Index of Species Information

SPECIES:  Juglans microcarpa


SPECIES: Juglans microcarpa
AUTHORSHIP AND CITATION : Tirmenstein, D. A. 1990. Juglans microcarpa. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: [].

ABBREVIATION : JUGMIC SYNONYMS : Juglans rupestris Juglans nana SCS PLANT CODE : JUMI JUMIS COMMON NAMES : little walnut Texas walnut Texas black walnut dwarf walnut river walnut Mexican walnut walnut nogal nogalillo nogalito namboca TAXONOMY : The currently accepted scientific name of little walnut is Juglans microcarpa Berl. [17]. Two varieties are delineated on the basis of morphological differences such as leaf and fruit size [26]. The following varieties are commonly recognized [17]: Juglans microcarpa var. microcarpa Juglans microcarpa var. stewartii Little walnut hybridizes with the closely related Arizona black walnut (Juglans major) [36], and populations which exhibit various intermediate characteristics have been reported [26]. At the eastern edge of its range, little walnut intergrades with black walnut (J. nigra) [3,31]. LIFE FORM : Tree, Shrub FEDERAL LEGAL STATUS : No special status OTHER STATUS : NO-ENTRY


SPECIES: Juglans microcarpa
GENERAL DISTRIBUTION : Little walnut grows from southwestern Kansas through Oklahoma to central New Mexico and Texas, south into northeastern Mexico [5,15,31]. In Texas, little walnut grows from the valley of the Colorado River west to the mountains of the Trans-Pecos [36]. The variety stewartii grows in the Chisos Mountains of Texas and extends into Mexico [26]. ECOSYSTEMS : FRES15 Oak - hickory FRES21 Ponderosa pine FRES28 Western hardwoods FRES31 Shinnery FRES32 Texas savanna STATES : AZ KS NM OK TX MEXICO BLM PHYSIOGRAPHIC REGIONS : 7 Lower Basin and Range 13 Rocky Mountain Piedmont 14 Great Plains KUCHLER PLANT ASSOCIATIONS : K060 Mesquite savanna K061 Mesquite - acacia savanna K062 Mesquite - live oak savanna K071 Shinnery K084 Cross Timbers K086 Juniper - oak savanna K087 Mesquite - oak savanna SAF COVER TYPES : 66 Ashe juniper - redberry (Pinchot) juniper 67 Mohrs ("shin") oak 68 Mesquite 235 Cottonwood - willow 240 Arizona cypress SRM (RANGELAND) COVER TYPES : NO-ENTRY HABITAT TYPES AND PLANT COMMUNITIES : Texas walnut occurs abundantly in a number of riparian woodland communities. It is included as a dominant or indicator in the following community type (cts) classifications: Area Classification Authority TX: Brewster Co. general veg. cts Carignan 1988 NM, TX: Guadalupe general veg. cts Gehlbach 1967 Escarpment Southwest riparian cts Szaro 1990a Southwest riparian cts Szaro 1990b TX: Brewster Co. general veg. cts Denyes 1956


