Sequoioideae

Sequoioideae, better known as redwoods, are a subfamily of conifers in the Cupressaceae family found across the northern hemisphere. They are home to the planet's largest and tallest trees, and they are among the most recognizable trees globally, often used as ornamentals. Their diversity was at its highest in the early Cenozoic. These fast-growing trees thrive in temperate climates, including Mediterranean and humid subtropical zones. There are three living species: the coast redwood (*Sequoia sempervirens*), the giant redwood (*Sequoiadendron giganteum*), and the dawn redwood (*Metasequoia glyptostroboides*). The coast and giant redwoods grow in the western United States, while the dawn redwood is native to China.

The three genera—*Sequoia* (coastal California and Oregon), *Sequoiadendron* (California's Sierra Nevada), and *Metasequoia* (China)—include the world's largest and tallest trees. These trees can live for thousands of years, but they face threats from logging, fire suppression, and burl poaching. Only *Sequoia* and *Sequoiadendron* produce truly massive trees. *Metasequoia* trees, represented by a single living species, are deciduous, much smaller (though still large compared to most trees), and can tolerate colder climates.

Studies of both physical and genetic traits strongly support the idea that Sequoioideae is a single evolutionary group (monophyletic). Most modern family trees place *Sequoia* as a close relative of *Sequoiadendron*, with *Metasequoia* as a more distant cousin. However, research by Yang et al. into the genetic makeup of Sequoioideae—specifically the polyploidy of *Sequoia*—produced a notable exception that questions this general consensus.

A 2006 study based on non-molecular evidence suggested a particular relationship among the living species. A 2021 molecular study found the same relationships within Sequoioideae but placed the subfamily as the sister group to Athrotaxidoideae (a group now only found in Tasmania), rather than to Taxodioideae. These two subfamilies are thought to have split during the Jurassic.

Reticulate evolution—where a new species arises from the merging of ancestral lineages—may have occurred in Sequoioideae. Polyploidy is common in flowering plants and ferns but rare in gymnosperms. *Sequoia sempervirens* is hexaploid (2n=6x=66). To understand this, Yang et al. used two single-copy nuclear genes, LFY and NLY, to build family trees. They found that *Sequoia* grouped with *Metasequoia* using the LFY gene but with *Sequoiadendron* using the NLY gene. This strongly suggests *Sequoia* originated from a hybridization event between *Metasequoia* and *Sequoiadendron*. The inconsistent relationships among the three genera may indicate reticulate evolution through hybrid speciation. However, the long history of these genera (with fossils from the Jurassic) makes it difficult to determine exactly when and how *Sequoia* arose, especially given an incomplete fossil record.

The three living species are: *Metasequoia glyptostroboides* (dawn redwood, south-central China); *Sequoiadendron giganteum* (giant sequoia or giant redwood, western slopes of California's Sierra Nevada); and *Sequoia sempervirens* (coastal redwood or California redwood, northern California coast and extreme southern Oregon).

Sequoioideae is an ancient group. The oldest described species, *Sequoia jeholensis*, comes from Jurassic deposits. Fossil wood called *Medulloprotaxodioxylon* from the late Triassic of China resembles *Sequoiadendron giganteum* and may represent an ancestral form, suggesting a Late Triassic Norian origin. The fossil record shows a huge range expansion during the Cretaceous, with Sequoioideae dominating northern latitudes as part of the Arcto-Tertiary Geoflora. Their fossils have been found in the Arctic Circle, Europe, North America, and across Asia and Japan. Cooling starting in the late Eocene and Oligocene, along with later ice ages, shrank their northern ranges. All three species still carry evolutionary adaptations to ancient environments, especially in their reproductive ecology, which ultimately confined each to refugial areas where they could survive. An extinct genus, *Austrosequoia* (Late Cretaceous to Oligocene of the Southern Hemisphere, including Australia and New Zealand), has been suggested as a member of the subfamily.

The entire subfamily is endangered, as assessed by the IUCN Red List.