BOTANICAL AND ECOLOGICAL CHARACTERISTICS
SPECIES: Prosopis velutina | Velvet Mesquite 



GENERAL BOTANICAL CHARACTERISTICS : 
Velvet mesquite is a deciduous, thorny shrub or small tree.  Plants may
grow into single-stemmed trees up to 50 feet (15 m) tall with a trunk
diameter up to about 2 feet (61 cm) or they may develop into an erect,
multistemmed bush [51,69].  The largest trees are often found along
water courses or floodplains where the deep root system has access to
year-round water.  In general, tree forms are found on alluvial soils
whereas smaller forms are found on tighter, rocky upland soils [51].

Velvet mesquite has a strong tendency for apical dominance and a
well-developed crown [89].  Undisturbed plants therefore develop into
single-stemmed trees.  If the aboveground growth is damaged or removed,
such as by freezing weather, drought, fire, trampling, browsing, or
cutting, dormant buds located on the underground stem initiate new
growth, resulting in the many-stemmed growth form.  In many semi-desert
grasslands the shrubby form predominates because years of browsing,
trampling, and repeated chemical or mechanical control attempts resulted
in many plants sprouting from the base [114].

The bark on older branches and the trunk is dark brown, rough and thick,
and separates into long narrow strips [75].  Young branches may be green
and photosynthetic [119].  Stout, yellow thorns are about 0.25 to 1 inch
(0.6-2.5 cm) long and generally occur in pairs on young branches [75].
The yellowish-green flowers occur on a 2 to 3 inch (5-7.6 cm) long
cylindrical, pedunculate, spikelike raceme [51,113].  The flattened,
straight or curved legume-type fruits are 3 to 8 inches (7.6-20 cm)
long, occur singly or in drooping clusters, and may be yellow, red,
black, or mottled [114].

Velvet mesquite is a facultative phreatophyte which extracts moisture
from a large volume of soil through a well-developed root system.  In
true desert environments plants are often restricted to wash areas where
the taproot can penetrate several meters to underground water.  Roots
probably do not extend much more than 50 feet (14 m) [67,86], although
they have been found at depths of 175 feet (53 m) [99].  Plants growing
on sites where the soil is shallow or moisture does not penetrate deeply
have an extensive system of lateral roots that reach several meters away
from the plant base1w.  Under these circumstances laterals are often
concentrated in a zone 6 to 12 inches (15-30 cm) below the soil surface
[90].  Plants can also extract water from soil held at high matric
forces [89].  These adaptations allow velvet mesquite to retain an
entire complement of leaves during all but the most severe droughts.

The primary botanical characteristics used to differentiate velvet
mesquite, honey mesquite, and western honey mesquite are the size,
shape, and hairness of the leaflets.  Velvet mesquite leaflets are
generally short, hairy, and closely spaced; those of honey mesquite are
long, linear, glabrous and widely spaced; those of western honey
mesquite are intermediate [6].  Several manuals and keys are available
to aid in proper identification [6,61,75,91].


RAUNKIAER LIFE FORM : 
   Phaneropyte


REGENERATION PROCESSES : 
Velvet mesquite reproduces sexually by producing an abundance of seeds.
Vegetative regeneration commonly occurs following damage to the
aboveground portion of the plant.

Seed production:  Velvet mesquite flowers are pollinated by insects,
predominately bees.  Although mesquite inflorescences contain hundreds
of flowers, generally only one to a few fruits develop per inflorescence
[113].  This ensures enough resources for proper development of a
limited number of fruits per inflorescence.  Insufficient soil moisture
can cause early spring flowers to be entirely aborted before pollination
occurs.  New flowers will develop if there is sufficient rainfall later
in the season.  Due to fluctuations in the weather, fruit production can
be quite variable from year to year for plants growing in semi-desert
grasslands [51].  Plants growing along washes or in riparian areas,
where they have access to permanent underground water, produce fruit
quite predictably from year to year.  The seeds are glossy brown, oval,
0.2 to 0.28 inch (5-7 mm) long and 0.12 to 0.2 inch (4-5 mm) wide [6].
There are about 13,418 seeds per pound (29,573/kg) [51].  The
reproductive potential of velvet mesquite is often greatly reduced by
seed-feeding insects.  The life cycle of Bruchid beetles (Algarobius
spp.) and the amical weevil (Bruchus amicus) are dependent upon mesquite
fruit.  Female beetles and weevils lay eggs on the pods and as the
larvae hatch they burrow into the fruits and feed on the seeds [71].  A
small, brown, circular spot often forms on the pod showing evidence of
larvae entry.  Fruit collected within a 25 mile (40 km) radius of Tucson
had as high as 70 to 80 percent of the seeds attacked by insect larvae
[51].  Although flower and pod attacking insects significantly reduces
the production of pods and viable seeds per plant, velvet mesquite
produces pods in such abundance that numerous viable seeds are still
produced [71].

