Dacrydium cupressinum, commonly known as rimu, is a species of conifer in the family Podocarpaceae, native to New Zealand. The tree usually reaches in height, but may reach up to and can have a stout trunk up to in diameter. It is dioecious, meaning individual trees are either male or female. Rimu is the country's most common native conifer, its range covers the North, South, and Stewart Islands. Rimu has an estimated lifespan of 600–800 years, although it may live as long as 1,200 years.
The name D. cupressinum was first published invalidly in 1786 by Daniel Solander and Georg Forster. However the species was first given a valid description in 1806 by Aylmer Lambert. The second part of the scientific name, cupressinum, was given because of the tree's similarity with cypress species. Rimu's fruits are consumed by various birds such as bellbirds, kererū, and the tūī. Rimu's fruits also provide an important source of food and vitamins for the native flightless parrot, the critically endangered kākāpō, which will only mate during years of heavy fruiting. Many different plants grow on the tree's trunk and branches, using it for support, which are called epiphytes and hemi-epiphytes. Rimu also plays host to numerous insect species.
In Māori culture, D. cupressinum is of great importance. Rimu had several medicinal purposes for Māori, and the British missionary, Richard Taylor, described the fruit as prized by the natives. In 1773, the British explorer, James Cook, brewed the first native beer at Tamatea / Dusky Sound, by extracting the young tips of the tree's branches. Rimu was once the most harvested timber, popular for flooring, furniture making, wagon building, and other uses, before it was protected by law. The tree's timber has very durable and resilient texture. Rimu's conservation status was assessed by the IUCN Red List in 2013 as "Least Concern", and its population trend was assessed as "Stable".
Description Dacrydium cupressinum (rimu) is a species of dioecious (either male or female) evergreen conifer in the family Podocarpaceae, and usually reaches in height, but may reach up to tall, and can have a stout trunk up to in diameter. The trunk is usually cylindrical and smooth. D. cupressinum grows in an erect (occasionally forked), and usually a monopodial manner, meaning it grows from the main stem. When reaching its maximum height, it is typically unbranched for of its height. The tree exhibits a pyramid-like shape in the early growth stages, and the crown of mature trees evolves into a domed shape, with the main branches spreading outwards or ascending.
Its bark is grey to dark-brown in colour, shedding in large, elongated, thick flakes. When the outer layers of bark flake off, a distinctive wavy pattern of parallel raised ridges is visible on the new bark beneath. These lines of ridges have been described as resembling the contour lines on a topographic map. Areas of bark between the ridges have small pointed projections with porous tissue that allow air to penetrate the bark, providing oxygen to the cells of the tree beneath the bark. The wood is typically a dark-red colour, gradually becoming a yellow-brown colour after exposure to sunlight. D. cupressinum has an estimated lifespan of 600–800 years, but can reach 1000 years old, although some specimens examined at the foothills of Mount Ruapehu suggest it may live as long as 1200 years.
Dacrydium cupressinum is characterised by its drooping and hanging foliage. Juvenile branches are slender, with hanging branchlets. In adult specimens, there are fewer branches, which are spreading, and also have slender and hanging branchlets. The tree's leaves vary in colour from dark-green to bronze-green, red-green, or in some cases orange. They are typically a yellowish-green colour and about 4 mm long, and arranged in an overlapping pattern (imbricated). Juvenile leaves measure 4–7–(10) millimetres in length and 0.5–1 mm in width. They are acute, long and narrow, slightly curved, and vaguely sickle-shaped (subfalcate) in character. Subadult leaves are ascending and are 4–6 mm long. Adult leaves are similar, but they are flattened down (appressed), rigid, and are triangular in shape.
Like all conifers, D. cupressinum does not have flowers, but instead has cones (strobili); with male and female examples first seen on subadult specimens. In male specimens, cones are solitary or paired, terminal, measuring 5–10 mm, and are oblong in shape, producing yellow pollen. The tree's ovules (female reproductive structures) are solitary and at the tip of up-curved branchlets. Its receptacles (fleshy fruit-holding structures) form a cup that is red or deep-orange in colour and measures about 1–2 mm in length. The pollen cones are found at the tips of branches and are oval to oblong in shape, measuring 7–10 mm long and 4 mm wide. Each pollen-bearing leaf (microsporophyll) has a pointed tip, is green in colour, and is 2–2.5 mm long. At its base, it holds two yellow pollen sacs.
Fruiting takes over a year to mature and often coincides with the presence of young female cones. The fruits are most commonly seen between February and May. D. cupressinum is a morphologically distinctive species, although it is sometimes misidentified with the seedlings of Manoao colensoi, which instead have glossier, coarser leaves. The tree's seed cones grow at the ends of short, curved branchlets or on the sides, measuring about 6–10 mm long. The bracts (specialised leaves) attached are short and barely visible when the cones mature into a swollen, orange to red colour. The tree's seeds are ovoid in shape, dark brown to black in colour, slightly flattened, are tiny at about 4–5 mm long, and they ripen to a black colour. D. cupressinum has a genetic diploid chromosome count of 20.
