Eucalyptus regnans
Eucalyptus regnans, known variously as mountain ash, giant ash or swamp gum, or stringy gum, is a species of very tall forest tree that is native to the Australian states of Tasmania and Victoria. It is a straight-trunked tree with smooth grey bark, but with a stocking of rough brown bark at the base, glossy green, lance-shaped to curved adult leaves, flower buds in groups of between nine and fifteen, white flowers, and cup-shaped or conical fruit. It is the tallest of all flowering plants; the tallest measured living specimen, named Centurion, stands 100 metres tall in Tasmania.
It often grows in pure stands in tall wet forest, sometimes with rainforest understorey, and in temperate, high rainfall areas with deep loam soils. A large number of the trees have been logged, including some of the tallest known. This species of eucalypt does not possess a lignotuber and is often killed by bushfire, regenerating from seed. Mature forests dominated by E. regnans have been found to store more carbon than any other forest known. The species is grown in plantations in Australia and in other countries. Along with E. obliqua and E. delegatensis it is known in the timber industry as Tasmanian oak.
Description
Eucalyptus regnans is a broad-leaved, evergreen tree that typically grows to a height of but does not form a lignotuber. The crown is open and small in relation to the size of the rest of the tree. The trunk is straight with smooth, cream-coloured, greyish or brown bark with a stocking of more or less fibrous or flaky bark that extends up to at the base. The trunk typically reaches a diameter of at breast height. Young plants and coppice regrowth have glossy green, egg-shaped leaves that are held horizontally, long and wide and petiolate. Adult leaves are arranged alternately along the stems, the same shade of glossy green on both sides, lance-shaped to broadly lance-shaped or sickle-shaped, long and wide, tapering to a reddish petiole long. The upper and lower surfaces of the leaves are dotted with numerous tiny, circular or irregularly-shaped oil glands. Secondary leaf veins arise at an acute angle from the midvein and tertiary venation is sparse.The flower buds are arranged in leaf axils in groups of between nine and fifteen on one or two unbranched peduncles long, the individual buds on pedicels long. Mature buds are oval, long and wide with a rounded operculum. Flowering occurs from March to May and the flowers are white. The fruit is a woody, cup-shaped or conical capsule long and wide on a pedicel long and usually with three valves near the level of the rim. The seeds are pyramid-shaped, long with the hilum at the end.
Seedlings have kidney shaped cotyledons, and the first two to three pairs of leaves are arranged in opposite pairs along the stem, then alternate.
Taxonomy
Eucalyptus regnans was first formally described in 1871 by Victorian botanist Ferdinand von Mueller in the Annual Report of the Victorian Acclimatisation Society. He gave the specific epithet from the Latin word meaning "ruling". Mueller noted that "his species or variety, which might be called Eucalyptus regnans, represents the loftiest tree in British Territory." However, until 1882 he considered the tree to be a form or variety of the Tasmanian black peppermint and called it thus, not using the binomial name Eucalyptus regnans until the Systematic Census of Australian Plants in 1882, and giving it a formal diagnosis in 1888 in Volume 1 of the Key to the System of Victorian Plants, where he describes it as "stupendously tall". Von Mueller did not designate a type specimen, nor did he use the name Eucalyptus regnans on his many collections of "White Mountain Ash" at the Melbourne Herbarium. Victorian botanist Jim Willis selected a lectotype in 1967, one of the more complete collections of a specimen from the Dandenong Ranges, that von Mueller had noted was one "of the tall trees measured by Mr D. Boyle in March 1867."Eucalyptus regnans is widely known as the mountain ash, due to the resemblance of its wood to that of the northern hemisphere ash. Swamp gum is a name given to it in Tasmania, as well as stringy gum in northern Tasmania. Other common names include white mountain ash, giant ash, stringy gum, swamp gum and Tasmanian oak. Von Mueller called it the "Giant gum-tree" and "Spurious blackbutt" in his 1888 Key to the System of Victorian Plants. The timber has been known as "Tasmanian oak", because early settlers likened the strength of its wood to that of English oak.
The brown barrel is a close relative of mountain ash, with the two sharing the rare trait in eucalypts of paired inflorescences arising from axillary buds. Botanist Ian Brooker classified the two in the series Regnantes. The latter species differs in having brown fibrous bark all the way up its trunk, and was long classified as a subspecies of E. regnans. The series lies in the section Eucalyptus of the subgenus Eucalyptus within the genus Eucalyptus.
