Species Brief
Juglans major, Arizona walnut
Written
July, 2026
Contributors
Ilana L. Abrahamson - 1st Compiler, Leila Duchac - 1st Editor, Taryn R. Brahmsteadt - 2nd Editor
Abrahamson, Ilana L., compiler. 2026. Species Brief: Juglans major, Arizona walnut. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Missoula Fire Sciences Laboratory (Producer). Available: https://research.fs.usda.gov/feis/species-briefs/jugmaj
DOI
10.2737/feis-species-brief-jugmaj
Scientific Name
Juglans major
Common Names
Arizona walnut
Arizona black walnut
Mexican walnut
mountain walnut
river walnut
western walnut
FEIS Lifeform Type
Tree
Kingdom
Plantae
Class
Magnoliopsida (Dicot)
Order
Juglandales
Family
Juglandaceae
Genus
Juglans
Nativity
Native
Invasiveness
Noninvasive
This Species Brief is a concise summary of the biological, ecological, and fire‑related characteristics of an individual species derived from a Fire Effects Information System (FEIS) Species Review. Table 1 is derived from a standardized template and highlights these characteristics.
| Attribute | Characteristic | Justification | Source |
|---|---|---|---|
| Biological characteristics | |||
| Life form | Tree | Arizona walnut is a small to large tree, usually 9 to 15 m tall and 0.3 to 1.2 m in basal diameter, with a single, straight trunk. | FEIS: Botanical Description [1] |
| Typical lifespan | 201+ years | Arizona walnut is long-lived, with the oldest individuals reaching about 400 years. | FEIS: Stand Structure [1] |
| Life cycle | Perennial | Arizona walnut is a long-lived tree. | FEIS: Stand Structure [1] |
| Drought tolerance | Moderate* | Arizona walnut is usually considered an obligate riparian species, yet its water needs are lower than associated riparian trees and it is somewhat drought tolerant. | FEIS: Site Characteristics [1] |
| Rooting depth | Deep | Arizona walnut is deep rooted. One study found a 10-year-old tree had a 14.3-cm-diameter taproot branching about 1 m belowground. Seedlings put most growth into taproots to reach moist soil layers. | FEIS: Botanical Description; Plant Growth and Mortality [1] |
| Bark thickness at maturity | Thick | The bark of mature Arizona walnut trees is described as thick. | FEIS: Botanical Description [1] |
| Sheds low branches | Insufficient information | The source does not provide explicit information about shedding of low branches. It typically has a single, straight trunk and a spreading upper crown. | FEIS: Botanical Description; Entire [1] |
| Reproduction and Regeneration | |||
| Adventitious buds | Root crown; Epicormic | Arizona walnut sprouts from the root crown after top-killing disturbances such as flood or fire, and may produce epicormic sprouts higher on the trunk when not completely top-killed. | FEIS: Vegetative Reproduction and Regeneration; Fire Adaptations and Plant Response to Fire [1] |
| Time to reproductive maturity | Insufficient information | The source does not provide explicit information about time to reproductive maturity. | FEIS: [1] |
| Soil seed bank longevity | Transient (<1 year) | Arizona walnut has a transient soil seed bank of nuts in litter and shallow soil. The nuts are short-lived because of their high lipid content. | FEIS: Seed Banking [1] |
| Serotinous cones | N/A | This attribute applies to conifers. Arizona walnut bears hard-shelled nuts in a thick husk. | FEIS: Botanical Description [1] |
| Seed dispersal mechanism | Gravity; Animal | Most Arizona walnut nuts fall under or near parent trees. Nuts may roll away from the parent tree on slopes, and rodents may disperse them on- and off-site. | FEIS: Seed Dispersal [1] |
| Seed dispersal distance | Short (mostly under parent plant) | Most nuts fall under or near parent trees. | FEIS: Seed Dispersal [1] |
| Optimal conditions for seedling establishment | Sun; Shade | Arizona walnut seedlings establish over wide light gradients, under open to closed canopies, though open, moist sites are most favorable. | FEIS: Seedling Establishment [1] |
| Successional Status | |||
| Shade tolerance | Shade tolerant | Arizona walnut is a shade-tolerant, mid- to late-successional tree whose advance regeneration grows in the understories of late-successional communities. | FEIS: Successional Status [1] |
| Successional status | Early; Mid; Late | Arizona walnut occurs in all stages of succession; it may pioneer on floodplains but is also a shade-tolerant, mid- to late-successional tree. | FEIS: Successional Status [1] |
| Fire | |||
