About Cupressus guadalupensis var. forbesii (Jeps.) Little
Cupressus guadalupensis var. forbesii, commonly called Guadalupe cypress, is a coniferous evergreen tree. When fully mature, it develops a broad, dome-shaped crown. It is variable in size: mature trees reach 12–20 metres (39–66 ft) tall, but 2000s observations note most native trees fall toward the lower end of this range. The first recorded scientific description noted the species could grow to "40 feet high or more".
Its trunk bark is fairly smooth, with thin layers peeling off, and ranges in color from cherry-red to dull gray brown. Larger branches have bark with the same texture and color as the trunk. First-year new shoots are yellow-green to brown. Foliage grows in dense sprays, colored dark blue-green or glaucous blue. Leaves are scale-like, 1.5–2 millimeters long, arranged in four rows on rounded (not flattened) shoots. Foliage has a soft texture; when crushed, it releases a light resinous aroma. Leaf scales are not shed individually; instead, entire small twigs are shed after two to three years. Terminal branchlets reach 2–3 centimeters in length and only 1 millimeter in thickness.
Seed cones are nearly spherical, with 8 to 10 horn-shaped scales that are narrow at the base, wide at the top, and end in a pointed tip. They measure 3–3.5 centimeters in diameter, are green while growing, and ripen to gray-brown or brown. Each cone holds approximately 70 to 100 brown seeds coated in a light waxy, glaucous layer. These seeds are much larger than those of other western cypress trees, weighing two to seven times as much. Closed ripe cones still attached to the tree open after exposure to fire heat. Unlike cones of the related Tecate cypress (Hesperocyparis forbesii), Guadalupe cypress cones will also open after drying for several months on the tree, with no fire exposure required. Each seed is about 5–6 millimeters long, with a small 1-millimeter wing structure. Cones and seeds reach full ripeness 15 to 18 months after fertilization.
Pollen-producing cones (strobili) have 14 to 18 scales, another characteristic botanists use to distinguish this species from Tecate cypress, which only has 10 to 14 pollen cone scales. Guadalupe cypress strobili are relatively large: about 6 millimeters long and 2–5 millimeters in diameter. Trees growing in California produce pollen from October to November, while cultivated trees in France and Italy produce pollen from December to March.
Lifespan is estimated by counting annual growth rings in dead tree trunks. The average estimated longevity is 150 to 160 years for a tree with a 50 centimeter trunk diameter. One 64-centimeter diameter trunk collected by botanical explorer Edward Palmer had 236 growth rings, indicating a 236-year lifespan.
Seedlings are very similar to those of all other western cypress species, producing three to five long, thin, needle-like seed leaves that start upright then spread out from the stem. Each seed leaf is usually 8 to 12 millimeters long. Juvenile leaves also resemble the needle-like seed leaves, but are shorter, less thick, and less fleshy in texture. Young trees begin producing adult foliage instead of new juvenile leaves in their second to fourth year of growth. Under favorable growing conditions, one-year-old young trees reach an average height of 40 to 45 centimeters, and reach 110 centimeters tall by the end of their second year. This rapid growth continues, and young trees can reach an estimated 9.7 meters tall in their first decade.
Guadalupe cypress (classified as Hesperocyparis guadalupensis) is endemic to Mexico, and is only found naturally on Guadalupe Island, a volcanic Pacific island west of Baja California. It grows at altitudes of 800–1,300 metres (2,600–4,300 ft). In the 21st century, Guadalupe cypress forms the largest tree community on the island, but only grows on the island's northwest high plateau. The island's climate is shaped by cold Pacific Ocean currents, creating conditions similar to mainland areas much farther north. Summers are warm to hot, while winters frequently bring fog. Fine cypress twigs collect airborne moisture, which drops to the ground or runs down trunks, making the area immediately under the trees wetter than areas without tall vegetation. Winter brings high winds to the island, but historically large cypress groves created sheltered, calm conditions inside the groves.
The introduction of goats to Guadalupe Island dramatically impacted all plant populations, including Guadalupe cypress. The exact release date of goats is unknown, but the first record of goats on the island dates to 1859. The pre-goat population of Guadalupe cypress is estimated to have been around 75,000 trees. In 1885, botanist Edward Lee Greene observed that while there was evidence of an extensive cypress forest on the island's northern side, only dead and fallen trees remained there by that time. A smaller grove on the island's east side also disappeared sometime after the 1920s. Available evidence also indicates an increase in fire frequency on the island. Though Guadalupe cypress is partially fire-adapted, the increasing number of fires is likely an additional cause of early tree mortality.
Along with Guadalupe palm (Brahea edulis) and Guadalupe pine (Pinus radiata var. binata), Guadalupe cypress is one of the dominant, defining plant species on the island, and acts as an ecological keystone. A 1950s study of Guadalupe Island bird life found that the ruby-crowned kinglet subspecies Corthylio calendula obscurus regularly occurs in the island's cypress groves. Another very common bird in these groves is the endemic house finch subspecies Haemorhous mexicanus amplus.
Though endangered in the wild, Guadalupe cypress has a long history of cultivation in California and other regions around the world. It was already being grown in the San Francisco area before 1878, though the exact introduction date is unknown. Despite its long history of use in gardens and parks, it is not a commonly cultivated species in the United States. Scientist William Dallimore reported that Guadalupe cypress seldom produces cones when cultivated, a observation that has been repeated in later reports. However, cultivated trees growing in Italy and France do produce seed crops, and the species is most often propagated from seed. It can also be grown successfully from cuttings and grafting, including grafting onto Cupressus sempervirens or Hesperocyparis glabra. It is a much more popular and common ornamental species in Europe than in North America.
Guadalupe cypress has very low frost tolerance, and is particularly noted for sensitivity to spring frosts. It is generally restricted to USDA Zone 9 or warmer, requiring a minimum low temperature between −6.5 and −1 °C (20 and 30 °F). However, the narrow fastigiate (column-like) cultivar 'Greenlee's Blue Rocket' is reported to tolerate temperatures as low as −9.5 °C (15 °F). It is well suited to coastal planting, as it resists damage from coastal sprays and winds. It is also highly adaptable to many different soil conditions. Unlike Monterey cypress, it is highly resistant to cypress canker; only one case of infection has been reported in scientific literature. In Europe, cultivated trees sometimes show signs of cypress bark beetle (Phloeosinus) infestation in the crown. There are no significant commercial plantations of Guadalupe cypress, and it is not widely used within its native habitat. The largest known individual Guadalupe cypress grows in a park in Fresno, California: it is 21 m (69 ft) tall, with a trunk diameter at breast height of 1.63 m (5.3 ft).
