstrated to
hybridize in experimental gardens. South America
is the likely origin for the genus, with creosote
bush ancestors arriving in North America 8.4 to
4.2 million years before present (Lia and others
2001) with current distribution occurring since the
last glaciation. Creosote bush is believed to be a
migrant through possible bird dispersal of the L.
divaricata populations (Mabry and others 1977).
There is a ploidy gradient from east to west (from
moister to dryer deserts) across the Southern
United States: Chihuahuan Desert populations are
diploid, Sonoran tetraploid, and Mojave hexaploid.
From a study of plant remains in packrat middens,
the plants appear to have spread slowly from lower
Rio Grande refugia in Texas north and west,
reaching the Southern Mojave about 8,500 years
ago, the Northern Sonoran about 6,400 years ago,
and the Northern Chihuahuan about 4,000 years
ago (Hunter and others 2001). Larrea divaricata
and diploid L. tridentata have homologous
genomes (Mabry and others 1977). Two varieties
have been recognized by one author: L. tridentata
var. tridentata and L. tridentata var. arenaria L.
Benson (Kartesz 1994).
Range.—Creosote bush occurs from Southern and
Western Texas through Southern New Mexico
(with a band up the Rio Grande valley to
Albuquerque), into Arizona, and Southern
California, as far north as Southern Nevada and
Southwestern Utah, and south well into Mexico.
Its elevational range is from below sea level
(Death Valley, CA) to about 1,700 m.
Ecology.—Creosote bush is a characteristic
dominant evergreen shrub of the Southwestern
warm Chihuahuan, Sonoran, and Mojave deserts.
On the northern fringes of the Mojave its range
just touches the Great Basin Desert (Mabry and
others 1977). It grows best in gravelly to sandy,
well drained soils of desert plains, benches, mesas,
alluvial fans, and rocky slopes. It does not occur
on alkali flats or pans. It roots deeply, to 3 m or
more, but most roots occur in the upper soil layers
(but below about 10 cm), and it will tolerate a
caliche layer. The shallow roots are able to utilize
small rainfall events. Creosote roots inhibited
further elongation of the roots of both adjacent
creosote bushes and the shrub Ambrosia dumosa
(A. Gray) Payne (Mahall and Callaway 1992).
Stem angle facilitates water stemflow to the roots,
which is maximal with stems greater than 65° and
almost none less than 45°. Creosote bushes in the
driest part of their range, for example, Death
Valley, generally have steeper angles, while there
is more variability in stem angle in the Chihuahuan
Desert. Shrubs with shallower stem angles take on
a more hemispherical shape as opposed to an
inverted cone shape. Shrub canopy architecture
also affects soil characteristics creating "islands of
fertility." Shrubs with hemispherical shapes collect
much more litter underneath, hence soil nitrogen is
greater under hemispherical shrubs than under
inverted cone shaped shrubs (Whitford 2002).
Creosote bushes often require a nurse shrub such
as the small shrub A. dumosa for seedling/sapling
establishment, and creosote bush itself will serve
as a nurse plant for certain other species (Whitford
2002). Creosote bush often forms monotypic
stands that extend for miles, although its density
varies greatly from site to site, with the greatest
densities occurring in the Chihuahuan Desert (up
to 8,000 plants/ha), and lesser densities in the
Sonoran (up to 4,000 per ha) and the Mojave (up
to 1,800 per ha) (Mabry and others 1977). Aboveground standing crop biomass tends to reflect
these density trends.