SPECIES: Juglans microcarpa
WOOD PRODUCTS VALUE : Wood of little walnut is dark brown, hard and heavy but not strong [36]. Sapwood is white [36]. Annual rings are poorly cemented and the wood frequently comes apart as it is being worked [20]. This characteristic limits its use [20]. However, wood is sometimes used to make cabinets, furniture, paneling, and veneer [36]. IMPORTANCE TO LIVESTOCK AND WILDLIFE : Little walnut provides some deer browse in parts of New Mexico [19]. The nuts are a valuable food source for many wildlife species including squirrels and other rodents [5,30,36]. The rock squirrel readily consumes nuts when available [30]. PALATABILITY : Nuts of little walnut are highly palatable to a wide variety of wildlife species [5]. NUTRITIONAL VALUE : The nut meat of little walnut is described as nutritious and of "high quality" [30]. COVER VALUE : Riparian woodlands dominated by little walnut host a diverse array of animal life [7]. These areas form particularly good habitat for many species of birds [7]. VALUE FOR REHABILITATION OF DISTURBED SITES : Little walnut has been widely used for shelterbelt plantings [6,38] and has shown promise for use in some types of rehabilitation projects. Under certain conditions, it can aid in soil stabilization [33]. Plants can be easily propagated from seed. Cleaned seed averages 92 per pound (203/ kg) [5]. Seed can be planted during the fall in sandy soils at depths of 1 to 2 inches (3-5 cm) [36]. Methods of propagating by seed have been examined in detail [5,36]. OTHER USES AND VALUES : Little walnut plants are sometimes cultivated for the sweet, edible nuts [17,30,31]. It is the primary rootstock used in Texas for trials of nonnative walnuts and has potential value for use in developing walnut cultivars [29]. It has been cultivated as an ornamental and shade tree in both the United States and Europe [36]. Plants were first cultivated in 1868 [5]. OTHER MANAGEMENT CONSIDERATIONS : Damage: The walnut husk fly infests ripening fruit of little walnut after late summer rains [20]. The amount of fruit damaged by this insect varies annually but generally tends to be less in exposed windy areas [20]. Mature little walnut appears to be relatively unaffected by even "devastating" flash floods [14]. Disease: Little walnut is highly susceptible to root or crown rot when periodically flooded [24]. It is less susceptible to the fungus-caused walnut anthracose than black walnut is [3].


SPECIES: Juglans microcarpa
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 : Phanerophyte 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 [5]. 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]. Seedling establishment: 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].


SPECIES: Juglans microcarpa
FIRE ECOLOGY OR ADAPTATIONS : Little walnut is most commonly associated with riparian woodlands which burn infrequently. These narrow canyon forests frequently contrast strikingly with adjacent desert grassland or shrubland communities [8]. Evidence suggests that recurrent fires in the much drier desert grassland types may have eliminated invading shrubs and trees [16]. Because of its affinity for moist sites, little walnut exhibits few specific adaptations to fire. Reestablishment presumably occurs through off-site seed. POSTFIRE REGENERATION STRATEGY : survivor species; surviving root crown or caudex off-site colonizer; seed carried by animals or water; postfire yr 1&2


SPECIES: Juglans microcarpa
IMMEDIATE FIRE EFFECT ON PLANT : Little documentation exists on the specific effects of fire on little walnut. Mature individuals of the closely related Arizona walnut are reportedly killed by hot wildfires [4]. Portions of the stumps or root collars of small black walnuts commonly survive fire [12]. DISCUSSION AND QUALIFICATION OF FIRE EFFECT : NO-ENTRY PLANT RESPONSE TO FIRE : The response of little walnut to fire has not been documented. Small individuals of the closely related black walnut often sprout freely from the stump after aboveground vegetation is killed or damaged by fire [12]. Fowells [12] reports that sprouts which develop high on older stumps often succumb to rot or decay, but those which develop from the root collar generally survive. Larger individuals of the Arizona walnut apparently did not sprout after a hot wildfire in a Southwestern riparian woodland [4]. DISCUSSION AND QUALIFICATION OF PLANT RESPONSE : NO-ENTRY FIRE MANAGEMENT CONSIDERATIONS : Bock and Bock [4] report that prescribed fire is "difficult to manage and potentially very destructive in established riparian woodlands of the Southwest." These relatively rare and fragile areas provide important food and cover for desert wildlife [28]. Because browse and cover are often limited in these areas, burning is not generally recommended [28].

References for species: Juglans microcarpa

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21. Legner, E. F.; Goeden, R. D. 1987. Larval parasitism of Rhagoletis completa (Diptera: Tephritidae) on Juglands microcarpa (Juglandaceae) in western Texas and southeastern NM. Proceedings of the Entomological Society of Washington. 89(4): 739-743. [11546]
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25. Matheron, M. E.; Mircetich, S. M. 1985. Relative resistance of different rootstocks of English walnut to six Phytophthora spp. that cause root and crown rot in orchard trees. Plant Disease. 69(12): 1039-1041. [11544]
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