|Redwood tree (Sequoideae)|
The redwood trees are a subfamily (Sequoioideae) in the family Cupressaceae, the cypress trees. There are three living genera in the subfamily. There were once more species of redwood trees, but most have become extinct.
These trees are pyrophytes, which means they have adapted to protect themselves from fire. Because fire is common in the regions where they grow, redwood trees have developed thick, fire-resistant bark. Their cones open only after a fire. Due to better fire control in modern times, these trees are endangered.
Redwood trees can grow to be very large. The largest species, Sequoiadendron giganteum, can reach up to 94.8 m tall and 17 m across. The tallest tree in the world is a Sequoia sempervirens named Hyperion. The largest tree in the world by volume is a Sequoiadendron giganteum named the General Sherman Tree, after William Tecumseh Sherman.
The three redwood subfamily genera are Sequoia and Sequoiadendron of California and Oregon, United States; and Metasequoia in China. The redwood species contains the largest and tallest trees in the world. These trees can live for thousands of years. This is an endangered subfamily due to habitat losses from fire ecology suppression, logging, and air pollution. Other threats to its existence include: climate change, illegal marijuana cultivation, and burl poaching.
Only two of the genera, Sequoia and Sequoiadendron, are known for massive trees. Trees of Metasequoia, from the single living species Metasequoia glyptostroboides, are much smaller.
Taxonomy and evolution
Multiple studies of both morphological and molecular characters have strongly supported the assertion that the Sequoioideae are monophyletic.
Most modern phylogenies place Sequoia as sister to Sequoiadendron and Metasequoia as the out-group. However, Yang et al. went on to investigate the origin of a peculiar genetic artifact of the Sequoioideae—the polyploidy of Sequoia—and generated a notable exception that calls into question the specifics of this relative consensus.
Evidence for reticulate evolution in Sequoioideae
Polyploidy has come to be understood as quite common in plants—with estimates ranging from 47% to 100% of flowering plants and extant ferns having derived from ancient polyploidy. Within the gymnosperms however it is quite rare. Sequoia sempervirens is hexaploid (2n= 6x= 66). To investigate the origins of this polyploidy Yang et al. used two single copy nuclear genes, LFY and NLY, to generate phylogenetic trees. Other researchers have had success with these genes in similar studies on different taxa.
Several hypotheses have been proposed to explain the origin of Sequoia's polyploidy: allopolyploidy by hybridization between Metasequoia and some probably extinct taxodiaceous plant; Metasequoia and Sequoiadendron, or ancestors of the two genera, as the parental species of Sequoia; and autohexaploidy, autoallohexaploidy, or segmental allohexaploidy.
Yang et al. found that Sequoia was clustered with Metasequoia in the tree generated using the LFY gene, but with Sequoiadendron in the tree generated with the NLY gene. Further analysis strongly supported the hypothesis that Sequoia was the result of a hybridization event involving Metasequoia and Sequoiadendron. Thus, Yang et al. hypothesize that the inconsistent relationships among Metasequoia, Sequoia, and Sequoiadendron could be a sign of reticulate evolution (in which two species hybridize and give rise to a third) among the three genera. However, the long evolutionary history of the three genera (the earliest fossil remains being from the Jurassic) make resolving the specifics of when and how Sequoia originated once and for all a difficult matter—especially since it in part depends on an incomplete fossil record.
Hubei and Hunan province, China
- The native habitat of Metasequoia glyptostroboides in Chongqing municipality in south-central China.
- The native habitat of Sequoiadendron giganteum trees is only on the western slopes of the Sierra Nevada range of California.
- The native habitat of Sequoia sempervirens trees is only in the Northern California coastal forests ecoregion, on the Northern California coast and several miles into Oregon.
Sequoioideae is an ancient taxon, with the oldest described Sequoioideae species, Sequoia jeholensis, recovered from Jurassic deposits. A genus Medulloprotaxodioxylon, reported from the late Triassic of China supports the idea of a Norian origin.
The fossil record shows a massive expansion of range in the Cretaceous and dominance of the Arcto-Tertiary Geoflora, especially in northern latitudes. Genera of Sequoioideae were found in the Arctic Circle, Europe, North America, and throughout Asia and Japan. A general cooling trend beginning in the late Eocene and Oligocene reduced the northern ranges of the Sequoioideae, as did subsequent ice ages. Evolutionary adaptations to ancient environments persist in all three species despite changing climate, distribution, and associated flora., especially the specific demands of their reproduction ecology that ultimately forced each of the species into refugial ranges where they could survive.
The entire subfamily is endangered. The IUCN Red List Category & Criteria assesses Sequoia Sempervirens as Endangered (A2acd), Sequoiadendron giganteum as Endangered (B2ab) and Metasequoia glyptostroboides as Endangered (B1ab).
The two California redwood species, since the early 19th century, and the Chinese redwood species since 1948, have been cultivated horticulturally far beyond their native habitats. They are found in botanical gardens, public parks, and private landscapes in many similar climates worldwide. Plantings outside their native ranges particularly are found in California, the coastal Northwestern and Eastern United States, areas of China, Germany, the United Kingdom, Australia and near Rotorua New Zealand. They are also used in educational projects recreating the look of the megaflora of the Pleistocene landscape.
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