Seed dispersal:  Animals - Pods are eaten and then dispersed by domestic
and wild animals.  Cattle and rodents are the primary dispersers of
velvet mesquite seed; numerous other animals also consume the pods [see
Importance To Livestock And Wildlife].  Studies involving honey mesquite
showed that when pods were fed to livestock, 97, 79, and 16 percent of
the seeds passed through the digestive tracts of horses, yearling
steers, and ewes with the greatest number of seeds passing through
between 42 and 60 hours after feeding [43].  When velvet mesquite seed
pods were fed to domestic sheep, 32 percent of the seed passed through
the digestive tract intact and sound.  Examination of cattle dung piles
on velvet mesquite-grasslands at a time when ripe pods were plentiful,
showed that each dung pile averaged 1,535 seeds, of which about 65
percent were sound [51].  Because it takes days for seeds to pass
through the digestive tracts of domestic animals, seeds are dispersed
great distances.  The digestive juices of domestic animals also kills
some seed-eating insects [89].  Many rodents collect and store velvet
mesquite seeds or pods in caches.  Merriam kangaroo rats often transport
seeds more than 100 feet (30 m) [51,103].  Seelings commonly germinate
from uneaten seed in rodent caches [103,105].  Sampling in 1948, 1950
and 1951 on the Santa Rita Experimental Range, showed that during these
years 37.4, 23.3, and 8.4 percent of seedlings emerging, respectively,
were from kangaroo rat caches [51].

Water - Velvet mesquite pods float and are carried downslope by flowing
water.  Intense summer rainstorms often cause overland flows of water on
areas with only minor slope gradients in the Sonoran Desert.  On the
Santa Rita Experimental Range, seedlings are abundant in the alluvium
along small water courses and in the alluvial fans formed by these
drainages [51].

Seed viability and germination:  Velvet mesquite seeds contain a bony,
protective endocarp.  Scarification of this hard seed coat must occur
before the seed can germinate.  Scarification of the seed occurs
naturally when seeds pass through the digestive system of animals.
Seeds remaining in pods not consumed by animals remain dormant until the
seed coat is broken by weathering.  Tests of various aged sound seed
showed that 1- to 45-month-old scarified seed displayed better than 94
percent germination, 1-month-old unscarified seed had only 20 percent
germination, and germination of 5- to 45-month-old unscarified seed
never exceeded 7 percent.  Most germination of velvet mesquite seeds
within pod segments buried 1 inch (2.5 cm) below the soil surface in
Arizona occurred within 3 years, with about 35, 9, and 1 percent
germination occurring 1, 2, and 3 years after planting [118].  The
protective endocarp allows seeds to remain viable for long periods.
Seeds in dry storage have remained viable for decades.  Germination of
stored seed has been reported as follows [51]:

Age in years            Germination of sound seed
                        Unscarified     Scarified

    11                     ----            97.5%
    44                      20%            60%
    50                      30%            60%

Planting of unscarified seeds in the field showed that germination of
seeds within pod segments was 2 to 3 times greater than that of hulled
seeds.  Over a 3-year period, 44.7 percent of seeds planted in pod
segments germinated, but only 15.8 percent of hulled seeds germinated
[118].

Velvet mesquite seeds germinate over a wide range of temperatures and
soil conditions.  Better than 80 percent germination was achieved at
temperatures ranging from 61 to 100 degrees F (16-38 C) [112].  High
germination occurs over a wide range of soil pH (4-10) and on soils of
high salinity [112].

Seedling emergence and establishment:  Velvet 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 normally die.  In
Arizona, seedling emergence is most common during July and August when
soil moisture is adequate due to summer rains, but may occur anytime
between March and November [51].  Recent research on honey mesquite in
Texas suggests that past grazing history has little influence on
mesquite establishment in grasslands, but that herbaceous defoliation
during the year of seed dispersal is a key factor [16].  In semi-desert
grasslands of Arizona, velvet mesquite seedling establishment was
similarly found to be much higher in grazed habitats and in areas
denuded by drought than in vigorous stands of perennial grasses [51].

In true desert environments, conditions that favor plant establishment
may occur only once every 5 to 10 years following intense rains [123].
Because velvet mesquite seeds can remain viable for several years, seeds
stored in the soil may germinate following such events.  In a crater in
northern Sonora, Mexico, velvet mesquite seedlings became established
following two unusually intense rainfalls in 1970 and 1972 which induced
ponding [120].

