Redwood

The redwoods, formally known as Sequoioideae, are a subfamily of conifers in the cypress family, Cupressaceae, found across the northern hemisphere. This group contains the planet’s largest and tallest trees, which are also among the most distinctive and are commonly planted as ornamentals. They grow quickly and thrive in temperate climates, including Mediterranean and humid subtropical zones.

The subfamily reached its greatest diversity in the early Cenozoic era. There are three living species: the coast redwood (Sequoia sempervirens), the giant redwood (Sequoiadendron giganteum), and the dawn redwood (Metasequoia glyptostroboides). The first two are native to the western United States—coastal California and Oregon for the coast redwood, and California’s Sierra Nevada for the giant redwood—while the dawn redwood comes from China.

Two of the genera, Sequoia and Sequoiadendron, are known for their massive size. The third, Metasequoia, is deciduous, grows much smaller (though still large by most standards), and can tolerate colder climates. These trees can live for thousands of years, but they face threats from logging, fire suppression, and burl poaching.

Taxonomy and evolution

Studies of both physical and genetic traits strongly indicate that the Sequoioideae form a single evolutionary group, or monophyletic clade. Most modern family trees place Sequoia as the closest relative of Sequoiadendron, with Metasequoia as an earlier offshoot. However, research by Yang and colleagues into the polyploidy of Sequoia—a rare condition in gymnosperms—has raised questions about this simple relationship. Sequoia sempervirens is hexaploid (2n=6x=66).

Possible reticulate evolution in Sequoioideae

Using two single-copy nuclear genes, LFY and NLY, Yang’s team found that Sequoia clustered with Metasequoia in one gene tree but with Sequoiadendron in the other. This supports the idea that Sequoia originated through reticulate evolution—specifically, hybrid speciation, where two species merge to form a third. Proposed hypotheses for the polyploidy include allopolyploidy from a hybrid between Metasequoia and an extinct relative, or between Metasequoia and Sequoiadendron, as well as autohexaploidy. The long evolutionary history of these genera, with fossils dating back to the Jurassic, makes it difficult to pinpoint exactly when and how Sequoia arose, especially given an incomplete fossil record.

Paleontology

The subfamily is ancient. The oldest described species, Sequoia jeholensis, comes from Jurassic deposits. Fossil wood from the late Triassic of China, Medulloprotaxodioxylon, resembles the giant redwood and may represent an ancestral form, suggesting a late Triassic origin for the group. During the Cretaceous, redwoods expanded dramatically across the Arctic, Europe, North America, Asia, and Japan.

A cooling trend from the late Eocene onward, along with ice ages, shrank their northern ranges. All three living species retain evolutionary adaptations to ancient environments, particularly in their reproductive ecology, which forced them into refugial areas where they survive today. An extinct genus, Austrosequoia, known from the Late Cretaceous to Oligocene of the Southern Hemisphere (Australia and New Zealand), has also been proposed as a member of this subfamily.

The entire subfamily is endangered.