BOTANICAL AND ECOLOGICAL CHARACTERISTICS
SPECIES: Juglans microcarpa | Little Walnut 



GENERAL BOTANICAL CHARACTERISTICS : 
Little walnut grows as a large, many-trunked shrub or small, clumped,
spreading, low-branched tree [15].  Plants may grow from 20 to 50 feet
(6-15 m) in height [5,30,31].  The strongly scented trunk is gray to
dark brown and becomes deeply fissured with age [18,36].  Twigs are
slender, orange-reddish, gray-brown, or gray and pubescent [31,36].
Little walnut is a phreatophyte with a long taproot which allows the
plant to obtain water from the water table [30].

Alternate, odd-pinnately compound leaves are 8 to 12 inches (20-30 cm)
long [18,36].  The 7 to 25 narrowly ovate to lanceolate leaflets are
serrate with low teeth [5,36].  The leaf base is cuneate to rounded and
the apex acute [36].  The upper surface is dark yellow to green, dull
and glabrous [31,36]; the lower surface is somewhat paler [31].  Leaves
are glabrous at maturity and aromatic when crushed [5,36].

Little walnut is monoecious [31].  Slender staminate catkins develop on
the wood of the previous year [31].  Yellow-green pistillate flowers are
borne singly or in clusters in short terminal spikes on the current
year's growth [5,31].  Fruit is globose, 0.5 to 0.8 inch (1.2-2.0 cm) in
diameter, brownish, and glabrous with age [5,18].  The fruit is borne
singly or in clusters of two or three [31].  The indehiscent husk or
shell is dark brown, thick, and fibrous [5,18,31].  The hard, dark brown
nut of little walnut [31] is the smallest of all walnuts (Juglans spp.)
[20].  It is globose to ovoid [25], deeply grooved longitudinally, and
0.8 to 0.9 inch (2-2.3 cm) in diameter [31].  The kernel is sweet and
oily [36].

The variety stewartii is characterized by slightly larger fruit (0.8 to
1 inch (21-25 mm) in length) and broader leaflets (0.6 to 0.9 inch
(15-23 mm) wide) [26].  The Arizona walnut is morphologically similar
to little walnut, and identification may be difficult where both species
occur together [30].


RAUNKIAER LIFE FORM : 
   Undisturbed State:  Phanerophyte (microphanerophyte)
   Undisturbed State:  Phanerophyte (mesophanerophyte)
   Burned or Clipped State:  Therophyte


REGENERATION PROCESSES : 
Seed:  Most walnuts bear abundant seed crops at irregular intervals
[12].  Little walnut first bears seed at approximately 20 years of age
[5].  Seed may be dispersed by animals or water.

Germination:  The seeds of most walnuts are characterized by a dormant
embryo [40].  Seed dormancy can be broken by stratification at 34 to 41
degrees F (1-5 degrees C) for 90 to 120 days [1,5].  Light is not
required for germination, and seeds germinate well when deeply buried in
moist soil [1].  Stratified seed generally germinates within 4 weeks,
but much variation has been noted [5].  Under natural conditions, seeds
germinate in the spring [5].  Results of laboratory tests are as follows
[5]:

        germination test conditions    germination energy      
        daily temperature        dur.           amount        days
        days        night       (days)          (percent)

        86           68         30-60            68           14

Germination capacity has averaged 46 percent in greenhouse experiments
[5].  Under natural conditions, seeds may be more likely to germinate on
high terraces than in ephemeral stream channels, which may be subject to
flooding [1].

Seedling establishment:  Seedling recruitment has been examined in
detail [1].  Once established, young plants generally grow rapidly [36].

Vegetative regeneration:  Epicormic branching of walnuts has been
reported [6].  The closely related black walnut reportedly stump-sprouts
after trees are cut or killed by fire [12].


SITE CHARACTERISTICS : 
Little walnut grows along rocky streambottoms, in canyons and arroyos,
and on first terraces of dry river beds [30,36].  It is particularly
common in arroyos of the Chihuahuan Desert [21].  In southeastern New
Mexico, little walnut grows along waterways that extend from the
foothills out onto the plains [20].  This plant commonly dominates
narrow riparian forests, which are often characterized by steep
limestone walls, and various river edge or creek bottom communities
[35,38].  Many sites have relatively high moisture availability.
Moisture is generally obtained from flowing or ephemeral streams and
flash floods [14,30].

Plant associates:  Common overstory associates of the Edwards Plateau of
Texas include pecan (Carya illinoensis), Texas persimmon (Diospyros
texana), netleaf hackberry (Celtis reticulata), and live oak (Quercus
virginiana) [38].  Species such as netleaf hackberry, cedar elm (Ulmus
crassifolia), and littleleaf sumac (Rhus microphylla) are particularly
common on drier sites [37].  Agarito (Mahonia trifoliolata), Ashe
juniper (Juniperus ashei), bald cypress (Taxodium distichum), American
sycamore (Platanus occidentalis), desert willow (Chilopsis linearis),
and Texas persimmon are common components of river edge or riparian
woodland communities [9,37,41].

Soils:  Little walnut commonly grows on shallow calcareous or alluvial
soils [35,37].  On many sites streambottom habitats are characterized
by gravelly soils, coarse sand, or exposed boulders [9,14].

Climate:  Little walnut grows in areas which receive less than 7 to 38
inches (<18 cm-72 cm) of precipitation annually [8,37].

Elevation:  Little walnut grows primarily in valleys at intermediate
elevations [26].  Generalized elevational ranges by geographic location
are as follows:

 Location  Elevation                             Authority

 TX-NM     3,950 to 5,250 feet (1,200-1,600 m)   Legner and Goeden 1987
 NM-TX     <5,200 feet (< 1,585 m)               Cottle 1931



SUCCESSIONAL STATUS : 
Little walnut is largely restricted to drainageways which support
riparian woodland communities.  These woodlands generally represent
climax or "postclimax" communities [8].


SEASONAL DEVELOPMENT : 
Flowering begins with or slightly after leaf emergence [5].  Male and
female flowers, which are borne on the same trees, mature at different
times, thus promoting cross-pollination [5].  Fruit ripens in late
summer or fall [5].  Generalized flowering and fruiting dates for little
walnut by geographic location are as follows:

  Location       Flowering     Fruit ripe      Authority

  SW             March-April     ----          Vines 1960
  Great Plains   March-April     ----          Great Plains Flora Assoc. 1986
 c Great Plains  May            October        Stephens 1973

Seed is typically dispersed in fall [5].  Some fruit falls from the
trees before the last week of August [21], but in many areas, most seed
falls during September and October [1].  Timing of fruit fall depends
largely on weather conditions such as wind and rain [21].

Related categories for Species: Juglans microcarpa | Little Walnut