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Millions of years ago, cypress woodlands containing one or more ancestral species of the cone-bearing genus Cupressus once dominated vast areas of California. During the past 20 million years, as mountains were uplifted and the climate became increasingly more arid, most of these extensive cypress woodlands vanished from the landscape. In some areas, the cypress were probably unable to compete with more drought resistant, aggressive species, such as impenetrable chaparral shrubs and desert scrub. Although cypress are fire-adapted with serotinous seed cones that open after a fire, they are vulnerable if the fire interval occurs too frequently, before the trees are old enough to produce a sufficient cone crop. Chaparral shrubs quickly resprout after a fast-moving brush fire from well-established subterranean lignotubers. This may explain why some cypress groves occur in very rocky, sterile sites with poor soils where the chaparral shrubs can't compete as well.

The present-day world distribution of cypress (Cupressus) includes North and Central America, Africa, Middle East and eastward along the Himalayas to China and North Vietnam. For thousands of years, Italian cypress (C. sempervirens) and mourning cypress (C. funebris) have been grown as ornamentals in the Mediterranean region and southern China. The beautiful, weeping Kashmir cypress (C. cashmeriana) has been cultivated around Buddhist Temples in Sikkim, Bhutan, Assam and nearby areas of Tibet and India. In Rome, the Church of Santa Sabina was built in the 5th century AD. The original door was made of Italian cypress with intricate carvings illustrating scenes from the Old and New Testaments. Whether or not cypress wood was used in the construction of Noah's Arc (Genesis 6:14) has been debated by biblical scholars for centuries. Italian cypress grew abundantly in the Middle East, and the seasoned timbers were very durable.

In California, cypress trees often grow on sterile, rocky outcrops. One of these rock types is serpentine (also called serpentinite) a greenish, shiny rock that is exposed throughout the Coast Ranges of central and northern California, and the Sierra Nevada foothills. According to the California Geological Survey, serpentine has been designated California's official state rock. Serpentine is a magnesium silicate rock with a waxy luster and a shiny, marblelike appearance. It varies from cream white through all shades of green to black. Higher grade, deeply-colored serpentines are used for animal carvings, particularly in Africa. Some polished serpentines resemble jade in color and are used in pendants and rings.

The distribution of some California cypress species, such as the Sargent cypress (Cupressus sargentii) and Macnab cypress (C. macnabiana) coincide nicely with serpentine outcrops. Other impoverished soil types include barren sandstone and metavolcanic rock. Cypress trees growing on rugged, rocky soils in the wild appear to be better anchored and able to withstand strong winds and rain, compared with top-heavy cultivated cypress in rain-soaked, loam soils.

Today this fascinating genus is represented by 10 species (or 8 species and 2 subspecies), confined to isolated groves scattered throughout the coastal and inland mountains, from the Mexican border to Oregon. Because some of these populations became isolated into "arboreal islands," gradual genetic changes over millions of years resulted in the present-day species and subspecies. It is quite likely that natural selection played a role in cypress speciation. Cypress of arid inland mountains and valleys (such as Piute cypress, Macnab cypress, Cuyamaca cypress, and Arizona cypress) have glandular (resinous) foliage and are more drought resistant. Coastal species (such as Monterey cypress, Gowen cypress, Santa Cruz cypress and Mendocino cypress) are generally nonglandular without resin glands on the leaf surfaces. Some phenotypic variability, particularly between different isolated groves of the same species may be due (in part) to genetic drift. These differences include slight variations in foliage, bark characteristics (exfoliating vs. persistent), and the general shape of seed cones. Differences attributed to genetic drift are analogous to racial differences in people, such as different blood type percentages and facial characteristics.

The relatively short period of isolation for Cupressus (cypress) species may be one of the reasons taxonomists disagree on the total number of species native to North America. In 1948, Carl B. Wolf published his "Taxonomic and Distributional Studies of the New World Cypresses" (El Aliso 1: 1-250). Dr. Wolf listed a total of 15 species, one in Baja California, one on Guadalupe Island off the coast of Baja California, one in Mexico and Central America, two in Arizona, and 10 in California. In 1953, the number of U.S. species was reduced to six by Dr. Elbert Little, Jr. in his Check List of Native and Naturalized Trees of the United States (USDA Agriculture Handbook No. 41). These numbers have fluctuated greatly in subsequent publications. In addition, the nursery trade has added several cultivated varieties, including at least four different cultivars for the Arizona cypress.

Evidence from DNA sequencing has further complicated the number of cypress species, including the transfer of other conifer genera into the genus Cupressus. For example, the Jepson Manual of California Plants lists ten species; however, two of these C. nootkatensis (Alaska cedar) and C. lawsoniana (Port Orford cedar) were formerly placed in the genus Chamaecyparis. These two genera are obviously closely related to Cupressus. In addition to their similar seed cones, they are also genetically compatible. The popular Leyland cypress (Cupressocyparis leylandii) used in landscaping is a bigeneric hybrid between the Monterey cypress (Cupressus macrocarpa) and the Alaska cedar (Chamecyparis nootkatensis).

Although coastal populations of Monterey cypress are resistant to corneum canker, trees planted in the interior valleys are very susceptible to this fungal disease. It is interesting to note that the Leyland cypress has also acquired this fungal susceptibility from its cypress parent. The cypress are a complex genus that still challenge taxonomists. It is possible that some of the disjunct species of Cupressus in California and Arizona have not been isolated long enough to warrant the status of a species. In fact, this is why most modern floras have consolidated four species of the southwestern United States and Baja California into subspecies of the Arizona cypress (C. arizonica). These species have been isolated long enough for genetic drift to occur, but perhaps not long enough for the development of distinct species populations.

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