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Maxwell [68] describes Joshua tree as a "delight to the eye and a fascinating feature of the western landscape." More specifically, however, Joshua tree is a 20- to 70-foot (5-20 m) tall, evergreen, tree-like plant. Trees exceeding 40 feet (10 m) are rare, and height is easily overestimated [51,62,72,76,77]. Tree size and growth form often vary with site and climate conditions [37,68,92]. Typically trees have 1 main stout stem that measures 1 to 3 feet (0.3-0.9 m) in diameter and have an expanded base [21,50,56,105,107]. Growth forms with several large stems are noted as well [92,107,110]. Trunks are fibrous, and the bark or periderm is "soft and cork like" [55,66,92]. Mature tree trunks typically measure 1 to 3 feet (0.3-0.9 m) in diameter. Bark plates measure 3 to 6 inches (7.5-15 cm) long by 1 to 2 inches (2.5-5 cm) in thickness [72].

Branching is often extensive on old plants, and rounded open crowns are common [37,50,62,92]. Young trees typically lack branches and are covered with persistent reflexed leaves [105]. Trees normally reach 3 to 9 feet (0.9-3 m) tall before branching [66]. Johnson [47] describes Joshua tree branching as "grotesque" and random. However, branching is formally referred to as dichotomous or almost dichotomous. Branches are formed following terminal bud death due to flowering or insect damage [50,66,68,88]. Branches are often 7 to 20 feet (2-5 m) or longer and fork at 2- to 3-foot (0.6-0.9 m) intervals. Inner branches are typically erect, and outer branches can be horizontal or drooping [21,50,62,72,110].

Joshua tree is slow growing and long lived [22,62]. Wallace and Romney [105] indicate that height, growth rings, or number of leaf blades may be used to age Joshua tree, but they caution that height may not accurately age "very mature" plants. Webber [107] reports that 21-year-old Joshua trees were unbranched, and the average annual growth rate was 5.9 cm/year. Other Y. b. var. jaegeriana plants grew an average of 11.7 cm/year. Johnson [47] indicates that large trees can be 300 years old, and Keith [55] suggests that Joshua tree has an average life span of 150 years. Little [62] suggests that Joshua tree is among the among the desert's "oldest living plants." An approximately 60-foot-tall (20 m) tree in California was an estimated 1,000 years old [62].

Leaves are clustered in rosettes at the branch ends. Clusters are commonly 1 to 5 feet (0.3-1.5 m) long and 1 to 2 feet (0.3-0.5 m) in diameter. Leaves are linear, needle shaped and measure 5.9 to 14 inches (15-35 cm) long by 0.3 to 0.6 inch (0.7-1.5 cm) wide. Enlarged bases attach the leaves to the branch. Leaf shape is slightly triangular and leaf margins are lined with small teeth. Spines measuring 0.3 to 0.5 inch (7-12 mm) occur at the leaf tips [6,21,37,50,51,62,76,77,107,110]. Leaf clusters are longer (3 to 5 feet (1-1.5 m)) on juvenile plants than on mature plants (1-3 feet (0.3-1 m)) [72]. Outer leaf layers are thick and waxy to reduce water loss [66]. Dead leaves are persistent and fold down, covering the branches and coating the trunks of young trees [47].

Joshua tree flowers occur in dense, heavy panicles that measure 8 to 20 inches (20-40 cm) long. Individual flowers are round to egg shaped and measure 1 to 2 inches (2.5-5 cm) by 0.4 to 0.8 inch (1-2 cm) wide [21,37,47,51,62,76,77,110]. Fruits are indehiscent capsules, which become spongy and dry with age. Egg-shaped capsules are 2 to 4 inches (6-10 cm) long and approximately 2 inches (5 cm) in diameter. Fruits develop at the base of the inflorescence while the upper portion is still in flower. Mature fruits contain 30 to 50 seeds, which are flat to thickened with smooth to undulate surfaces. Seeds are 0.3 to 0.4 inch (7-11 mm) long [3,21,47,50,61,62,72,76,77,107]. Fruit clusters often weigh over 9 pounds (4 kg), while a single capsule frequently weighs over 8.8 ounces (250 g). Fruits borne on erect branches are not easily detached [61]. Average individual seed weight ranged from 0.0025 to 0.0035 ounce (0.07-0.1 g) based on several seed collections in the Southwest [3]. In Los Angeles County, California, the average fruit length was 2.7 inches (69 mm), the number of seeds per locule averaged 26, and individual seed weight averaged 99 mg [53].


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