BOTANICAL AND ECOLOGICAL CHARACTERISTICS
SPECIES: Yucca brevifolia | Joshua Tree 



GENERAL BOTANICAL CHARACTERISTICS : 
Joshua tree is a large, erect, evergreen, arborescent monocot
[10,26,34,43].  It is generally single-stemmed, but plants with two or
three stems also occur [26,43].  Joshua tree is the largest nonriparian
plant of the Mohave Desert, reaching heights of 16 to 49 feet (5-15 m);
the huge reddish-brown to gray trunks can grow to 2 or 4 feet (0.6-1.2
m) in diameter [10,21,28,34].  Erratic branching generally begins 3 to
10 feet (1-3 m) above the ground [10,26] and is often caused by the
yucca-boring weevil which destroys the growing tips [21].  Branches are
erect, ascending or spreading, and form a dense, compact, rounded top
[10,26].  The soft, corklike bark is rough and fissured [21,26].  The
inflorescence is a crowded, brittle often glabrous panicle [26].
Globose flowers are greenish-white or gray and papery at maturity, with
an odor described as "unpleasantly mushroom-like" [26,43].

The exact age of Joshua trees may be difficult to determine since
annuanl rings are not produced [21].  The overall shape, however, can
provide a general range.  Juvenile Joshua trees are generally
unbranched; middle-aged plants are forked and dense [26].  Older trees
generally have a single stem and an open crown [26].

Varieties:  Important differences in morphology and general growth form
of varieties are as follows [28,33,43]:

     var. brevifolia - tall, stout stem, branches mostly 3 to 10
                       feet (1-3 m) above the ground.
     var. jaegeriana - smaller, 10 to 20 feet (3-6 m) tall, branches
                       mostly 2.3 to 3.3 feet (7-10 dm) above ground,
                       more compact.
     var. herbertii -  many stems forming clumps up to 33
                       feet (10 m) in diameter, long rhizomes.


RAUNKIAER LIFE FORM : 
   Phanerophyte
   Therophyte


REGENERATION PROCESSES : 
Seed and seed dispersal:  Seeds are contained in dry, spongy,
indehiscent baccate fruits which average 3 inches (69 mm) in length
[18,26,45].  Fruit first develops near the base of the inflorescence
while the upper part is still in flower, and averages 25 to 40 per
cluster.  Annual fruit production varies greatly under natural
conditions [26].  Generally, fruit is produced only in wetter years.  On
extremely harsh sites the Joshua tree flowers rarely, if at all [21].
Seeds are dispersed chiefly by wind and animals.  Birds frequently open
the fruit, exposing seeds for subsequent wind dispersal [26].  A number
of desert rodents are known to cache Joshua tree seeds [21] and may also
disperse seed.  Fruit can persist on the tree, but usually disintegrates
rapidly [26].

Pollination:  One of the most interesting and well-studied aspects of
yucca ecology centers around the symbiotic relationship between yuccas
and their yucca moth pollinators.  The Joshua tree relies solely on the
yucca moth (Tegeticula synethetica) for pollination.  Seed production is
totally dependent on the availability of this pollinator, which in the
larval stage, feeds on a small percentage of seeds (generally around 7%)
[18].  In years of extremely low pollinator availablity, sexual
reproduction may be very limited.

Germination:  Most yucca seeds germinate well when temperature and
moisture conditions are favorable [19].  Laboratory experiments indicate
good germination potential for Joshua tree seed, with viability
approaching 96 percent [2].  Germination can begin within only 3 days if
seed is soaked in water for 24 hours prior to planting [43].
Germination capacity may be severely reduced if seeds are subjected to
high temperatures for even brief periods.

Seedling establishment:  Seedlings are uncommon on many harsh sites and
even under laboratory conditions, only 24 percent of the seed actually
produced viable seedlings [2].  Some researchers believe that sexual
reproduction was much more important during more favorable climatic
regimes, such as during the late Pleistocene, when summers were cooler
and annual precipitation greater.  Vegetative reproduction is now the
most important mode of regeneration on many sites.

Growth:  Joshua tree grows an average of 3 inches (8 cm) annually for
the first 10 years, then slows to 1.5 inches (1.3 cm) [21].

