The evolution of the flora in this region is the outcome of tens of millions of years of evolutionary process, along with drastic climatic swings that triggered the expansion and recession of innumerable plant species. Notably, like the Klamath-Siskiyou Mountains as a whole, the Siskiyou Crest region was largely ice free during the last ice age, allowing plant species to continue evolving and find refuge, when much of the rest of the continent was covered in glaciers and ice sheets.
As climatic patterns came and went, species were isolated in unique microclimates that allowed them to persist, even as their larger, more contiguous populations receded. Due to this pattern, an astonishing array of plant species meet their geographic limits in these mountains, including conifer species like Pacific silver fir (Abies amabilis), subalpine fir (Abies lasiocarpa) and Alaska yellow-cedar (Callitropsis nootkatensis), which are found at the southern end of their range in cool, moist mountain habitats similar to the High Cascades of Oregon. The area also supports the westernmost populations of Western juniper, big sagebrush, and mountain mahogany from the high deserts of the Great Basin and interior West, as well as the northernmost populations of many chaparral and oak woodland species that extend into the region from the more arid, Mediterranean climate of the California foothills. Overall, the flora of the Siskiyou Crest, in both southwest Oregon and northwest California, is part of the extremely diverse California Floristic Province, at the northern extent of the province, where it begins to merge with the lush forests and flora of the Pacific Northwest.
The region supports numerous paleoendemic species that lived in the Klamath-Siskiyou Mountains during the Little Ice Age and became stranded as the last survivors of a once widespread flora that colonized the region roughly 65 million years ago. Portions of this ancient flora still exist today on the Siskiyou Crest, represented by species such as Baker cypress (Hesperocyparis bakeri), Brewer spruce (Picea breweriana), Port Orford-cedar (Chamaecyparis lawsoniana), Sadler oak (Quercus sadleriana), and Marshall’s gooseberry (Ribes marshallii).
Many botanists have described the flora of the region as a botanical museum due to the high occurrence of paleoendemic species in the region because of its ancient evolutionary history.
Yet, the Siskiyou Crest region has not just provided habitat niches for ancient ice age relicts; in fact, the region is also actively evolving new species known as neoendemics. This adaptive evolutionary process promotes new species that are often found in very small geographic areas and in isolated habitats. This speciation often occurs in response to unique soil conditions or genetic isolation.
Many of these species are currently evolving on the serpentine or ultramafic soils of the region and are actively diverging into new species such as, Siskiyou willowherb (Epilobium siskiyouense), Siskiyou paintbrush (Castilleja miniata ssp. elata), Copeland’s speedwell (Veronica copelandii), Oregon bleeding heart (Dicentra formosa ssp. oregana), brook trillium (Pseudotrillium rivale), Vollmer’s lily (Lilium pardalinum ssp. vollmeri), Serpentine catchfly (Silene serpentinicola), and McDonald’s rockcress (Arabis mcdonaldiana). These stark serpentine landscapes, dominated by barren rock, twisted shrubs, nutrient starved pine and cedar trees, carnivorous cobra lily fens, and colorful rock gardens, are emblematic of the Siskiyou Crest region, and are a stronghold of its extraordinary biodiversity.
The diversity of the Siskiyou Crest is globally significant and defines the rugged borderlands between southwest Oregon and northwest California. Protecting the Siskiyou Crest and its world-class biodiversity, requires world-class protections, as well as innovative and comprehensive permanent protection.
 DellaSala, DA, Reid, SB, Frest, TJ, Strittholt, JR, Olson, DM Natural Areas Journal [Nat. Areas J.]. Vol. 19, no. 4, pp. 300-319. Oct 1999. A Global Perspective on the Biodiversity of the Klamath-Siskiyou Ecoregion. https://www.researchgate.net/profile/David-Olson-12/publication/291850761_A_global_perspective_on_the_biodiversity_of_the_Klamath-Siskiyou_Ecoregion/links/5925cafc458515e3d450d82e/A-global-perspective-on-the-biodiversity-of-the-Klamath-Siskiyou-Ecoregion.pdf
Todt. Don. Clues to Past Environments: Relict and Disjunct Plant Distribution Along the California-Oregon Border. Living with the Land: The Indians of Southwestern Oregon. Ed. Hannon, Nan & Olmo, Richard. Southern Oregon Historical Society Press. 1990.
 Sawyer, John. 2006. Northwest California: A Natural History. University of California Press. Berkley & Los Angeles, California. 2006.
“The 70 endemics growing only on serpentine substrates in the Siskiyou Mountains outnumber endemics on any other serpentine outcrop in North America.”