Chihuahuan Desert
populations tend to show a greater size or age class
structural diversity than those in other deserts
(Mabry and others 1977). The resinous coating on
the leaves helps retard water loss. During extreme
droughts creosote bush will produce smaller,
tougher leaves, and will also shed many of its
older leaves. It quickly produces new shoot
growth after substantial summer rains. Creosote
bush has the C3 photosynthetic pathway, with
peak photosynthetic activity (Pmax) in the spring
and during summer monsoon seasons especially
when pre-dawn water potentials are high
(Whitford 2002). Pmax is as high as that found in
C4 plants (Fitter and Hay 1987), and creosote
shrubland has a similar community energy balance
as the C4 desert grasslands they have invaded
(Dugas and others 1996). Creosote bush has
demonstrated a threshold for stomatal closure at 5.8 MPa, has maintained full turgor at -3.0 MPa,
with positive net photosynthesis occurring at -8 to
-16 MPa (Fitter and Hay 1987, Mabry and others
1977). It can tolerate leaf water potentials below 50 MPa. It is able to maintain a relatively stable
photosynthetic rate all year in Death Valley, with
increases associated with spring temperatures and
moisture availability (Mabry and others 1977). It
has high water use efficiency and ability to resorb
some nutrients, especially phosphorus, 72 to 86
percent, while nitrogen resorption is within the
general range for other evergreen shrubs, 47 to 57
percent. Creosote bush stem and leaf orientations
maximize light collection during cooler and
moister early morning periods. Creosote bush has
a greater ability to withstand xylem cavitation
during droughts than riparian species (Pockman
and Sperry 2000). An Arizona population of
creosote bush experiences freezing-induced xylem
vessel cavitation and consequent disruption of
water transport at temperatures from -11 to -20 °C
(Pockman and Sperry 1997). These temperatures
correspond to 20+ year minimum isotherms and
also correspond with the northern distribution in
both the Sonoran and Mojave Deserts. A few
studies have examined the effects of
environmental pollutants on creosote bush. Foliage
demonstrates leaf injury when exposed to acute
levels of sulfur dioxide (SO2) (Olyszyk and others
1987) and elevated CO2 concentrations may
ameliorate the effects of heat stress (Hamerlynck
and others 2000). Creosote foliage may
accumulate heavy metals from industrial pollution
as demonstrated in a 1980 to 1995 study from El
Paso, TX, and Ciudad Juarez, Mexico (Mackay
and others 1998).
Reproduction.—Creosote
bush
blooms
periodically during the growing season, blooming
correlated first with seasonality, secondarily with
the timing of moisture availability and other local
environmental factors. Peak blooming periods
occur in spring (February to May) and again in late
summer into the fall (August to December). A
single mature shrub may produce more than 2,000
flowers (or about 50 g of flowers) during the
growing season. Numerous insects visit the
flowers, but bees are the most effective pollinators.
Many species of bees are known to visit the
flowers; some are generalists but some are
specialists and forage solely on creosote bush
(Minckley and others 2000). It is an outcrossing
species but is always self-compatible (Mabry and
others 1977) with localized populations having
higher levels of inbreeding in the Chihuahuan
Desert. The flowers are borne solitary in the axils,
are 2 to 3 cm in diameter, are perfect, with five
yellow clawed petals, obovate to spatulate, 6 to 10
mm long, and 5 mm wide. There are five
yellowish green, unequal, deciduous sepals, 5 to 8
mm long. There are 10 stamens with winged
filaments, and the ovary has five locules. The
petals twist at a 90 degree angle after pollination.
The fruit is a roundish capsule, about 5 to 7 mm in
diameter, covered with a dense concentration of
white to reddish hairs, and separates into five
indehiscent one-seeded carpels at maturity (Carter
1997, Correll and Johnston 1970, Kearney and
others 1951, Martin and Hutchins 1980-81). Both
fruits and flowers are commonly found on the
plant at the same time. Flowering may begin on
plants as young as 4 to 6 years, although heavy
flowering and fruiting generally occurs after 8 to
13 years. Greenhouse-raised seedlings may flower
at 2 years of age. Fruit production ranges from 39
to 278 fruits per 100 g of branches or 120 to 1,710
per plant (Young and Young 1992). There are
about 370 seeds/g. The seeds have mechanical
dormancy and scarification aids germination
(Young and Young 1992). Germination success
varies by population, latitudinal and local
environmental conditions with reported success
rates of 2 to 70 percent. Salt (NaCl) at 10,000 ppm
(-0.78 MPa) resulted in 0 percent germination, and
seed germinated equally well at pH's from 7 to 10
(Barbour 1968), although some studies have
shown some variation from these pH's (reviewed
in Baskin and Baskin 2001). Leachates from fruit
coats inhibit germination of some plant species
[Bouteloua eriopoda (Torr.) Torr. but not
Muhlenbergia porteri Scribn. ex Beal], but not of
creosote bush seeds (Baskin and Baskin 2001).
