• GENERAL BOTANICAL CHARACTERISTICS
• RAUNKIAER LIFE FORM
• REGENERATION PROCESSES
• SITE CHARACTERISTICS
• SUCCESSIONAL STATUS
• SEASONAL DEVELOPMENT

GENERAL BOTANICAL CHARACTERISTICS:

Alligator juniper is the largest southwestern juniper, but most commonly grows as a medium-sized evergreen tree that reaches 20 to 40 feet (6.1-12.2 m) at maturity [82,139]. It can attain heights of up to 65 feet (20 m) and maximum diameters of 7 feet (2.1 m) [69, 85]. A single short but heavy massive trunk is more typical than multiple stems [10,103]. Alligator juniper has a "dense and spreading canopy of deep green foliage" [103]. The plant receives its name from the thick, checkered, furrowed bark which is divided into scales resembling the back of an alligator [62,103,139]. Mature alligator junipers often have a large portion of dead wood intermixed with living wood. Chojnacky [23] estimated dead wood at <15% total volume in alligator juniper.

Cones generally contain 3 to 5 seeds [125]. The hard, mealy "berries," which mature during the second year, are approximately ½ inch (1.3 cm) in diameter [82].

Alligator juniper is noted for its slow growth rate. It ceases growth when moisture conditions are unfavorable but begins growing again with adequate moisture [54]. This characteristic greatly enhances the ability of alligator juniper to survive in harsh, arid environments. A diameter growth rate of approximately 0.6 inch (1.5 cm) per decade is typical for young trees, with growth slowing to 0.4 inch (0.1 cm) per decade after the tree reaches 170 years of age [90]. Maximum longevity is reported at 500 years [85].

RAUNKIAER LIFE FORM:

Phanerophyte

REGENERATION PROCESSES:

Alligator juniper is monoecious. Cones open during February or March and mature from August to October of their second year [66]. Cone crop production exhibits significant annual variation [91].

Juniper seeds have low germination rates due to impermeable to semipermeable, thick, hard seedcoats, chemical inhibitors, and dormant embryos [56,64,106]. Juniper seeds remain dormant in the soil until favorable moisture conditions promote germination [64]. "Major episodes" of alligator juniper establishment can occur during wet years [37]. Germination varies from 16 to 45%, with an average of 16% of alligator juniper seeds retaining viability after 9 years [6,64,66,125]. Approximately 30% of all alligator juniper seed stored in a warehouse for 5 years germinated [64]. Two to 3 years may be required for germination [30]. Pack [106] found that high temperatures, alternating temperatures, repeated freezing and thawing, removal of the seedcoat, and application of hydrogen peroxide, dilute acids, carbon dioxide, or light had little influence on the germination of juniper seeds. Johnsen and Alexander [66] note that the most common treatment for alligator juniper seed involves cold stratification at 41 degrees Fahrenheit (5^oC) for a period of 30 to 120 days.

Shrubs and trees often moderate harsh environmental conditions so juniper seedlings are commonly found beneath a canopy [56]. Animal dispersal is known to be extremely important in alligator juniper. Up to 95% of juniper reproduction may be attributed to birds on harsh New Mexico sites [44,108]. Digestive processes may encourage germination [9,39]. Juniper seed is dispersed by gravity, run-off, birds, or mammals including coyote, bighorn sheep, mice, rabbits, and livestock [19, 99,122]. Birds such as Bohemian waxwing, American robin, wild turkey, and jays disperse juniper seed. Brightly colored alligator juniper "berries" can be seen by birds at some distance and large quantities of this readily available food source may be consumed [9,41].

Alligator juniper is noted for its prolific sprouting ability [62]. This species is capable of sprouting from shallow roots, the root crown, or epicormic buds located along the branches or trunk [22,47,50,86,133]. Alligator juniper generally sprouts prolifically from dormant basal, root, or stem buds whenever the top is damaged, and consequently is not significantly reduced by most mechanical treatments. Sprouting often occurs after removal or death of the main trunk. Research indicates that younger trees sprout more readily than older plants [6] and that sprouting ability declines as stump diameters increases [47]. Alligator junipers 2 feet (0.6 m) or larger in diameter rarely sprout [6]. Jameson and Johnsen [62] report the following sprouting by stump diameter:

		trees with sprouts
stump diameter (in.)	# trees observed	#	%
1-6	156	140	90
7-12	27	18	67
13-30	37	22	59
31-48	2	0	0

Stumps larger than 12 inches (31 cm) in diameter generally produce sprouts only from multi-stemmed trees. Trees as young as 1 year in age may produce basal sprouts. Dormant buds typically begin growth soon after the top of the plant is damaged, but growth of dormant buds is often limited when side branches are damaged. Sprouts derived from the root crown are most common [62].

SITE CHARACTERISTICS:

Alligator juniper grows in semi-arid to subhumid montane "island" ecosystems in the Chihuahuan Desert and adjacent dry areas [37] as a subdominant in ponderosa pine forests of New Mexico, in riparian forest and riparian woodlands of Arizona [18,33]. Annual precipitation in pinyon-juniper ranges from 12 to 22 inches (305-559 mm) [49]. Alligator juniper is well adapted to survive short-term droughts, but mortality can occur after more than 6 years of drought [37]. Alligator juniper soils are variable but tend to be shallow, rocky and of low fertility [86]. Clay content averages approximately 18 to 27%. Some researchers report that alligator juniper favors soils which are slightly to moderately alkaline [81].

Alligator juniper most commonly grows between 4,000 and 6,000 ft (1,220-1,830 m) [30]. Elevational ranges of alligator juniper are as follows [46,82,86]:

     4,426 to 7,915 ft (1,350-2,414 m) from AZ to NM
     5,100 to 6,000 ft (1,556-1,830 m) from NM to TX

SUCCESSIONAL STATUS:

Succession and stand dynamics are poorly documented in evergreen woodlands in which alligator juniper occurs[77]. Pieper and others [110] note that "successional patterns where alligator juniper is a major potential component of the vegetation have not been determined." Climax alligator juniper stands have been reported on ridges and canyon rims [51]. In southwestern New Mexico, alligator juniper is considered a "minor climax species" [90]. However, this species has been included as a codominant indicator of climax vegetation in several habitat type classifications [73]. A generalized successional pathway for pinyon-juniper communities after fire has been described as follows [56]:

community description	postfire year
skeletal forest & bare soil	0
annuals	2
perennial grass/forb	4
shrub	25
shrub/open tree	100
climax pinyon-juniper	300

SEASONAL DEVELOPMENT:

Alligator juniper begins annual leader elongation in the early spring when soil temperatures warm to approximately 50 degrees Fahrenheit (10^oC) [54]. Phenological development in Arizona was as follows:

Phenological stage                                      Date

bark begins to slip                                       April 1
pollen shedding and female cones open      April 8
approximate start of leader elongation        April 20
1st conspicuous formation of male cones   August 19
bark begins to stick                                    September 15
leader elongation ceases                             October 19

Throughout its range, alligator juniper flowers from February through March [66,86]. Lymbery and Pieper [86] report the following phenological sequence in the Sacramento Mountains of New Mexico:

Year          Month                        Stage of development

1               February-March          cones open; pollen disperses
2               August-October          cones mature; seed disperses

Related categories for SPECIES: Juniperus deppeana | Alligator Juniper