| Primary fire response(s) or adaptation(s) | Recover/Resprout; Resist | Arizona walnut sprouts from the root crown or higher on the trunk after partial to complete top-kill by low- to moderate-severity fire. Limited research suggests it has relatively low postfire mortality (7-23%). Trees most often survive as mature trees or sprout after top-kill. | FEIS: Fire Adaptations and Plant Response to Fire; Immediate Fire Effects [1] |
| Germination is fire stimulated | Rarely/No | Arizona walnut nuts are dormant when shed, and cold stratification over winter breaks dormancy. Germination coincides with spring or summer rains, not fire cues. | FEIS: Germination [1] |
| Flowering/fruiting is fire stimulated | Insufficient information | The source does not provide explicit information about fire-stimulated flowering or fruiting. | FEIS: [1] |
| Season when it is most sensitive to fire | Insufficient information | The source did not provide information regarding when Arizona walnut is most sensitive to fire. | FEIS: [1] |
| Plant flammability | Insufficient information | Information on the flammability of Arizona walnut was lacking in the literature. | FEIS: Fuel Characteristics [1] |
| Early colonizer/recolonizer of burned or disturbed sites | Sometimes | Arizona walnut can establish in early postfire environments. Seedlings may establish from surviving trees (on-site seeds) or from rodent-dispersed seeds (off-site seeds); however, there are few data on its fire response. | FEIS: Successional Status; Fire Adaptations and Plant Response to Fire [1] |
| Effect of fire exclusion on population | Insufficient information | The source does not provide explicit information about the effect of fire exclusion on Arizona walnut populations. | FEIS: [1] |
| Is this species typically managed with prescribed fire? | Rarely/No | Little information was available on prescribed fire in southwestern riparian communities. While fire management in southwestern riparian areas may include fuel reduction and prescribed fire treatments that promote or maintain native vegetation and reduce or eradicate invasives, information is limited. One study recommends against burning in late-successional riparian woodlands because fire in those communities can be destructive and difficult to manage. | FEIS: Fire Management Considerations [1] |
| Predominant historical fire frequencies of ecosystem where it most commonly occurs | Insufficient information | It is unknown how frequent fires were within desert riparian areas historically. Fires may have been infrequent in areas with periodic flooding that removed understory vegetation, but more common in communities with wider riparian areas and more developed vegetation. Fires burning from adjacent uplands may have burned out when reaching riparian areas. However, specific information is lacking. | FEIS: Fire Regimes [1] |
| Predominant historical fire severity where it most commonly occurs | Insufficient information | Information on historical fire regimes in southwestern riparian areas is limited due to lack of data, and the variability of historical fire regimes within and among desert riparian areas is not well known. Most fires were likely patchy and of low severity due to the patchiness of vegetation; however, mixed- and high-severity fires likely also occurred. | FEIS: Fire Regimes [1] |
| Predominant historical fire regime group where it most commonly occurs | Insufficient information | LANDFIRE classifies Arizona walnut's riparian range in fire regime groups 1 (frequent, mixed-severity fire along larger rivers) 3, (moderately frequent, mixed-severity), and group 5 (infrequent, replacement-severity fire in smaller stringers). However, these general models are not specific to riparian areas with Arizona walnut and may not accurately describe historical fire regimes where Arizona walnut occurs. | FEIS: Fire Regimes [1] |
| Alters historical fuel characteristics of ecosystems where it occurs | Rarely/No | Arizona walnut is native and does not alter fuels itself; altered fuels and fire regimes in its communities are attributed to nonnative invaders such as tamarisk and Russian-olive with different fuel and fire traits. | FEIS: Fire Management Considerations [1] |
1. Fryer, Janet L. 2022. Juglans major, Arizona walnut. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Missoula Fire Sciences Laboratory (Producer). Available: https://research.fs.usda.gov/feis/species-reviews/jugmaj
Last updated September 1, 2026