Phytochemistry In terms of chemical composition, the foliage of D. cupressinum contains various sesquiterpenes, with their levels showing significant variation between specimens, which is similar to the differences observed in diterpene levels. There are twelve known sesquiterpenes in the tree that have been identified by a 1985 study, but none of which show any structural similarity to lauren-1-ene. The distinctive diterpene lauren-1-ene was first discovered in the foliage of the tree, however, the concentration of this compound, along with other diterpenes, also shows significant variation between D. cupressinum specimens.
Dacrydium cupressinum contains high levels of the unique sesquiterpene terpene compounds of 9-epi-β-caryophyllene. The tree also has a distinctive flavonoid glycoside profile; the tree possesses flavonol-3-O-rhamnoglucosides and also, to a lesser extent, 3-0-methyl-myricetin glycoside compounds.
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Taxonomy Evolution
Dacrydium cupressinum is the only member of the genus Dacrydium native to New Zealand. A 1998 study on the phylogeny of the Podocarpaceae family examined the evolution of D. cupressinum and various other species. The research identified it as sister to a clade (group) consisting of various other Podocarpaceae species, including members of the genus Dacrycarpus and Podocarpus. This discovery revealed that D. cupressinum shares a common biological ancestor with these genera, while maintaining its evolutionary uniqueness.
A 2020 study suggested that the Dacrydium genus originated in New Caledonia, based on a historical biogeographic reconstruction. Despite this, macrofossil evidence strongly supports a more broader Australasian origin, and another study also suggests that Dacrydium originated in Australasia during the Late Cretaceous epoch and dispersed into Southeast Asia in the Early Oligocene. Stull et al. 2021 investigated the role of plant evolution, analysing how ecological changes have shaped biodiversity and trait development over long timescales. In the study, they revealed that polyploidy is a primary driver of microevolution in plants, but its broader impact on macroevolutionary diversification and phenotypic innovation remains uncertain. In the study, researchers produced an ancestral reconstruction of gymnosperms using RNA sequencing. D. cupressinum is the type species of the genus Dacrydium.
History The name D. cupressinum was first published invalidly in 1786 by the European naturalist Georg Forster, using a description by Daniel Solander, in a publication entitled, De Plantis Esculentis Insularum Oceani Australis Commentatio Botanica. The British botanist Aylmer Lambert provided the first valid description of the species in his 1806 work, A Description of the Genus Pinus. The name, Thalamia cupressina, comes from a later 1826 description of D. cupressinum where Kurt Sprengel attempted to place the taxon, D. cupressinum into the genus, Thalamia. His placement has not been accepted by other plant taxonomists, and hence the name given by Sprengel becomes a homotypic synonym. In 1855, French botanist Élie-Abel Carrière described the species Dacrydium cupressiforme, in his work . What he thought to be a new species is now recognised as being Dacrydium cupressinum and hence is a heterotypic synonym of the species described earlier in 1806.
Etymology The etymology (word origin) of D. cupressinums genus name, Dacrydium, comes from the Greek (δάκρυδον), which translates to 'tear-drop' or simply 'tear'; one possibility, suggested by Dutch botanist Aljos Farjon, stated that the genus name could be in reference to the resinous drops exuded by the tree. However, English-born botanist Thomas Kirk, in his 1889 revision titled The Forest Flora of New Zealand, instead hypothesised that Dacrydium is in reference to the weeping habit of the trees. The specific epithet (second part of the scientific name), , translates simply to 'cypress'; which refers to the tree's similarity with Cupressus species. D. cupressinum is commonly known as rimu. The word rimu (or limu) is typically used in Polynesian languages to describe seaweeds and mosses; it is likely that the name was applied to the tree due to the similarity of the tree's leaves to seaweed.
Ecology
Dacrydium cupressinums seeds are dispersed by gravity and by fruit-eating animals (frugivores). These seeds are well-adapted to be dispersed by birds; primary dispersers of the seeds include bellbirds, kererū, and the tūī. Brown creepers, fantails, grey warblers, riflemen, silvereyes, and tomtits have been observed in association with the tree. Other birds, such as white-eyes and house sparrows, feed on the tree's receptacles (fleshy fruit-like structures). The tree is known to be browsed by deer and possums, which browse the seedlings and consume the foliage and branchlets, respectively. Excrements of the wētā are frequently found in seed traps, which could indicate that wētā consume the seeds of the tree. The seeds have been recorded being destroyed by chaffinches, mice, rats, and wētā.
Dacrydium cupressinums fruits are an important food and vitamin resource for the native flightless parrot, the kākāpō, which feeds on the fruits. The fruits are high in calcium and vitamins, and the kākāpō may have adapted to survive prolonged periods of limited access to these resources. The kākāpō will only mate during years of heavy fruiting.