Profile of the Joshua Tree
by Richard Katz
The Joshua tree is an unusual tree-like species of Yucca, a member of
the Lily Family (Liliaceae), and sometimes considered in the Agave
Habitat and range
It grows uniquely in the desert southwest of the United States, principally
in California's Mohave Desert, Antelope Valley and the surrounding area,
and also in parts of Arizona, Nevada, and the southwest corner of Utah.
It is found typically in the high desert, around 4000 feet (1200 m) elevation
where there are freezing temperatures during winter nights, and very hot,
dry summers. The small annual rainfall occurs primarily in the winter,
with occasional dustings of snow. Thus, while this is a severe climate,
it is not quite as hot or dry as the lower elevation Sonoran deserts.
The Joshua tree is not a true "tree," in
that it does not produce a trunk with annual rings. For the first several
decades of its life, the
Joshua Tree grows as a vertical stem with no branches. It grows very slowly,
only 1/2 to 3 inches (1 to 7 cm) per year, typically reaching 5-10 feet
(1.5 to 3 meters) before the first blossoms appear. At this point, the
Joshua tree literally takes a new turn in its development. After flowering,
the blossoms drop off, leaving a length of dried stalk. New leaves grow
beneath this dead portion, and a new branch begins its growth in another
Unlike a typical tree branch, this new stem grows rigidly in a totally
different direction, at an angle, horizontally, or even down towards the
ground. Each branching stem also abruptly ends its growth after blossoming,
and further branches veer off in new directions. As well as ending in blossoming,
branching may occur where a stem has been damaged by insects.
After many years, some Joshua trees develop a complex system of twisted
branches growing in many directions. Others develop a more harmonious tree
shape, while still others remain mostly vertical. The amazing variety of
shapes and growth patterns imparts an unusual individuality to each tree.
The largest Joshua tree on record was 80 feet (24 meters) tall and was
estimated to be about 1000 years old. Joshua trees typically grow more
than 20 feet tall (6 meters). They may take 60 years to come to maturity,
and can live more than 500 years.
Young Joshua Trees have soft, tender leaves which
make them vulnerable to desert animals. Therefore, they do best when they
are seeded next to
a “nurse plant” which protects them when they are first growing.
After they are about 10-12 inches (25-30 cm) tall they develop their characteristic
sharp, pointed leaves, sword-like in their intensity. These not only afford
protection, and because of their form, they minimize the loss of water
through evaporation. During infrequent rain showers, the concave shape
of the leaves captures the water and directs it to the trunk and down to
the roots. As well, the mass of foliage tends to create a still-air zone
around the plant, further reducing the evaporative effects of the drying
Leaves are generally 5-12 inches (12-30 cm) long. The younger ones remain
green, but as they age the leaves fade to gray and become a fibrous residue
which droops and finally covers the branch or trunk in a protective coating.
As mentioned above, the leaves are somewhat concave, which allows rain
to collect and be drawn into the plant, where it can make its way down
the trunk to the roots. Thus the leaves embody this plant's ability to
survive in the harsh, dry desert climate.
Flowers and pollination
The flowers begin as greenish buds clustered near the end of the branches.
The six tepals (3 petals and 3 similar sepals) then open up, to reveal
a pod-like ovary, surrounded by six stamens, with one pistil. The flowers
are waxy, white-to-greenish-white, and 2-3 inches long (5-7 cm). The flowers
emit a sweet fragrance reminiscent of coconut.
The Joshua Tree pollen is so sticky that it cannot be carried by the wind.
Pollination is accomplished only with the help of the tiny Yucca moth,
with which this species has evolved a symbiotic relationship. This moth
(Pronuba yuccasella) collects the pollen from a number of flowers and then
chooses one flower as a home for its offspring. It deposits the pollen
on the flower’s stigma in the process of laying eggs in the flower’s
ovary. The newly hatched larvae will feed on the seeds (consuming only
about 20 out of 200 in each ovary).
After the Joshua Tree flower is pollinated, the ovary swells with seeds
and the flower falls away. The ripened fruit then falls to the ground where
the seeds are scattered, often taken up by the wind. Very few will be able
to germinate in the harsh desert conditions, but it is by this process
of seed propagation that the new plant colonies are established.
The Joshua Tree can also propagate by shoots from its roots or crown, and
this is particularly important when the upper part of the tree is destroyed
Seasonal and Diurnal Cycles
The blooming cycle of the Joshua Tree is totally dependant on climatic
conditions. Depending on the timing and intensity of winter rains, blossoming
can occur any time from March to May, and can very from very sparse to
a rare abundance of blossoms in relative wet years.
Joshua Tree has a special relationship with the night. The leaves actually
remain dormant during the day, and save their respiration cycle (exchange
of carbon dioxide and oxygen) for the night-time hours when the air is
cooler, and moisture can be conserved. It is also at night, after dusk,
when the Pronuba moth pollinates the flowers. A number of observers have
noted that the Joshua Tree flowers, which never appear fully open, reveal
themselves more to the night, as well as releasing their fragrance into
the night air. Joshua Tree is in its full glory in a moonlit desert night,
as its white flowers glow in the eerie luminescence of reflected light.
History and Lore
Archaeological evidence indicates that the Joshua Tree dates back to
the Pliocene Epoch, some two million years ago, when the climate
was more moist
and warm, and its habitat more extensive.
Native Americans knew the Joshua tree as a source of food and fiber.
The flowers were considered a sweet delicacy, roasted over a fire. The
were used for baskets and cordage.
Early European settlers often saw the Joshua Tree as grotesque and
misshapen. However, the Mormons, trying to find their way back from
Utah, used the Joshua Tree as a guide. They gave the plant its name
in the early 19th century, because it reminded them of the biblical
Joshua, with his arms upraised in prayer. They saw this plant as a
spiritual sign of welcome into the Promised Land.
The Joshua Tree (Yucca brevifolia) is the largest of the Yucca species.
Often considered part of the Agave family (Agavaceae), or grouped
with Agave (Century Plant) in the Lily family (Liliaceae), the Joshua
has the same linear, sharply pointed leaves as Agaves and other Yuccas.
species name brevifolia describes the shortness of the leaves. Joshua
Tree differs from other Yucca not only in its size and longevity
500 years), but also in their irregular growth patterns (described
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