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Lowland ribbonwood (Plagianthus regius), commonly known as ribbonwood, Māori name Mānatu, is a quick-growing non-threatened endemic New Zealand tree from the Malvaceae family, with its source lineage being traced to Australia.

Lowland ribbonwood has had several name changes over the years; it was once called Plagianthus betulinus and before that Phillippodendrum regium. Lowland ribbonwood is also known as houi, houhi ongaonga, whauwhai, puruhi.

Description
Lowland ribbonwood development is heteroblastic meaning the plant's form and features change as it develops. Lowland ribbonwood has two very distinctive forms, juvenile and adult, where the juvenile form is quite different from the adult form. The juvenile plant has a divaricating growth form, with the plant growing in a tangled way with many close slender interlocking branches looking more like a shrub than a tree. Leaves are small (5-15mm long by 3-10mm wide), round and widely dispersed along the branches. The juvenile leaves have a soft texture with rounded toothed margins. At the base of the leaf stalk, small noticeable stipules form when young. It is thought that the divaricated growth form of lowland ribbonwood evolved soon after they became established in New Zealand.



Unlike its tangled juvenile form, the adult or mature lowland ribbonwood usually develops a straight trunk with wide spreading branches, though lower branches and leaves on some adult trees' still maintain their interlocking juvenile features. An adult/mature lowland ribbonwood can grow up to 15m with a trunk diameter of around 1m and is the largest of the ribbonwoods. Its bark is greyish in colour and appears rough. Flowers mature in early spring and are small (3-4mm) but numerous, hanging in large drooping clusters with a yellowish/green hue. Compared to the female flowers which are only slightly scented, the male flowers have a sweet scent. The leaves on a mature lowland ribbonwood are much larger than those of the juveniles (2.5-7.5cm long by 2-5cm wide) and are covered on both sides with soft star-shaped hairs. The leaves are thinner than in their juvenile form and are narrow at the tip with distinctive pointed tooth margins, with the underside of the leaves being paler than the upper surface. The arrangement of the leaves on the stem are alternating rather than opposite.

Distribution
Lowland ribbonwood grow widely throughout New Zealand from Kaitaia to Stewart Island. However, their distribution is more sparse in the regions of Auckland and Northland. A subspecies, Plagianthus reguis subsp. chatamicus, can be found in low lying forests on the Chatham Islands and differs from Plagianthus reguis by lacking the divaricating juvenile growth form. Lowland ribbonwood prefers coastal and lowland forest habitats up to around 500m above sea level. They tend to be found on river banks and alluvial terraces, preferring moist, fertile, free draining soils. Lowland ribbonwood often coexists with lacebark (Hoheria), another member of the Malvaceae family. Lowland ribbonwood is now not as common as it once was because much of its preferred habitat has been lost to agricultural development.

Life Cycle
Lowland ribbonwood has a distinct juvenile phase that gradually changes form (4 to 8 years) into an adult tree. Once the lowland ribbonwood reaches maturity, it starts to produce flowers. The male and female flowers are dioecious, meaning that they are generally found on separate trees, although some unisexual flowers can be found on male trees. Being dioecious is a rare feature in the Malvaceae family. Flowering generally occurs in spring (September to November) with fruit developing over the summer (December to February). Male flowers are larger than female flowers and once male flowers have released their pollen, their flowers fall to the ground. Pollination is usually via insects. The female flower's ovary matures into 1 or 2 small dry rounded fruits each containing a single seed. The fruit is dehiscent, meaning it splits open to release the seed inside. The fruit is wind and gravity dispersed, breaking open upon hitting the ground, and often falling to the ground on mass. Seeds usually germinate over a relatively short period of 2-5 months. Germination occurs in the autumn with an abundance of young seedlings appearing in early spring. Moreover, seed loss occurs because lowland ribbonwood seed is vulnerable to small insects boring holes into them.

Another distinctive aspect of the lowland ribbonwood lifecycle is that it is deciduous. Lowland ribbonwood is just one of 11 native deciduous trees. Being deciduous in winter is an unusual characteristic of endemic New Zealand flora where most trees are evergreen. Due to a warmer climate in the Auckland and Northland regions compared to the rest of New Zealand, lowland ribbonwood in these regions tend to retain their leaves all year round.

Interactions
Lowland ribbonwood interacts in many ways with its surrounding environment. Traits such as small leaves, deciduousness, and divaricated growth form are thought to have evolved in response to both climatic conditions and soil types. Lowland ribbonwood has adapted to its environment of fertile soils, dry windy summers and cool winters, by producing small, thin, high nutrient rich leaves that are short-lived. Leaf growth is maximised over summer but hard to maintain during the frosty winter months, thus leading to deciduousness over winter. Furthermore, juvenile lowland ribbonwood may have evolved a divaricating growth form as a defence mechanism against browsing herbivores such as the moa.

Lowland ribbonwood is the host plant for the rare New Zealand aphid (Paradoxaphis plagianthi). During spring, lowland ribbonwood provides a plentiful food source for the New Zealand aphid, which feeds on the new leaf growth. Lowland ribbonwood provides valuable nutrients to aphids such as nitrogen. During autumn and spring, nitrogen levels in leaves of lowland ribbonwood trees tend to be at their highest; therefore, in addition to leaf growth, nitrogen levels in leaves may also contribute to increase in aphids during these periods. Studies have shown that a low abundance of lowland ribbonwood in the ecosystem leads to a lower abundance of aphids. During winter when lowland ribbonwood has lost its leaves, few aphids are present. In addition, to being the host of Paradoxaphis plagianthi, lowland ribbonwood is also an attractive tree for native birds with the leaves, flowers, and fruit providing an abundant food source. Lowland ribbonwood is a particular favourite of Kereru.

Moreover, lowland ribbonwood is a host plant of two fungi, Puccinia plagianthi and Cryptosporiopsis hoheriae (previously known as Myxosporium hoheriae). Puccinia plagianthi is a rust fungus that can be seen on both the leaves and the stem. This fungus causes both sides of the leaf to develop pale yellow and dark brown blotches. It is also associated with large areas of distortions on the stem. This fungus is distributed throughout the North Island and South Island. Overall, Puccinia plagianthi has little impact on the plant's health. Unlike Puccinia plagianthi, Cryptosporiopsis hoheriae is a wound pathogen that can be harmful to the lowland ribbonwood as it can cause sizeable branch dieback.

Further Information
The actual wood from lowland ribbonwood was not of significant value to early European settlers or Māori, though for a while lowland ribbonwood was used for rails and palings due to the wood being straight and tough. However, due to its quick perishability, it is no longer used. In the 19th century, the fibres of the lowland ribbonwood were once considered ideal for manufacturing paper, but paper production never took off. Moreover, the strong fibrous inner bark that is easily peeled off in strips was once used by both Māori and Europeans as twine for fishing nets, as well as to make rope. In the past, the inner fibres were also used by gardeners for tying up plants. In addition, when food was scarce Māori would sometimes eat the fibrous inner bark of lowland ribbonwood.