Vegetative regeneration:  Joshua tree can sprout from the roots and from
underground rhizomes [9,20,26,42].  Joshua tree rhizomes are
fast-growing and numerous, and possess many scalelike leaves [43].
Specific characteristics differ by variety as follows [43]:

     var. herbertii - aerial stems connected by underground
                      rhizomes 1.6-4.3 feet (0.2-1.3 m) in
                      length which quickly grow to the
                      surface.

      var. jaegeriana - rhizome development may be related to
                        precipitation, and is believed to be
                        stimulated by damage or injury to the
                        stem.


SITE CHARACTERISTICS : 
Joshua tree grows on desert plains, alluvial fans, slopes, ridges,
bajadas, mesas, or foothills [25,35,43].  Trees are often smaller and
less common away from the base of desert mountain ranges [32]; the
species is generally absent along the eastern edge of the Mohave Desert
[7].

Associated species:  The Joshua tree grows in open desert scrub,
pinyon-juniper woodlands, and in desert grasslands [7,19,35].  Much
variation has been reported on Joshua tree sites [30], but the following
species are common associates:  Nevada ephedra (Ephedra nevadensis),
broom snakeweed (Gutierrezia sarothrae), blackbrush (Coleogyne
ramosissima), creosote bush (Larrea spp.), bursage (Ambrosia dumsa),
California buckwheat (Eriogonum fasciculatum), Wright eriogonum
(Eriogonum wrightii), desert sage (Salvia carnosa), catclaw acacia
(Acacia greggii), and oaks (Quercus spp.) [7,15,16,30,32,34,43].  The
following grasses frequently grow in the understory of Joshua tree
woodlands:  big galleta (Hilaria rigida), galleta (Hilaria jamesii), and
bush muhly (Muhlenbergia porterii) [7,16].

Soils:  Joshua trees have been reported on coarse sand, very fine silt,
gravel, or sandy loam [8,20,43].  Many sites have bimodal soils with
both coarse sands and fine silts [20].

Climate:  Annual preciptation in many Joshua tree woodlands averages
only 5 inches (13 cm) or less [32], although some sites receive as much
as 8 to 10 (20-25 cm) inches [25].  Most precipitation occurs in winter
or early spring with very little rainfall during the summer [32].  The
distribution of this species is limited by low temperatures at higher
elevations but is largely unaffected by high temperatures [34].  The
Joshua tree can tolerate temperature extremes [6,34].  Evidence suggests
that the Joshua tree was much more widely distributed in more humid
prehistoric times [20].  During moister full-glacial periods of the late
Pleistocene, the range of the Joshua tree extended to the valley floor
of the Mohave Desert in areas where it is now restricted to uplands
above 5,576 feet (1,700 m) [45].

Elevation:  Elevational range of Joshua tree is 2,000 to 6,000 feet
(610-1,829 m) in California [42], 2,000 to 6,900 feet (610-2,103 m) in
Nevada [35], and 2,624 to 7,216 feet (800-2,200 m) in Utah [46].


SUCCESSIONAL STATUS : 
The slow-growing, long-lived Joshua tree is an important constituent of
a number of climax desert communities.  It is generally not well
represented in most seral communities.


SEASONAL DEVELOPMENT : 
Joshua tree requires sufficient precipitation for flowering to occur.
On some extremely harsh, arid sites, flowering rarely if ever occurs
[20].  Start of flowering is probably controlled primarily by daylength
[1].  Variation in flowering dates according to taxonomic variety and
geographic location has been noted.  Generalized flowering and fruiting
dates are as follows [11,26,28,35]:

Location     Beginning of    End of      Variety        Fruiting
             flowering       flowering   

   CA        March           May         brevifolia     ----
   CA        ----            ----        jaegeriana     April
   NV        April           May         ----           ----
   UT        March           March       ----           ----

Annual variation in floral development has been reported.  The following
data, collected over a 3-year period at a Nevada site, illustrate the
range of annual phenological development in this species [1]:

Stage           1971         1972           1973

bud             March        February       March
flower          April        March          April
fruit           May-June     May-June       May-June

Most annual vegetative growth of the Joshua tree occurs during a
5-month-long winter-spring growing season [34].

Related categories for Species: Yucca brevifolia | Joshua Tree