Seeds stored dry in paper bags at room
temperature for as long as 8 years showed some
germinability (Barbour 1968).
Growth and Management.—Growth response
and productivity can be rapid after rain, but there
is not a strong pattern of productivity and rainfall,
while nitrogen has been shown as an important
factor in creosote bush productivity (Whitford
2002). Maximum leaf longevity is 16 months.
Maximal root growth occurs in the fall. Branches
will root if covered with soil. Ring counts of stems
have indicated that creosote bush is long-lived, at
least 30 to 50 years, but ring counts may not be
reliable.
Individual shrubs may sometimes
reproduce by fragmentation or separation of the
root crowns along with new basal sprouting,
causing the plant to spread out and away from the
center. Such fragmentation may contribute to longterm survival estimated at more than 100 years to
several thousand years. Creosote bush shrubland
has been expanding over the past 100 years or
more due to overgrazing of desert grasslands by
livestock, although climate trends have also been
implicated. One experiment to test this involved
transplanting seedling creosote bushes into
fertilized, irrigated, or unmodified ungrazed black
grama (B. eriopoda) grassland, a creosote
shrubland, or a heavily grazed grassland. The only
site in which seedlings survived was in the heavily
grazed grassland (Whitford 2002). Prior to heavy
livestock grazing creosote bush was generally
restricted to well-drained gravelly or sandy soils of
steep slopes. Creosote bush and mesquite
(Prosopis glandulosa Torrey) invasion likely
occurred in episodes in conjunction with both
reduced grass cover and drought (Grover and
Musick 1990). Localized decreases have also been
reported. In an area of Big Bend National Park,
previously overgrazed by livestock, the dry period
from 1960 to 1967 showed increased creosote
bush cover, while the wetter period from 1967 to
1981 showed a decrease in creosote bush cover
and increased cover of perennial grasses, forbs,
and most other shrubs (Wondzell and Ludwig
1995). At a site in southern New Mexico creosote
shrub size has decreased over a 10-year period,
possibly due to longer term climate change (Miller
and Huenneke 1996). Certain creosote bush
populations in the Sonoran Desert of Mexico
declined by 50 to 90 percent, perhaps due to
prolonged droughts from 1936 to 1964 (Turner
1990). Creosote bush shrublands have been
experimentally treated with a variety of herbicides
for shrub management. Several liquid-applied
herbicides (2,4-D, dicamba, picloram, 2,3,6-TBA)
either singly or in combination with other
herbicides have demonstrated greater than 50
percent root kill (Herbel and Gould 1995). But
individual shrub treatment with dry herbicides
(bromacil, fenuron) or aerially applied tebuthiruon
pellets have given greater control, 80 to 90
percent.
Perennial grass and annual forb
production increased substantially on tebuthiurontreated plots.
Benefits and Disadvantages.—Creoste bush is
unpalatable as livestock forage and is usually
toxic, sometimes causing death (Gay and Dwyer
1998, Kearney and others 1951, Stubbendieck and
others 1997, USDA 1937). Jackrabbits are one of
the few mammals that will consume the plant to a
certain degree, largely based on the water content
of a particular plant (Whitford 2002). Some
woodrat populations will also consume creosote
bush, demonstrating differentially developed
tolerances to the resin (Mangione and others
2000). Woodrats consume creosote leaves and
stems in the Mojave Desert when necessary to
maintain water balance but suffer ill effects such
as loss of body mass (Karasov 1989). Both rabbits
and woodrats will peel open the stems to get water.
