BOTANICAL AND ECOLOGICAL CHARACTERISTICS
SPECIES: Prosopis pubescens | Screwbean Mesquite 



GENERAL BOTANICAL CHARACTERISTICS : 
Screwbean mesquite is a deciduous, thorny shrub or small tree with
slender branches.  Plants range from 6.5 to 33 feet (2-10 m) tall.  The
trunk diameter on larger trees often reaches about 8 to 12 inches (20-30
cm) [28,38].  The smooth, thick, gray to reddish brown bark separates in
long strips [28].  Single or paired, sharp, straight, white thorns occur
at the base of the leaf stalks and are about 0.33 to 0.5 inch (8-12 mm)
long [38].  The alternate, bipinnately compound leaves are 1.5 to 3
inches (3.8-7.6 cm) long and contain five to eight pairs of leaflets
which are 0.25 to 0.38 inch (6-10 mm) long and 0.13 inch (3mm) wide
[28].  The yellow flowers occur in 2-inch-long (5 cm) cylindrical spikes
[32].  The fruit is a yellow to brown, 1- to 2-inch-long (2.5-5 cm) pod,
which is tightly coiled into a springlike cylinder.

Screwbean is easily distinguished from other common mesquites, such as
velvet mesquite (P. velutina), honey mesquite (P. glandulosa var.
glandulosa), and western honey mesquite (P. glandulosa var. torreyana),
by its odd, screwlike pods and its considerably smaller leaves which
have comparatively few, small leaflets [21].


RAUNKIAER LIFE FORM : 
   Phanerophyte


REGENERATION PROCESSES : 
Screwbean mesquite reproduces sexually by producing an abundance of
seeds.  Information regarding this species' vegetative regenerative
capabilities is lacking.  Other mesquites (Prosopis spp.) commonly
sprout from buds located on an underground stem following injury to the
aboveground portion of the plant [13,15].

Seed production:  Mesquite flowers are pollinated by insects,
predominately bees [35].  Screwbean mesquite's fruit production is
probably rather consistent from year to year because plants are often
found in riparian situations where they have access to subsurface water.

Seed dispersal:  Mesquite pods are dispersed primarily by domestic and
wild animals.  A large percentage of mesquite seeds pass through the
digestive tracts of domestic animals intact and viable [13,15].  Many
species of rodents collect and store mesquite beans in caches and
uneaten seeds commonly.  Mesquite pods may be carried downslope by
running water [15].

Seed viability and germination:  Mesquite seeds contain a bony,
protective endocarp.  Scarification of this hard seedcoat must occur
before the seed can germinate.  Scarification occurs naturally when
seeds pass through the digestive system of animals.  Seeds remaining in
pods not consumed by animals remain dormant until the seedcoat is broken
by weathering.  Seeds remain dormant for long periods.  Sixty percent
viability was reported for 44-year-old velvet mesquite seeds taken from
herbarium specimens [36].

Seedling establishment:  Mesquite seeds must be covered with a small
amount of soil or dung for seedlings to become established.  Seeds that
germinate on the soil surface usually die [15].


SITE CHARACTERISTICS : 
Screwbean mesquite is generally restricted to river bottoms,
floodplains, washes, oases, and irrigation ditches where it tends to
form thickets [20,21,33].

Shade tolerance:  Screwbean mesquite apparently has moderate shade
tolerance which allows plants to grow beneath cottonwoods (Populus spp.)
[8].

Soils:  Screwbean mesquite occurs on many soil textures, including
gravels, sands, sandy-loams, and clays [8,37].  It often occurs on
saline soils [23].

Elevation:  Elevational ranges for the following Southwestern states are
given below [23,32,41]:

        below 4,000 feet (1,219 m) in AZ
        below 2,500 feet (762 m) in CA
        from 2,400 to 5,450 feet (730-900 m) in UT


SUCCESSIONAL STATUS : 
NO-ENTRY


SEASONAL DEVELOPMENT : 
Screwbean mesquite flowers in the spring.  The fruits mature between
July and October and fall from the plant in autumn [10,37].

Related categories for Species: Prosopis pubescens | Screwbean Mesquite