Hesperocyparis stephensonii

The Cuyamaca Cypress only verifiably exists in the headwaters area of King Creek in the Cuyamaca Mountains of the Peninsular Ranges system, south of Cuyamaca Peak within San Diego County in extreme Southern California.^[3][4]

Trees were reported growing as low as 3,000 feet (910 m) in elevation in 1998, but the presence of these individuals today has not been verified.^[5] Most individual trees occur at 4,500–5,100 feet (1,400–1,600 m) within the Pacific Southwest Research Station's King Creek Research Natural Area, in the Cleveland National Forest.^[6][7][8]

Hesperocyparis stephensonii is a tree that can attain heights of 10 to 16 m (33 to 52 ft).^[6] It usually forms a spreading tree with a central leader, only slightly taller than it is wide.

The tree's female cones are about 10 mm in diameter, while cone scales are normally 6–8 mm. Often, but not always, they have conspicuous umbos 3–4 mm, which are high and conical. There are normally 100-125 seeds per cone, not at all glaucous. 3-4 cotyledons are usually present. It is the only western cypress species in California to release pollen in the summertime.^[7][4]

Hesperocyparis stephensonii was not given a scientific description until 1948. In that year Carl Brandt Wolf published a paper and gave it the name Cupressus stephensonii.^[2][11] At that time it was standard for species classified in Hesperocyparis to be classified as part of a larger Cupressus genus that covered both new and old world species. Though somewhat distinct there was disagreement on its correct classification with it being described as a variety of Cupressus arizonica in 1966 and in 1982 as a subspecies of the same.^[2]

In the 2000s new genetic research called into question the classification of New World cypress trees. In 2009 a paper was published by Jim A. Bartel that reclassified most of the North American species into the new genus Hesperocyparis. By the year 2017 this classification was being used by the USDA Natural Resources Conservation Service PLANTS database,^[12] California Native Plant Society,^[13] and University and Jepson Herbaria.^[14]

In the past there has been considerable debate about the status of Hesperocyparis stephensonii as a species and some sources continue to use classifications such as Cupressus arizonica var. stephensonii.^[6][15] As of 2024^[update] Hesperocyparis stephensonii is recognized as an accepted species by Plants of the World Online,^[2] World Flora Online,^[16] and the Gymnosperm Database.^[6]

• Hesperocyparis forbesii — nearby endemic Cypress/Hesperocyparis sp.
• Flora of the California chaparral and woodlands
• Natural history of the Peninsular Ranges