Once a particular shrub is browsed by a jackrabbit,
that same shrub is much more likely to be browsed
again than another shrub that has never been
browsed (Ernest 1994). Younger leaves have a
higher concentration of resins that affect relative
leaf palatability and digestibility. Resin content
may be as high as 26 percent in young leaves,
dropping to 11 to 16 percent in older leaves, with
insects showing preference for the older leaves
(Mabry and others 1977). A phenoloxidase system
in the leaves appears to make digestibility of the
resinous leaves even more difficult. Chemical
treatment of creosote foliage to remove the resins
results in nutritious edible forage, but due to
economics this practice is not often implemented
(Mabry and others 1977). Numerous species of
arthropods set up habitation in creosote bush. In
particular males of the grasshopper species,
Ligurotettix coquilletti McNeil, select a particular
shrub as their territory and defend it against other
male grasshoppers (Mabry and others 1977). The
mounds created by kangaroo rats (Dipodomys
spp.) appear to have a positive effect on creoste
bush survival, flowering, and fruiting, while
creosote bush itself has a negative effect on
kangaroo rat populations (Chew and Whitford
1992). Creosote bush exposure to 0.1 to 0.2 ppm
ozone
consistently
reduced
the
toxic
nordihydroguaiaretic acid content of leaves, which
could result in potentially greater levels of
herbivory (Gonzalez and others 1988). Powdered
creosote leaves added to stored pinto beans made
them significantly less attractive to the bean
weevil (Zabrotes subfasciatus Boheman) (CortezRocha and others 1993).
Pharmaceutical Chemistry.—Although a toxic
plant for livestock and many wildlife, it has value
due to this same toxicity because of antiseptic and
other potential medicinal uses. Native Americans
of the Southwestern deserts have used this plant in
teas, tinctures, and salves, as a poultice to retard
bacterial growth, as an emetic, expectorant, and
diuretic to treat venereal disease, tuberculosis,
bowel cramps, and rheumatism (Kearney and
others 1951, Mabry and others 1977). It has been
used for stiff limbs, sores, skin ailments,
snakebites, menstrual cramps, and chicken pox
(Bowers and Wignall 1993, Mabry and others
1977). Creosote bush may cause dermatitis in
certain people with allergic skin sensitivity.
Secretions and contents of the lac scale insect
found on creosote bush have been used for
repairing pottery, fastening arrow points, water
proofing baskets, and making mosaics (Kearney
and others 1951, Warnock 1974). One of the
principal compounds in creosote bush resin is the
anti-oxidant nordihydroguaiaretic acid (NDGA).
This is a phenolic aglycone and comprises 5 to 10
percent of leaf dry weight, 80 percent of all
phenolics in the resin, and once was used as an
anti-oxidant in foods, pharmaceuticals, and
industrial materials (Mabry and others 1977).
NDGA has demonstrated an ability to inhibit
tumors and cancers but does have toxic side effects
to both animals and humans. Its use as a
chemoprevention for skin cancer and protection
against the effects of ultaviolet light has been
investigated (Gonzales and Bowden 2002). The
botanical dietary supplement "chapparal" has been
associated with nonviral toxic hepatitis and
contains lignans similar to estrogens (Obermeyer
and others 1995) and may cause acute liver
damage (Sheikh and others 1997). NDGA has
shown potential to inhibit the human
immunodeficiency virus (HIV) (Gnabre and others
1995). NDGA (masoprocol) lowered glucose and
triglyceride levels in rats and may have potential in
Type II diabetes treatment (Reed and others 1999).
Numerous
flavanoid
aglycones,
flavanoid
glycosides, wax esters, saponins, and volatile
constituents such as terpenes, have been identified
from creosote bush.
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_________________________________________
James E. Nellessen, Botanist, Biologist and
Environmental Scientist, Taschek Environmental
Consulting, 8901 Adams St., NE, Albuquerque,
NM 87113