Seedling development:  Soil moisture stress and browsing by rodents and
insects often result in very slow development of velvet mesquite
seedlings.  Seedlings exhibit more rapid growth on coarse-textured soils
than fine-textured soils.  Velvet mesquite seedlings rapidly develop a
deep root system which allows them to reach water and nutrients for
later growth before upper soil layers become dry [13].  Top and root
growth of seedlings found on upland sites at the Santa Rita Experimental
Range that were protected from browsing was as follows [51]:

age in months       average maximum plant        average maximum root
                          height                      penetration 
                   (inches)  (centimeters)      (inches)  (centimeters)

    9                2.1         5.3              20.1        51.0
   14                3.0         7.6              27.1        68.8
   24                3.7         9.4              ----        ----
   38                4.5        11.4              33.4        84.8
   50                5.0        12.7              ----        ----

Another study found that on upland sites, 94 percent of all velvet
mesquite plants established over a 17-year period were under 3.0 feet
(0.9 m) tall [50].  

Vegetative regeneration:  Velvet mesquite plants have numerous perennial
dormant buds located along an underground stem.  These dormant buds
generally occur from the basal portion of the plant to about 6 inches
(15 cm) below the soil surface [95].  When aboveground growth is damaged
or killed, new sprouts arise from the bud zone.


SITE CHARACTERISTICS : 
In southern Arizona, precipitation and temperature change along an
elevational gradient to produce distinct vegetation zones.  Annual
precipitation ranges from only a few inches at the lowest elevations to
about 19 inches (480 mm) at the highest elevations [55].  Velvet mesquite
occurs primarily below 5,500 feet (1,676 m) in the three lowest
elevation vegetation zones described below:

  (1) The paloverde-bursage-cacti association is low elevation desert and
      occurs from about 1,000 to 3,000 feet (305-914 m) in elevation and
      receives about 7 to 12 inches (17.5-30 cm) of annual precipitation
      [55].  In this zone velvet mesquite's abundance is influenced by
      edaphic factors.  The processes of weathering and erosion has
      resulted in a soil particle gradient along the bajadas.  This
      gradient is from coarser textured soils on the upper bajada to
      finer textured soils on the lower bajada and valley fill [11].
      Velvet mesquite is often restricted to the coarse soils of the
      upper bajada, which have more available soil moisture than the
      finer textured soils [10].  Plants also occupy washes,
      intermittent streams, or playas of the lower bajada where they
      may have access to underground water, but are generally absent
      from the middle slopes of the bajada.

  (2) Desert grasslands dominated by grama grasses (Bouteloua spp.) and
      threeawns (Aristida spp.) occur between about 3,000 and 5,000 feet
      (914-1,524 m) in elevation and receive about 12 to 16 inches
      (300-400 mm) of annual precipitation.  Velvet mesquite occurs in
      various densities throughout desert grasslands.  On some sites it
      occurs as scattered plants forming mesquite savannas, but on
      others its persistence has converted many grasslands to "brushy
      ranges".  In southeastern Arizona, velvet mesquite-grasslands were
      found primarily on level or rolling upland areas with coarse- or
      medium-textured soils [30].  
 
  (3) Oak woodlands with velvet mesquite generally occur between about
      3,650 and 5,500 feet (1,113-1,676 m) in elevation [55].  

Velvet mesquite woodlands are found along major water courses and their
tributaries.  These bosques attain their maximum development on alluvial
deposits where plants may reach over 50 feet (15 m) in height and
comprise over 95 percent of the trees [86,87].  Bosques may also occur,
but are less common, on fine-textured soils and in side canyons [87].
Bosques are commonly on a terrace 5 to 20 feet (1.5-6 m) above the river
channel [55,86].  


SUCCESSIONAL STATUS : 
The geographical range of velvet mesquite has probably changed very
little since settlement times, but its abundance within its range has
increased dramatically.  Its widespread occurrence in desert grasslands,
and to a lesser degree in oak woodlands, is a relatively recent event.
Velvet mesquite thickets and scattered plants were occasionally found in
desert grasslands before settlement times but are widespread today [55].
Velvet mesquite's dramatic invasion into grasslands has been attributed
to a combination of (1) overgrazing by livestock which reduced
herbaceous fuels and thus reduced the frequency and intensity of range
fires and (2) the concurrent dispersal of mesquite seed by livestock
into grazed habitats.  Both factors are discussed below.

