Metasequoia glyptostroboides
Metasequoia glyptostroboides, the dawn redwood, is a fast-growing, endangered deciduous conifer. It is the sole living species of the genus Metasequoia, one of three genera in the subfamily Sequoioideae of the family Cupressaceae. It now survives in the wild only in wet lower slopes and montane river and stream valleys in the border region of Hubei and Hunan provinces and Chongqing municipality in south-central China, notably in Lichuan county in Hubei. Although the shortest of the redwoods, it can grow to in height.
In 1941, the genus Metasequoia was reported by paleobotanist as a widely distributed extinct genus based on fossils, before attracting considerable attention a few years later when small populations were found alive in central China. It is a well-known example of a living fossil species. Modern dawn redwood appears identical to its late Cretaceous ancestors. The tree faces considerable risks of extinction in its wild range due to deforestation; however, it has been planted extensively in arboreta worldwide, where it has proved a popular and fast-growing ornamental plant. If the species had not been discovered when it was, it might have become extinct before being investigated.
History
Although it was commonly known from the fossil record from across the northern hemisphere, the dawn redwood was considered extinct until the mid-twentieth century. When the genus Metasequoia was first described in 1941, it was from Mesozoic Era fossils, none of which were less than 150 million years old. While studying fossil samples of the family Cupressaceae, Dr. Shigeru Miki from Kyoto University identified a divergent leaf form. He realized he had discovered a new genus, which he named Metasequoia, meaning "like a sequoia".In the same year, Kan Duo, professor of Forest Management, National Central University, Chongqing, observed an enormous living specimen while performing a survey in Sichuan and Hubei provinces. Though unaware of Miki's new genus, he recognized the unique traits of the tree. Unfortunately, while he collected some plant material, he did not attempt to identify or publish his findings. This tree formed part of a local shrine, where villagers called it . Shuǐshān continues to be the common name of dawn redwood in Chinese.
In 1943, Wang Zhan of the National Bureau of Forest Research, the Ministry of Agriculture and Forests, Chongqing, collected samples from an unidentified tree in the village of in Lichuan County, Hubei province—now believed to be the same tree Kan discovered. The samples were determined to belong to a tree yet unknown to science, but World War II postponed further study.
Professors Zheng Wanjun and Hu Xiansu made the pivotal connection between Miki's fossil genus and the living samples in 1946. The name Metasequoia viva was initially used, but was eventually changed to the specific epithet "glyptostroboides", after its resemblance to the Chinese swamp cypress.
In July 1947, the Arnold Arboretum of Harvard University provided $250 to fund an expedition by Zheng Wanjun's assistant Hua Jingchan to collect seeds for the arboretum from the Metasequoia type tree in Moudao, and trees in the nearby Metasequoia Valley. Hua's collecting trip returned with several kilos of seed that were distributed over the next few months for growth trials to Chinese institutions; the Arnold Arboretum, Missouri Botanic Garden, and elsewhere in the United States; botanic gardens at Kew, Edinburgh, and other locations in the United Kingdom; as well as botanic gardens in continental Europe and worldwide.
Description
Metasequoia glyptostroboides leaves are opposite, long, and bright fresh green, turning a foxy reddish brown in autumn. The pollen cones are long, produced on long spikes in early spring; they are produced only on trees growing in regions with hot summers. The cones are globose to ovoid, in diameter with 16–28 scales arranged in opposite pairs in four rows, each pair at right angles to the adjacent pair; they mature in about 8–9 months after pollination.While the bark and foliage are similar to another closely related genus of redwoods, Sequoia, M. glyptostroboides differs from the coast redwood in that it is deciduous, like Taxodium distichum. Similar to T. distichum, older trees may form wide buttresses on the lower trunk. M. glyptostroboides is a fast-growing tree, exceeding in height and in trunk diameter by the age of 50, in cultivation. The trunk forms a distinctive "armpit" under each branch. The bark is vertically fissured and tends to exfoliate in ribbon-like strips.
The largest dawn redwood recorded was an isolated specimen in China about tall and wide. This tree was killed by a lightning strike in 1951. Several dawn redwoods of this height still live in the eastern part of Metasequoia Valley, where the tree was discovered. The tree's true potential size is much larger, as logs up to wide at the base have been discovered in rice paddies.
The thickest and tallest dawn redwoods listed by Monumental Trees are both at Longwood Gardens, outside Kennett Square, Pennsylvania, United States. The widest has a girth at breast height of, average diameter, and is tall. The tallest is tall, and has a girth at breast height of, average diameter. Both trees were planted in 1948 and measured in 2018.
