Sequoioideae

Sequoioideae is a subfamily of coniferous trees found in the northern hemisphere. These trees belong to the larger family known as Cupressaceae. They are famous because they include the largest and tallest trees on our planet. Because of their impressive size and beauty, people often plant them as ornamental trees in gardens. These massive plants are incredibly long-lived and can survive for thousands of years. Studying them helps scientists understand how life changes over vast amounts of time.

There are three distinct genera, or groups, within this subfamily. The first is Sequoia, which grows along the coasts of California and Oregon. The second is Sequoiadendron, found on the western slopes of the Sierra Nevada mountains. The third group is Metasequoia, which is located in China. While Sequoia and Sequoiadendron are known for being massive, Metasequoia is different. These trees are deciduous, meaning they lose their leaves. They grow much smaller than their cousins but can survive in colder climates.

Scientists use molecular studies to understand the complex history of these trees. Molecular studies examine the genetic instructions inside cells to see how species are related. A major mystery involves the polyploidy of the Sequoia genus. Polyploidy occurs when an organism has more than two sets of chromosomes. Sequoia sempervirens is hexaploid, meaning it has six sets of chromosomes. Researchers like Yang et al. investigated this using two specific nuclear genes called LFY and NLY. This genetic complexity suggests a very unusual way of evolving.

One fascinating theory involves a process called reticulate evolution. This happens when different ancestor lineages merge to create a new taxon. Specifically, researchers suggest that Sequoia might have resulted from hybrid speciation. This is when two different species hybridize to give rise to a third species. By looking at the LFY and NLY genes, Yang et al. found inconsistent relationships. The LFY gene clustered Sequoia with Metasequoia. However, the NLY gene clustered it with Sequoiadendron. This evidence supports the idea that Sequoia is a hybrid of these two groups.

The history of Sequoioideae stretches back to ancient times. The oldest described species, Sequoia jeholensis, was recovered from Jurassic deposits. Some fossil wood called Medulloprotaxodioxylon was found in the late Triassic of China. This wood looks like the modern Sequoiadendron giganteum. This similarity suggests the subfamily may have an origin as far back as the Late Triassic Norian period. During the Cretaceous period, these trees saw a massive expansion in their range. They once dominated the Arcto-Tertiary Geoflora in many northern latitudes.

In the past, redwoods were much more widespread than they are today. They were found in the Arctic Circle, Europe, North America, and throughout Asia and Japan. However, a general cooling trend began during the late Eocene and Oligocene epochs. This cooling, along with subsequent ice ages, reduced their northern ranges. The trees had to adapt to survive in specific refugial ranges. These are smaller areas where a species can survive even when the surrounding environment changes. Their ancient adaptations still help them survive in these specific locations today.

Today, the members of this subfamily face serious survival challenges. All three genera are currently listed as endangered on the IUCN Red List. Sequoia sempervirens and Sequoiadendron giganteum are both classified as Endangered. The Metasequoia glyptostroboides is also listed as Endangered. Major threats to these giants include logging and fire suppression. People also engage in burl poaching, which harms the trees. In California, extreme wildfires recently destroyed about one-fifth of all giant sequoias over a two-year period. Even in Britain, where 500,000 redwoods exist, most were originally brought from the US during the Victorian era.