Fire - The role that range fires played in controlling velvet mesquite
density in desert grasslands prior to settlement by Europeans is
unclear.  Some researchers feel that fires had the ability to keep
velvet mesquite at very low densities within grasslands [60].  Fire has
killed up to 50 percent of velvet mesquite plants smaller than 1 inch
(2.5 cm) in basal stem diameter [51].  In desert grasslands it often
takes velvet mesquite 10 to 20 years to reach this size [51].  Prior to
grazing by livestock, herbaceous fuels were probably sufficiently
abundant to carry a hot fire.  Recurrent fires every 10 to 20 years
would have killed many plants and kept others in a low stature,
nonflowering state [51,60].

Seed dispersal - Some researchers hypothesize that low densities of
mesquites in Southwestern grasslands prior to the introduction by
livestock resulted primarily from limited seed dispersal [16].  Since
mesquites evolved with New World megafauna, such as camelids,
stegomastodons, notoungulates, and edentates [89], dispersal of mesquite
seeds became very restricted when most of these herbivores became
extinct at the end of the Pleistocene.  With the introduction of
livestock by European settlers, mesquite invaded grasslands as cattle
transported seed from plants which were primarily found in draws and
drainageways.  Livestock deposited the seeds into grazed habitats.
Studies have shown that vigorous stands of grass significantly reduce
velvet mesquite seedling establishment and survival when compared to
nearby stands weakened by grazing [51].

Saltcedar has become established along many rivers of the Southwest.
In some areas it has invaded and replaced velvet mesquite bosque
communities.  Salt cedar spread seems to be related to the altering of
natural river flow by dams [130].

Flooding of rivers can cause the destruction or formation of mesquite
bosque communities.  If channel widening and clearing occur bosques can
be destroyed by the undercutting and collapse of the terrace.  Following
the receding of floodwaters, velvet mesquite seedlings may become
established on freshly deposited alluvium.  When seedlings successfully
establish themselves on river bars, they further enhance the
accumulation of alluvium, which in time, progressively elevates the
surface above the river, allowing the development of the mesquite bosque
[88].


SEASONAL DEVELOPMENT : 
Spring bud break in velvet mesquite can vary from year to year and from
site to site.  Bud break seems to depend on both photo and thermal
periods and rarely occurs until after the last spring frost [29,84].
Bud break and flowering generally begins earlier in warmer locations
such as at lower elevations, lower latitudes, and in warm microhabitats
[27].  Following bud burst, twig elongation and leaf growth are rapid
and are generally completed in about 4 weeks [24].  Inflorescences
emerge and develop simultaneously with the leaves.  Flowers develop and
are fertilized about 25 days after they begin growth.  Insufficient soil
moisture can cause spring flowers to be entirely aborted before
pollination occurs. Flowers can also be destroyed by hail, heavy rain,
or strong winds [51].  New flowers will form after abortion or
destruction if favorable conditions later arise.  Pods mature about 7 to
9 weeks after bud burst [24,51].  In Arizona, fruits generally mature in
July and drop from the plant in September; however, mature fruits have
been observed from June through October [51].  Leaves usually drop by
late December [24], but during warm winters, plants may retain their
leaves until just before new ones begin growing in the spring [89].  If
summer rainfall is high, velvet mesquite can develop a smaller number of
second cohort of leaves [113].  A second set of flowers and fruit are
also possible with abundant summer rainfall.

Variation in the phenological development of velvet mesquite plants
growing at different elevations on the Santa Rita Experimental Range in
Arizona over a 4-year period is summarized below [51]:

year and                              elevation (feet)
phenological stage      3,000        3,500        4,000        4,500       
                 
   1949
leaf bud burst        late March   early April  late March   late April
flowering begins      early April  mid-April    mid-April    late April
flowering ends        early May    early June   mid-May      early July
first pods mature     mid-May      early June   mid-May      early July
all pods mature       early July   early July   early July   early Aug 

   1950
leaf bud burst        mid-March    late March   mid-March    mid-March
flowering begins      late March   late March   mid-March    late March
flowering ends        late April   late April   late April   mid-May
first pods mature     late April   mid-May      late April   mid-May
all pods mature       early July   mid-July     late July    mid-July

   1951
leaf bud burst        mid-April    mid-April    mid-April    mid-April
flowering begins      early May    mid April    early April  mid-April
flowering ends        early June   early June   mid-May      early June
first pods mature     early June   early June   late May     early June
all pods mature       early July   early July   late June    late July

   1952
leaf bud burst        mid-April    mid-April    mid-April    mid-April
flowering begins      mid-April    mid-April    mid-April    mid-April
flowering ends        mid-May      early June   early May    mid-May
first pods mature     early June   early June   late May     late May
all pods mature       early July   mid-July     mid-July     late July

Related categories for Species: Prosopis velutina | Velvet Mesquite