Ecotypic variation
has developed in a variety of microhabitats. Three variations have been observed based on the size of their cones, large, medium, and small. The large cone type is found growing on mountain slopes, develops a broad canopy, seeds more readily and is more drought tolerant. The small cone type is found near stream banks, is more sensitive to drought, and develops a more uniform trunk.| Ecological Type | Large cone | Medium cone | Small cone |
| Growth Rate | fast | medium | slow |
| Ecological preference | mountain slope, drought tolerant | mountain slope, medium drought tolerance | ditches and creeks, drought intolerant |
| Bark Thickness | thick | medium | thin |
| Bark Color | gray | brownish gray | brown |
| Stem Furrow | distinct | very distinct | indistinct |
| Leaf Color | yellowish green | green | dark green |
| Leaf Density | low | medium | high |
| Branch Canopy | wide | medium | narrow |
| Branch Distribution | sparse | medium | dense |
| Branch Angle | >90 | medium | <50 |
| Cone Size | 2.2 x 2.0 cm | 2.0 x 1.8 cm | 1.5 x 1.4 cm |
| Number of seeds per 500 grams | 128 | 161 | 280 |
| Number of seeds per cone | 106 | 85 | 62 |
| Seed size | 0.6 x 0.5 cm | 0.53 x 0.48 cm | 0.48 x 0.42 cm |
| Weight in grams per 1000 seeds | 2.96 | 2.84 | 2.40 |
| Seed germination rate | 15 | 21 | 18 |
Related species
Together with Sequoia sempervirens and Sequoiadendron giganteum of California, M. glyptostroboides is classified in the subfamily Sequoioideae of the family Cupressaceae. Although it is the only living species in its genus, three fossil species are known as well. The other Sequoioideae and several other genera have been transferred from the Taxodiaceae to the Cupressaceae based on DNA analysis.Conservation
Studies carried out between 2007 and 2009 counted 5,371 trees primarily in Lichuan, Hubei, with much smaller groups in Shizhu, Chongqing and Longshan, Hunan.The floodplain of Metasequoia Valley in Hubei had already been turned to rice paddies by the time of the tree's discovery, but was once probably a more extensive dawn redwood forest. Such a forest would have been similar to bald cypress forests in the United States, with many similar species growing in association. Nearly 3,000 trunks were found in the floor of the valley, ranging from wide at the base. Additionally, houses made of Metasequoia wood 200–300 years old still exist and probably date back to the original settlement of the valley.
Since its discovery, the dawn redwood has become something of a national point of pride, and it is protected under Chinese law and also planted widely. However, it is still listed as endangered in the wild. Cutting of trees or branches is illegal, but the demand for seedlings drives cone collection to the point that natural reproduction is no longer occurring in the dawn redwood forest. In addition, the landscape has been highly modified by human use, and a 1980 expedition found that the habitat has significantly degraded since the tree's discovery. Most of the other vegetation has been cut, and most of the area is no longer suitable ground for seedlings. Between 1950 and 1980 several hundred were logged in Metasequoia Valley, some over wide.
A survey in the 2000s counted and measured all wild dawn redwoods. The remaining healthy trees range from to wide at breast height, in height, and estimated 41–265 years in age. The average size was tall and wide at breast height, with an estimated age of about 95 years. No seedlings were found, and if they ever do appear it is legal to remove and transplant them. The species will continue to live in yards, parks and on roadsides all over China, but the M. glyptostroboides forest ecosystem could disappear when its mature trees die.
Cultivation
Since the tree's rediscovery, the dawn redwood has become a popular ornamental tree in parks and gardens worldwide.File:A642, Metasequoia glyptostroboides, the dawn redwood, Morris Arboretum, Chestnut Hill, Philadelphia, 2018.jpg|thumb|Grove of dawn redwoods in the Morris Arboretum, Philadelphia
Dawn redwood seed arrived at the Arnold Arboretum on January 5, 1948, and was shipped to arboreta around the world soon after. By 1951, the species had entered the commercial horticultural trade in the United States, and following media excitement about the new discovery, including in the San Francisco Chronicle of March 25, 1948, sales soon took off.
Dawn redwood has proved an easy tree to grow in temperate regions, and is now widely planted as an ornamental tree. Planted trees have already reached at least up to in diameter at breast height and tall, despite being in cultivation for only 70 years. This rapid rate of growth has led to consideration for using the tree in forestry plantations. It has been discovered that M. glyptostroboides will thrive in standing water, much like bald cypress, and if left branched to the ground in full sun, will develop the large, contorted boles that have made bald cypress famous. Limbing or pruning at an early age will prohibit this formation later on.
In cultivation, M. glyptostroboides is hardy to USDA Zone 5, making it hardy down to lows of. It is tolerant of soggy, waterlogged soils; in the wild it is adapted to growing on flood plains. Until it is established in a specific site, it is vulnerable to drought and inadequate water availability. The dawn redwood is recommended for urban areas in the Midwest, Southeast, and East Coast of North America, as its fast growth rate and tolerance for air pollution make it adaptable and able to thrive where other species might suffer. The species tends to struggle without irrigation in arid climates such as the American West unless planted directly on or adjacent to a body of water such as a pond or stream. This species is also highly susceptible to damage from contact with heavy amounts of winter de-icing salt.
In the 1980s, it was discovered that many of the second-generation trees in cultivation suffered from inbreeding depression, which could lead to increased susceptibility to disease and reproductive failure. Many sources claim that the original 1947 seedlots came from as few as one tree; however, this has proven to be false. The original seeds did have a wide range of source trees, and the inbreeding depression is more likely to come from self-pollination by isolated trees. However, the total cultivated population still had less genetic variation than the wild ones, and more widespread seed-collecting expeditions in China in the 1990s sought to resolve this problem and restore genetic diversity to cultivated M. glyptostroboides.