Population genetics
across its range of chloroplast DNA yielded 41 haplotypes, divided broadly into Victorian and Tasmanian groups, but also showing distinct profiles for some areas such as East Gippsland, and north-eastern and south-eastern Tasmania, suggesting the species had persisted in these areas during the Last Glacial Maximum and recolonised others. There was some sharing of haplotypes between populations of the Otway Ranges and north-western Tasmania, suggesting this was the most likely area for gene flow between the mainland and Tasmania in the past. Further analysis of the same chloroplast genetic markers by researchers at The Australian National University suggests that there is more natural haplotype diversity in the Central Highlands of Victoria than previously observed. More recently, next-generation sequencing of nuclear DNA identified very little population genetic structure throughout the range of the species, with a considerable proportion of the entire species genetic variation found within any given population of mountain ash. This suggests that gene flow is likely to be occurring over long distances, and that the lengthy generation times of the species has precluded the development of substantial genetic differentiation between Tasmania and the mainland. Further comparison of chloroplast and nuclear DNA markers confirmed the expectation of extensive pollen dispersal but limited seed dispersal, leading to patterns of strong differentiation in chloroplast markers and weak differentiation at nuclear markers.Genome-wide sequencing of numerous mountain ash populations suggests that hybridisation with messmate occurs frequently, with all populations currently studied having at least one hybrid individual present. In many cases these hybrids show no obvious morphological signs of hybridisation, although some individuals do show intermediate phenotypes in characteristics such as the oil gland density in leaves and the structure and height of rough bark on the trunk. Morphology is generally now considered to be a poor method of identifying hybrid individuals as it does not always accurately reflect the genetic makeup of an individual. A good example of this is a population of purported mountain ash on Wilson's Promontory in Victoria, which are morphologically more similar to mountain ash but genetically much more closely related to messmate. Other populations with high levels of hybridisation include those on Bruny Island and the Tasman Peninsula in Tasmania. It is not surprising that the populations with the highest level of hybridisation occur on islands, promontories and peninsulas, as these areas are likely to occur on the edge of the ecological niche of mountain ash, and the small patches of mountain ash still remaining at these sites are probably experiencing pollen swamping from the more dominant messmate trees. Hybrids between mountain ash and red stringybark have been observed in the Cathedral Range in Victoria. These trees resemble mountain ash in appearance though they lack the paired inflorescences, and have the oil composition of red stringybark.
Distribution and habitat
Eucalyptus regnans occurs across a 700 km by 500 km region in the southern Australian states of Victoria and Tasmania. The species grows mostly in cool, mountainous areas that receive rainfall over per year. E. regnans reaches its highest elevations of about ASL on the Errinundra Plateau in north-eastern Victoria, and its lowest elevations near sea-level in some southern parts of its Tasmanian distribution.In Victoria, stands of tall trees are found in the Otway, Dandenong, Yarra and Strzelecki ranges as well as Mount Disappointment and East Gippsland. However, the distribution is much reduced. Most of the E. regnans forest across Gippsland was cleared for farmland between 1860 and 1880, and in the Otway Ranges between 1880 and 1900, while severe bushfires hit in 1851, 1898 and 1939. In Tasmania, E. regnans is found in the Huon and Derwent River valleys in the southeast of the state.
In the Otways, the species is found in wet forest in pure stands or growing in association with mountain grey gum, messmate and Victorian blue gum. Other trees it grows with include manna gum, shining gum, myrtle beech and silver wattle The mountain ash-dominated forest can be interspersed with rainforest understory, with such species as southern sassafras, celery-top pine, leatherwood and horizontal. The mountain ash is most suited to deep friable clay loam soils, often of volcanic origin; in areas of poorer soils, it can be confined to watercourses and valleys.
Ecology
Tree growth and stand development
Eucalyptus regnans is a very fast growing tree, with mean height growth rates in young stands ranging from to per year. In fact, some individuals grow at more than per year for the first 20 years of their lives. However, growth rates slow with age, and eventually turn negative as old trees senesce and the tops of the canopy are damaged in high winds, lightning strikes or during fires. Mean tree height after 8 years is about 15 m, and after 22 years is about 33 m. After 50 years, trees are typically about tall. In young stands, mean stem diameter growth is approximately 0.8 to 2 cm per year, with half of the total stem diameter growth occurring in the first 90 years of life.A number of environmental factors influence the growth and maturation of E. regnans, with research showing that the amount of incident solar radiation is positively associated with height and stem diameter growth, and that the amount of sunlight received is strongly negatively correlated with the level of precipitation.
In the absence of disturbance events such as high-intensity fire, individual trees can survive for hundreds of years, with the oldest known individuals identified as being 500 years old. Historically, low-frequency and high-intensity wildfires would prevent many stands from reaching this age, with fires killing mature overstorey trees and a new cohort developing from canopy-stored seedbanks. Despite this, natural variation in the spatial scale and frequency of wildfires meant that 30-60% of pre-European E. regnans forests would have been considered old growth. In addition, studies of older E. regnans forests have shown that low-intensity fires lead to the development of younger cohorts of trees without killing the parent trees, which leads to the presence of multiple age classes in old-growth forests.
As E. regnans forests mature, they start to develop characteristics that are representative of old-growth stands, such as large hollows, long strips of decorticating bark, an abundance of tree ferns and rainforest trees, buttressing at the base of E. regnans trunks, large clumps of mistletoe in the canopy, large fallen logs, and thick mats of moisture-retaining mosses.